THROUGH THE TELESCOPE

AGENTS

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PLATE I.

The 40-inch Refractor of the Yerkes Observatory.

THROUGH
THE TELESCOPE
BY
JAMES BAIKIE, F.R.A.S.

WITH 32 FULL-PAGE ILLUSTRATIONS FROM PHOTOGRAPHS
AND 26 SMALLER FIGURES IN THE TEXT

LONDON
ADAM AND CHARLES BLACK
1906

TO

C. N. B. and H. E. B.

PREFACE

The main object of the following chapters is to give a brief and simple description of the most important and interesting facts concerning the heavenly bodies, and to suggest to the general reader how much of the ground thus covered lies open to his personal survey on very easy conditions. Many people who are more or less interested in astronomy are deterred from making practical acquaintance with the wonders of the heavens by the idea that these are only disclosed to the possessors of large and costly instruments. In reality there is probably no science which offers to those whose opportunities and means of observation are restricted greater stores of knowledge and pleasure than astronomy; and the possibility of that quickening of interest which can only be gained by practical study is, in these days, denied to very few indeed.

Accordingly, I have endeavoured, while recounting the great triumphs of astronomical discovery, to give some practical help to those who are inclined to the study of the heavens, but do not know how to begin. My excuse for venturing on such a task must be that, in the course of nearly twenty years of observation with telescopes of all sorts and sizes, I have made most of the mistakes against which others need to be warned.

The book has no pretensions to being a complete manual; it is merely descriptive of things seen and learned. Nor has it any claim to originality. On the contrary, one of its chief purposes has been to gather into short compass the results of the work of others. I have therefore to acknowledge my indebtedness to other writers, and notably to Miss Agnes Clerke, Professor Young, Professor Newcomb, the late Rev. T. W. Webb, and Mr. W. F. Denning. I have also found much help in the Monthly Notices and Memoirs of the Royal Astronomical Society, and the Journal and Memoirs of the British Astronomical Association.

The illustrations have been mainly chosen with the view of representing to the general reader some of the results of the best modern observers and instruments; but I have ventured to reproduce a few specimens of more commonplace work done with small telescopes. I desire to offer my cordial thanks to those who have so kindly granted me permission to reproduce illustrations from their published works, or have lent photographs or drawings for reproduction—to Miss Agnes Clerke for Plates XXV.-XXVIII. and XXX.-XXXII. inclusive; to Mrs. Maunder for Plate VIII.; to M. Loewy, Director of the Paris Observatory, for Plates XI.-XIV. and Plate XVII.; to Professor E. B. Frost, Director of the Yerkes Observatory, for Plates I., VII., XV., and XVI.; to M. Deslandres, of the Meudon Observatory, for Plate IX., and the gift of several of his own solar memoirs; to the Astronomer Royal for England, Sir W. Mahony Christie, for Plate V.; to Mr. H. MacEwen for the drawings of Venus, Plate X.; to the Rev. T. E. R. Phillips for those of Mars and Jupiter, Plates XX. and XXII.; to Professor Barnard for that of Saturn, Plate XXIV., reproduced by permission from the Monthly Notices of the Royal Astronomical Society; to Mr. W. E. Wilson for Plates XXIX. and XXXII.; to Mr. John Murray for Plates XVIII. and XIX.; to the proprietors of Knowledge for Plate VI.; to Mr. Denning and Messrs. Taylor and Francis for Plate III. and Figs. 6 and 20; to the British Astronomical Association for the chart of Mars, Plate XXI., reproduced from the Memoirs; and to Messrs. T. Cooke and Sons for Plate II. For those who wish to see for themselves some of the wonders and beauties of the starry heavens the two Appendices furnish a few specimens chosen from an innumerable company; while readers who have no desire to engage in practical work are invited to skip Chapters I. and II.

CONTENTS

LIST OF ILLUSTRATIONS

PRINTED SEPARATELY FROM THE TEXT

LIST OF ILLUSTRATIONS

PRINTED IN THE TEXT

THROUGH THE TELESCOPE

CHAPTER I

THE TELESCOPE—HISTORICAL

The claim of priority in the invention of this wonderful instrument, which has so enlarged our ideas of the scale and variety of the universe, has been warmly asserted on behalf of a number of individuals. Holland maintains the rights of Jansen, Lippershey, and Metius; while our own country produces evidence that Roger Bacon had, in the thirteenth century, 'arrived at theoretical proof of the possibility of constructing a telescope and a microscope' and that Leonard Digges 'had a method of discovering, by perspective glasses set at due angles, all objects pretty far distant that the sun shone on, which lay in the country round about.'

All these claims, however, whether well or ill founded, are very little to the point. The man to whom the human race owes a debt of gratitude in connection with any great invention is not necessarily he who, perhaps by mere accident, may stumble on the principle of it, but he who takes up the raw material of the invention and shows the full powers and possibilities which are latent in it. In the present case there is one such man to whom, beyond all question, we owe the telescope as a practical astronomical instrument, and that man is Galileo Galilei. He himself admits that it was only after hearing, in 1609, that a Dutchman had succeeded in making such an instrument, that he set himself to investigate the matter, and produced telescopes ranging from one magnifying but three diameters up to the one with a power of thirty-three with which he made his famous discoveries; but this fact cannot deprive the great Italian of the credit which is undoubtedly his due. Others may have anticipated him in theory, or even to a small extent in practice, but Galileo first gave to the world the telescope as an instrument of real value in research.

The telescope with which he made his great discoveries was constructed on a principle which, except in the case of binoculars, is now discarded. It consisted of a double convex lens converging the rays of light from a distant object, and of a double concave lens, intercepting the convergent rays before they reach a focus, and rendering them parallel again (Fig. 1). His largest instrument, as already mentioned, had a power of only thirty-three diameters, and the field of view was very small. A more powerful one can now be obtained for a few shillings, or constructed, one might almost say, for a few pence; yet, as Proctor has observed: 'If we regard the absolute importance of the discoveries effected by different telescopes, few, perhaps, will rank higher than the little tube now lying in the Tribune of Galileo at Florence.'

FIG. 1.—PRINCIPLE OF GALILEAN TELESCOPE.

Galileo's first discoveries with this instrument were made in 1610, and it was not till nearly half a century later that any great improvement in telescopic construction was effected. In the middle of the seventeenth century Scheiner and Huygens made telescopes on the principle, suggested by Kepler, of using two double convex lenses instead of a convex and a concave, and the modern refracting telescope is still constructed on essentially the same principle, though, of course, with many minor modifications (Fig. 2).

FIG. 2.—PRINCIPLE OF COMMON REFRACTOR.

The latter part of the seventeenth century witnessed the introduction of telescopes on this principle of the most amazing length, the increase in length being designed to minimize the imperfections which a simple lens exhibits both in definition and in colour. Huygens constructed one such telescope of 123 feet focal length, which he presented to the Royal Society of London; Cassini, at Paris, used instruments of 100 and 136 feet; while Bradley, in 1722, measured the diameter of Venus with a glass whose focal length was 212¼ feet. Auzout is said to have made glasses of lengths varying from 300 to 600 feet, but, as might have been expected, there is no record of any useful observations having ever been made with these monstrosities. Of course, these instruments differed widely from the compact and handy telescopes with which we are now familiar. They were entirely without tubes. The object-glass was fastened to a tall pole or to some high building, and was painfully manœuvred into line with the eye-piece, which was placed on a support near the ground, by means of an arrangement of cords. The difficulties of observation with these unwieldy monsters must have been of the most exasperating type, while their magnifying power did not exceed that of an ordinary modern achromatic of, perhaps, 36 inches focal length. Cassini, for instance, seems never to have gone beyond a power of 150 diameters, which might be quite usefully employed on a good modern 3-inch refractor in good air. Yet with such tools he was able to discover four of the satellites of Saturn and that division in Saturn's ring which still bears his name. Such facts speak volumes for the quality of the observer. Those who are the most accustomed to use the almost perfect products of modern optical skill will have the best conception of, and the profoundest admiration for, the limitless patience and the wonderful ability which enabled him to achieve such results with the very imperfect means at his disposal.

The clumsiness and unmanageableness of these aerial telescopes quickly reached a point which made it evident that nothing more was to be expected of them; and attempts were made to find a method of combining lenses, which might result in an instrument capable of bearing equal or greater magnifying powers on a much shorter length. The chief hindrance to the efficiency of the refracting telescope lies in the fact that the rays of different colours which collectively compose white light cannot be brought to one focus by any single lens. The red rays, for example, have a different focal length from the blue, and so any lens which brings the one set to a focus leaves a fringe of the other outstanding around any bright object.

In 1729 Mr. Chester Moor Hall discovered a means of conquering this difficulty, but his results were not followed up, and it was left for the optician John Dollond to rediscover the principle some twenty-five years later. By making the object-glass of the telescope double, the one lens being of crown and the other of flint glass, he succeeded in obtaining a telescope which gave a virtually colourless image.

This great discovery of the achromatic form of construction at once revolutionized the art of telescope-making. It was found that instruments of not more than 5 feet focal length could be constructed, which infinitely surpassed in efficiency, as well as in handiness, the cumbrous tools which Cassini had used; and Dollond's 5-foot achromatics, generally with object-glasses of 3¾ inches diameter, represented for a considerable time the acme of optical excellence. Since the time of Dollond, the record of the achromatic refractor has been one of continual, and, latterly, of very rapid progress. For a time much hindrance was experienced from the fact that it proved exceedingly difficult to obtain glass discs of any size whose purity and uniformity were sufficient to enable them to pass the stringent test of optical performance. In the latter part of the eighteenth century, a 6-inch glass was considered with feelings of admiration, somewhat similar to those with which we regard the Yerkes 40-inch to-day; and when, in 1823, the Dorpat refractor of 9⁶⁄10 inches was mounted (Fig. 3), the astronomical world seemed to have the idea that something very like finality had been reached. The Dorpat telescope proved, however, to be only a milestone on the path of progress. Before very long it was surpassed by a glass of 12 inches diameter, which Sir James South obtained from Cauchoix of Paris, and which is now mounted in the Dunsink Observatory, Dublin. This, in its turn, had to give place to the fine instruments of 14·9 inches which were figured by Merz of Munich for the Pulkowa and Cambridge (U.S.A.) Observatories; and then there came a pause of a few years, which was broken by Alvan Clark's completion of an 18½-inch, an instrument which earned its diploma, before ever it left the workshop of its constructor, by the discovery of the companion to Sirius.

FIG. 3.—DORPAT REFRACTOR.

The next step was made on our side of the Atlantic, and proved to be a long and notable one, in a sense definitely marking out the boundary line of the modern era of giant refractors. This was the completion, by Thomas Cooke, of York, of a 25-inch instrument for the late Mr. Newall. It did not retain for long its pride of place. The palm was speedily taken back to America by Alvan Clark's construction of the 26-inch of the Washington Naval Observatory, with which Professor Asaph Hall discovered in 1877 the two satellites of Mars. Then came Grubb's 27-inch for Vienna; the pair of 30-inch instruments, by Clark and Henry respectively, for Pulkowa (Fig. 4) and Nice; and at last the instrument which has for a number of years been regarded as the finest example of optical skill in the world, the 36-inch Clark refractor of the Lick Observatory, California. Placed at an elevation of over 4,000 feet, and in a climate exceptionally well suited for astronomical work, this fine instrument has had the advantage of being handled by a very remarkable succession of brilliant observers, and has, since its completion, been looked to as a sort of court of final appeal in disputed questions. But America has not been satisfied even with such an instrument, and the 40-inch Clark refractor of the Yerkes Observatory is at present the last word of optical skill so far as achromatics are concerned (Frontispiece). It is not improbable that it may also be the last word so far as size goes, for the late Professor Keeler's report upon its performance implies that in this splendid telescope the limit of practicable size for object-glasses is being approached. The star images formed by the great lens show indications of slight flexure of the glass under its own weight as it is turned from one part of the sky to another. It would be rash, however, to say that even this difficulty will not be overcome. So many obstacles, seemingly insuperable, have vanished before the astronomer's imperious demand for 'more light,' and so many great telescopes, believed in their day to represent the absolute culmination of the optical art, are now mere commoners in the ranks where once they were supreme, that it may quite conceivably prove that the great Yerkes refractor, like so many of its predecessors, represents only a stage and not the end of the journey.

FIG. 4.—30-INCH REFRACTOR, PULKOWA OBSERVATORY.

Meanwhile, Sir Isaac Newton, considering, wrongly as the sequel showed, that 'the case of the refractor was desperate,' set about the attempt to find out whether the reflection of light by means of suitably-shaped mirrors might not afford a substitute for the refractor. In this attempt he was successful, and in 1671 presented to the Royal Society the first specimen, constructed by his own hands, of that form of reflecting telescope which has since borne his name. The principle of the Newtonian reflector will be easily grasped from Fig. 5. The rays of light from the object under inspection enter the open mouth of the instrument, and passing down the tube are converged by the concave mirror AA towards a focus, before reaching which they are intercepted by the small flat mirror BB, placed at an angle of 45 degrees to the axis of the tube, and are by it reflected into the eye-piece E which is placed at the side of the instrument. In this construction, therefore, the observer actually looks in a direction at right angles to that of the object which he is viewing, a condition which seems strange to the uninitiated, but which presents no difficulties in practice, and is found to have several advantages, chief among them the fact that there is no breaking of one's neck in the attempt to observe objects near the zenith, the line of vision being always horizontal, no matter what may be the altitude of the object under inspection. Other forms of reflector have been devised, and go by the names of the Gregorian, the Cassegrain, and the Herschelian; but the Newtonian has proved itself the superior, and has practically driven its rivals out of the field, though the Cassegrain form has been revived in a few instances of late years, and is particularly suited to certain forms of research.

FIG. 5.—PRINCIPLE OF NEWTONIAN REFLECTOR.

FIG. 6.—LORD ROSSE'S TELESCOPE.

At first the mirrors of reflecting telescopes were made of an alloy known as speculum metal, which consisted of practically 4 parts of copper to 1 of tin; but during the last half-century this metal has been entirely superseded by mirrors made of glass ground to the proper figure, and then polished and silvered on the face by a chemical process. To the reflecting form of construction belong some of the largest telescopes in the world, such as the Rosse 6-foot (metal mirrors), Fig. 6, the Common 5-foot (silver on glass), the Melbourne 4-foot (metal mirrors, Cassegrain form), and the 5-foot constructed by Mr. Ritchey for the Yerkes Observatory. Probably the most celebrated, as it was also the first of these monsters, was the 4-foot telescope of Sir William Herschel, made by himself on the principle which goes by his name. It was used by him to some extent in the discoveries which have made his name famous, and nearly everyone who has ever opened an astronomical book is familiar with the engraving of the huge 40-foot tube, with its cumbrous staging, which Oliver Wendell Holmes has so quaintly celebrated in 'The Poet at the Breakfast Table' (Fig. 7).

FIG. 7.—HERSCHEL'S 4-FOOT REFLECTOR.

CHAPTER II

THE TELESCOPE—PRACTICAL

Having thus briefly sketched the history of the telescope, we turn now to consider the optical means which are most likely to be in the hands or within the reach of the beginner in astronomical observation. Let us, first of all, make the statement that any telescope, good, bad, or indifferent, is better than no telescope. There are some purists who would demur to such a statement, who make the beginner's heart heavy with the verdict that it is better to have no telescope at all than one that is not of the utmost perfection, and, of course, of corresponding costliness, and who seem to believe that the performance of an inferior glass may breed disgust at astronomy altogether. This is surely mere nonsense. For most amateurs at the beginning of their astronomical work the question is not between a good telescope and an inferior one, it is between a telescope and no telescope. Of course, no one would be so foolish as willingly to observe with an inferior instrument if a better could be had; but even a comparatively poor glass will reveal much that is of great interest and beauty, and its defects must even be put up with sometimes for the sake of its advantages until something more satisfactory can be obtained. An instrument which will show fifty stars where the naked eye sees five is not to be despised, even though it may show wings to Sirius that have no business there, or a brilliant fringe of colours round Venus to which even that beautiful planet can lay no real claim. Galileo's telescope would be considered a shockingly bad instrument nowadays; still, it had its own little influence upon the history of astronomy, and the wonders which it first revealed are easily within the reach of anyone who has the command of a shilling or two, and, what is perhaps still more important, of a little patience. The writer has still in his possession an object-glass made out of a simple single eyeglass, such as is worn by Mr. Joseph Chamberlain. This, mounted in a cardboard tube with another single lens in a sliding tube as an eye-piece, proved competent to reveal the more prominent lunar craters, a number of sunspots, the phases of Venus, and the existence, though not the true form, of Saturn's ring. Its total cost, if memory serve, was one shilling and a penny. Of course it showed, in addition, a number of things which should not have been seen, such as a lovely border of colour round every bright object; but, at the same time, it gave a great deal more than thirteen pence worth of pleasure and instruction.

Furthermore, there is this to be said in favour of beginning with a cheap and inferior instrument, that experience may thus be gained in the least costly fashion. The budding astronomer is by nature insatiably curious. He wants to know the why and how of all the things that his telescope does or does not do. Now this curiosity, while eminently laudable in itself, is apt in the end to be rather hard upon his instrument. A fine telescope, whatever its size may be, is an instrument that requires and should receive careful handling; it is easily damaged, and costly to replace. And therefore it may be better that the beginner should make his earlier experiments, and find out the more conspicuous and immediately fatal of the many ways of damaging a telescope, upon an instrument whose injury, or even whose total destruction, need not cause him many pangs or much financial loss.

It is not suggested that a beginning should necessarily be made on such a humble footing as that just indicated. Telescopes of the sizes mainly referred to in these pages—i.e., refractors of 2 or 3 inches aperture, and reflectors of 4½ to 6 inches—may frequently be picked up second-hand at a very moderate figure indeed. Of course, in these circumstances the purchaser has to take his chance of defects in the instrument, unless he can arrange for a trial of it, either by himself, or, preferably, by a friend who has some experience; yet even should the glass turn out far from perfect, the chances are that it will at least be worth the small sum paid for it. Nor is it in the least probable, as some writers seem to believe, that the use of an inferior instrument will disgust the student and hinder him from prosecuting his studies. The chances are that it will merely create a desire for more satisfactory optical means. Even a skilled observer like the late Rev. T. W. Webb had to confess of one of his telescopes that 'much of its light went the wrong way'; and yet he was able to get both use and pleasure out of it. The words of a well-known English amateur observer may be quoted. After detailing his essays with glasses of various degrees of imperfection Mr. Mee remarks: 'For the intending amateur I could wish no other experience than my own. To commence with a large and perfect instrument is a mistake; its owner cannot properly appreciate it, and in gaining experience is pretty sure to do the glass irreparable injury.'

Should the beginner not be willing or able to face the purchase of even a comparatively humble instrument, his case is by no means desperate, for he will find facilities at hand, such as were not thought of a few years ago, for the construction of his own telescope. Two-inch achromatic object-glasses, with suitable lenses for the making up of the requisite eye-pieces, are to be had for a few shillings, together with cardboard tubes of sizes suitable for fitting up the instrument; and such a volume as Fowler's 'Telescopic Astronomy' gives complete directions for the construction of a glass which is capable of a wonderful amount of work in proportion to its cost. The substitution of metal tubes for the cardboard ones is desirable, as metal will be found to be much more satisfactory if the instrument is to be much used. The observer, however, will not long be satisfied with such tools as these, useful though they may be. The natural history of amateur astronomers may be summed up briefly in the words 'they go from strength to strength.' The possessor of a small telescope naturally and inevitably covets a bigger one; and when the bigger one has been secured it represents only a stage in the search for one bigger still, while along with the desire for increased size goes that for increased optical perfection. No properly constituted amateur will be satisfied until he has got the largest and best instrument that he has money to buy, space to house, and time to use.

Let us suppose, then, that the telescope has been acquired, and that it is such an instrument as may very commonly be found in the hands of a beginner—a refractor, say, of 2, 2½, or 3 inches aperture (diameter of object-glass). The question of reflectors will fall to be considered later. Human nature suggests that the first thing to do with it is to unscrew all the screws and take the new acquisition to pieces, so far as possible, in order to examine into its construction. Hence many glasses whose career of usefulness is cut short before it has well begun. 'In most cases,' says Webb, 'a screw-driver is a dangerous tool in inexperienced hands'; and Smyth, in the Prolegomena to his 'Celestial Cycle,' utters words of solemn warning to the 'over-handy gentlemen who, in their feverish anxiety for meddling with and making instruments, are continually tormenting them with screw-drivers, files, and what-not.' Unfortunately, it is not only the screw-driver that is dangerous; the most deadly danger to the most delicate part of the telescope lies in the unarmed but inexperienced hands themselves. You may do more irreparable damage to the object-glass of your telescope in five minutes with your fingers than you are likely to do to the rest of the instrument in a month with a screw-driver. Remember that an object-glass is a work of art, sometimes as costly as, and always much more remarkable than, the finest piece of jewellery. It may be unscrewed, carefully, from the end of its tube and examined. Should the examination lead to the detection of bubbles or even scratches in the glass (quite likely the latter if the instrument be second-hand), these need not unduly vex its owner's soul. They do not necessarily mean bad performance, and the amount of light which they obstruct is very small, unless the case be an extreme one. But on no account should the two lenses of the object-glass itself be separated, for this will only result in making a good objective bad and a bad one worse. The lenses were presumably placed in their proper adjustment to one another by an optician before being sent out; and should their performance be so unsatisfactory as to suggest that this adjustment has been disturbed, it is to an optician that they should be returned for inspection. The glass may, of course, be carefully and gently cleaned, using either soft chamois leather, or preferably an old silk handkerchief, studiously kept from dust; but the cleaning should never amount to more than a gentle sweeping away of any dust which may have gathered on the surface. Rubbing is not to be thought of, and the man whose telescope has been so neglected that its object-glass needs rubbing should turn to some other and less reprehensible form of mischief. For cleaning the small lenses of the eye-pieces, the same silk may be employed; Webb recommends a piece of blotting-paper, rolled to a point and aided by breathing, for the edges which are awkward to get at. Care must, of course, be taken to replace these lenses in their original positions, and the easiest way to ensure this is to take out only one at a time. In replacing them, see that the finger does not touch the surface of the glass, or the cleaning will be all to do over again.

FIG. 8.

a, O.G. in perfect adjustment; b, O.G. defectively centred.

Next comes the question of testing the quality of the objective. (The stand is meanwhile assumed, but will be spoken of later.) Point the telescope to a star of about the third magnitude, and employ the eye-piece of highest power, if more than one goes with the instrument—this will be the shortest eye-piece of the set. If the glass be of high quality, the image of the star will be a neat round disc of small size, surrounded by one or two thin bright rings (Fig. 8, a). Should the image be elliptical and the rings be thrown to the one side (Fig. 8, b), the glass may still be quite a good one, but is out of square, and should be readjusted by an optician. Should the image be irregular and the rings broken, the glass is of inferior quality, though it may still be serviceable enough for many purposes. Next throw the image of the star out of focus by racking the eye-piece in towards the objective, and then repeat the process by racking it again out of focus away from the objective. The image will, in either case, expand into a number of rings of light, and these rings should be truly circular, and should present precisely the same appearance at equal distances within and without the focus. A further conception of the objective's quality may be gained by observing whether the image of a star or the detail of the moon or of the planets comes sharply to a focus when the milled head for focussing is turned. Should it be possible to rack the eye-tube in or out for any distance without disturbing the distinctness of the picture to any extent, then the glass is defective. A good objective will admit of no such range, but will come sharply up to focus, and as sharply away from it, with any motion of the focussing screw. A good glass will also show the details of a planet like Saturn, such as are within its reach, that is, with clearness of definition, while an inferior one will soften all the outlines, and impart a general haziness to them. The observer may now proceed to test the colour correction of his objective. No achromatic, its name notwithstanding, ever gives an absolutely colourless image; all that can be expected is that the colour aberration should have been so far eliminated as not to be unpleasant. In a good instrument a fringe of violet or blue will be seen around any bright object, such as Venus, on a dark sky; a poor glass will show red or yellow. It is well to make sure, however, should bad colour be seen, that the eye-piece is not causing it; and, therefore, more than one eye-piece should be tried before an opinion is formed. Probably more colour will be seen at first than was expected, more particularly with an object so brilliant as Venus. But the observer need not worry overmuch about this. He will find that the eye gets so accustomed to it as almost to forget that it is there, so that something of a shock may be experienced when a casual star-gazing friend, on looking at some bright object, remarks, as friends always do, 'What beautiful colours!' Denning records a somewhat extreme case in which a friend, who had been accustomed to observe with a refractor, absolutely resented the absence of the familiar colour fringe in the picture given by a reflector, which is the true achromatic in nature, though not in name. The beginner is recommended to read the article 'The Adjustment of a Small Equatorial,' by Mr. E. W. Maunder, in the Journal of the British Astronomical Association, vol. ii., p. 219, where he will find the process of testing described at length and with great clearness.

In making these tests, allowance has, of course, to be made for the state of the atmosphere. A good telescope can only do its best on a good night, and it is not fair to any instrument to condemn it until it has been tested under favourable conditions. The ideal test would be to have its performance tried along with that of another instrument of known good quality and of as nearly the same size as possible. If this cannot be arranged for, the tests must be made on a succession of nights, and good performance on one of these is sufficient to vindicate the reputation of the glass, and to show that any deficiency on other occasions was due to the state of the air, and not to the instrument. Should his telescope pass the above tests satisfactorily, the observer ought to count himself a happy man, and will until he begins to hanker after a bigger instrument.

The mention of the pointing of the telescope to a star brings up the question of how this is to be done. It seems a simple thing; as a matter of fact, with anything like a high magnifying power it is next to impossible; and there are few things more exasperating than to see a star or a planet shining brightly before your eyes, and yet to find yourself quite unable to get it into the field of view. The simple remedy is the addition of a finder to the telescope. This is a small telescope of low magnifying power which is fastened to the larger instrument by means of collars bearing adjusting screws, which enable it to be laid accurately parallel with the large tube (Fig. 10). Its eye-piece is furnished with cross-threads, and a star brought to the intersection of these threads will be in the field of the large telescope. In place of the two threads crossing at right angles there may be substituted three threads interlacing to form a little triangle in the centre of the finder's field. By this device the star can always be seen when the glass is being pointed instead of being hidden, as in the other case, behind the intersection of the two threads. A fine needle-point fixed in the eye-piece will also be found an efficient substitute for the cross-threads. In the absence of a finder the telescope may be pointed by using the lowest power eye-piece and substituting a higher one when the object is in the field; but beyond question the finder is well worth the small addition which it makes to the cost of an instrument. A little care in adjusting the finder now and again will often save trouble and annoyance on a working evening.

The question of a stand on which to mount the telescope now falls to be considered, and is one of great importance, though apt to be rather neglected at first. It will soon be found that little satisfaction or comfort can be had in observing unless the stand adopted is steady. A shaky mounting will spoil the performance of the best telescope that ever was made, and will only tantalize the observer with occasional glimpses of what might be seen under better conditions. Better have a little less aperture to the object-glass, and a good steady mounting, than an extra inch of objective and a mounting which robs you of all comfort in the using of your telescope. Beginners are indeed rather apt to be misled into the idea that the only matters of importance are the objective and its tube, and that money spent on the stand is money wasted. Hence many fearful and wonderful contrivances for doing badly what a little saved in the size of the telescope and expended on the stand would have enabled them to do well. It is very interesting, no doubt, to get a view of Jupiter or Saturn for one field's-breadth, and then to find, on attempting to readjust the instrument for another look, that the mounting has obligingly taken your star-gazing into its own hands, and is now directing your telescope to a different object altogether; but repetition of this form of amusement is apt to pall. A radically weak stand can never be made into a good one; the best plan is to get a properly proportioned mounting at once, and be done with it.

FIG. 9.—SMALL TELESCOPE ON PILLAR AND CLAW STAND.

For small instruments, such as we are dealing with, the mounting generally adopted is that known as the Altazimuth, from its giving two motions, one in altitude and one in azimuth, or, to use more familiar terms, one vertical and the other horizontal. There are various types of the Altazimuth. If the instrument be of not more than 3 feet focal length, the ordinary stand known as the 'pillar and claw' (Fig. 9) will meet all the requirements of this form of motion. Should the focal length be greater than 3 feet, it is advisable to have the instrument mounted on a tripod stand, such as is shown in Fig. 10. In the simpler forms of both these mountings the two motions requisite to follow an object must be given by hand, and it is practically impossible to do this without conveying a certain amount of tremor to the telescope, which disturbs clearness of vision until it subsides, by which time the object to be viewed is generally getting ready to go out of the field again. To obviate this inconvenience as far as possible, the star or planet when found should be placed just outside the field of view, and allowed to enter it by the diurnal motion of the earth. The tremors will thus have time to subside before the object reaches the centre of the field, and this process must be repeated as long as the observation continues. In making this adjustment attention must be paid to the direction of the object's motion through the field, which, of course, varies according to its position in the sky. If it be remembered that a star's motion through the telescopic field is the exact reverse of its true direction across the sky, little difficulty will be found, and use will soon render the matter so familiar that the adjustment will be made almost automatically.

FIG. 10.—TELESCOPE ON TRIPOD, WITH FINDER AND SLOW MOTIONS.

A much more convenient way of imparting the requisite motions is by the employment of tangent screws connected with Hooke's joint-handles, which are brought conveniently near to the hands of the observer as he sits at the eye-end. These screws clamp into circles or portions of circles, which have teeth cut on them to fit the pitch of the screw, and by means of them a slow and steady motion may be imparted to the telescope. When it is required to move the instrument more rapidly, or over a large expanse of sky, the clamps which connect the screws with the circles are slackened, and the motion is given by hand. Fig. 10 shows an instrument provided with these adjuncts, which, though not absolutely necessary, and adding somewhat to the cost of the mounting, are certainly a great addition to the ease and comfort of observation.

FIG. 11.—EQUATORIAL MOUNTING FOR SMALL TELESCOPE.

The Altazimuth mounting, from its simplicity and comparative cheapness, has all along been, and will probably continue to be, the form most used by amateurs. It is, however, decidedly inferior in every respect to the equatorial form of mount. In this form (Fig. 11) the telescope is carried by means of two axes, one of which—the Polar axis—is so adjusted as to be parallel to the pole of the earth's rotation, its degree of inclination being therefore dependent upon the latitude of the place for which it is designed. At the equator it will be horizontal, will lie at an angle of 45 degrees half-way between the equator and either pole, and will be vertical at the poles. At its upper end it carries a cross-head with bearings through which there passes another axis at right angles to the first (the declination axis). Both these axes are free to rotate in their respective bearings, and thus the telescope is capable of two motions, one of which—that of the declination axis—enables the instrument to be set to the elevation of the object to be observed, while the other—that of the polar axis—enables the observer to follow the object, when found, from its rising to its setting by means of a single movement, the telescope sweeping out circles on the sky corresponding to those which the stars themselves describe in their journey across the heavens. This single movement may be given by means of a tangent screw such as has already been described, and the use of a telescope thus equipped is certainly much easier and more convenient than that of an Altazimuth, where two motions have constantly to be imparted. To gain the full advantage of the equatorial form of mounting, the polar axis must be placed exactly in the North and South line, and unless the mounting can be adjusted properly and left in adjustment, it is robbed of much of its superiority. For large fixed instruments it is, of course, almost universally used; and in observatories the motion in Right Ascension, as it is called, which follows the star across the sky, is communicated to the driving-wheel of the polar axis by means of a clock which turns the rod carrying the tangent screw (Plate [II.]). These are matters which in most circumstances are outside the sphere of the amateur; it may be interesting for him, however, to see examples of the way in which large instruments are mounted. The frontispiece, accordingly, shows the largest and most perfect instrument at present in existence, while Plate II., with Figs. [4] and [12], give further examples of fine modern work. The student can scarcely fail to be struck by the extreme solidity of the modern mountings, and by the way in which all the mechanical parts of the instrument are so contrived as to give the greatest convenience and ease in working. Comparing, for instance, Plate II., a 6-inch refractor by Messrs. Cooke, of York, available either for visual or photographic work, with the Dorpat refractor (Fig. [3]), it is seen that the modern maker uses for a 6-inch telescope a stand much more solid and steady than was deemed sufficient eighty years ago for an instrument of 9⁶⁄10 inches. Attention is particularly directed to the way in which nowadays all the motions are brought to the eye-end so as to be most convenient for the observer, and frequently, as in this case, accomplished by electric power, while the declination circle is read by means of a small telescope so that the large instrument can be directed upon any object with the minimum of trouble. The driving clock, well shown on the right of the supporting pillar, is automatically controlled by electric current from the sidereal clock of the observatory.

PLATE II.

6-inch Photo-Visual Refractor, equatorially mounted. Messrs. T. Cooke & Sons.

We have now to consider the reflecting form of telescope, which, especially in this country, has deservedly gained much favour, and has come to be regarded as in some sense the amateur's particular tool.

FIG. 12.—8-INCH REFRACTOR ON EQUATORIAL MOUNTING.

As a matter of policy, one can scarcely advise the beginner to make his first essay with a reflector. Its adjustments, though simple enough, are apt to be troublesome at the time when everything has to be learned by experience; and its silver films, though much more durable than is commonly supposed, are easily destroyed by careless or unskilful handling, and require more careful nursing than the objective of a refractor. But, having once paid his first fees to experience, the observer, if he feel so inclined, may venture upon a reflector, which has probably more than sufficient advantages to make up for its weaker points. First and foremost of these advantages stands the not inconsiderable one of cheapness. A 10½-inch reflector may be purchased new for rather less than the sum which will buy a 4-inch refractor. True, the reflector has not the same command of light inch for inch as the refractor, but a reflector of 10½ inches should at least be the match of an 8-inch refractor in this respect, and will be immeasurably more powerful than the 4-inch refractor, which comes nearest to it in price. Second stands the ease and comfort so conspicuous in observing with a Newtonian. Instead of having almost to break his neck craning under the eye-piece of a telescope pointed to near the zenith, the observer with a Newtonian looks always straight in front of him, as the eye-piece of a reflector mounted as an altazimuth is always horizontal, and when the instrument is mounted equatorially, the tube, or its eye-end, is made to rotate so that the line of vision may be kept horizontal. Third is the absence of colour. Colour is not conspicuous in a small refractor, unless the objective be of very bad quality; but as the aperture increases it is apt to become somewhat painfully apparent. The reflector, on the other hand, is truly achromatic, and may be relied upon to show the natural tints of all objects with which it deals. This point is of considerable importance in connection with planetary observation. The colouring of Jupiter, for instance, will be seen in a reflector as a refractor can never show it.

Against these advantages there have to be set certain disadvantages. First, the question of adjustments. A small refractor requires practically none; but a reflector, whatever its size, must be occasionally attended to, or else its mirrors will get out of square and bad performance will be the result. It is easy, however, to make too much of this difficulty. The adjustments of the writer's 8½-inch With reflector have remained for months at a time as perfect as when they had been newly attended to. Second, the renewal of the silver films. This may cause some trouble in the neighbourhood of towns where the atmosphere is such as to tarnish silver quickly; and even in the country a film must be renewed at intervals. But these may be long enough. The film on the mirror above referred to has stood without serious deterioration for five years at a time. Third, the reflector, with its open-mouthed tube, is undoubtedly more subject to disturbance from air currents and changes of temperature, and its mirrors take longer to settle down into good definition after the instrument has been moved from one point of the sky to another. This difficulty cannot be got over, and must be put up with; but it is not very conspicuous with the smaller sizes of telescopes, such as are likely to be in the hands of an amateur at the beginning of his work. There are probably but few nights when an 8½-inch reflector will not give quite a good account of itself in this respect by comparison with a refractor of anything like equal power. On the whole, the state of the question is this: If the observer wishes to have as much power as possible in proportion to his expenditure, and is not afraid to take the risk of a small amount of trouble with the adjustments and films, the reflector is probably the instrument best suited to him. If, on the other hand, he is so situated that his telescope has to be much moved, or, which is almost as bad, has to stand unused for any considerable intervals of time, he will be well advised to prefer a refractor. One further advantage of the reflecting form is that, aperture for aperture, it is very much shorter. The average refractor will probably run to a length of from twelve to fifteen times the diameter of its objective. Reflectors are rarely of a greater length than nine times the diameter of the large mirror, and are frequently shorter still. Consequently, size for size, they can be worked in less space, which is often a consideration of importance.

FIG. 13.—FOUR-FOOT REFLECTOR EQUATORIALLY MOUNTED.

The mountings of the reflector are in principle precisely similar to those of the refractor already described. The greater weight, however, and the convenience of having the body of the instrument kept as low as possible, owing to the fact of the eye-piece being at the upper end of the tube, have necessitated various modifications in the forms to which these principles are applied. Plates [III.] and [IV.], and Fig. 13, illustrate the altazimuth and equatorial forms of mounting as applied to reflectors of various sizes, Fig. 13 being a representation of Lassell's great 4-foot reflector.

PLATE III.

20-inch Reflector, Stanmore Observatory.

And now, having his telescope, whatever its size, principle, or form of mounting, the observer has to proceed to use it. Generally speaking, there is no great difficulty in arriving at the manner of using either a refractor or a reflector, and for either instrument the details of handling must be learned by experience, as nearly all makers have little variations of their own in the form of clamps and slow motions, though the principles in all instruments are the same. With regard to these, the only recommendation that need be made is one of caution in the use of the glass until its ways of working have been gradually found out. With a knowledge of the principles of its construction and a little application of common-sense, there is no part of a telescope mounting which may not be readily understood. Accordingly, what follows must simply take the form of general hints as to matters which every telescopist ought to know, and which are easier learned once and for all at the beginning than by slow experience. These hints are of course the very commonplaces of observation; but it is the commonplace that is the foundation of good work in everything.

If possible, let the telescope be fixed in the open air. Where money is no object, a few pounds will furnish a convenient little telescope-house, with either a rotating or sliding roof, which enables the instrument to be pointed to any quarter of the heavens. Such houses are now much more easily obtained than they once were, and anyone who has tried both ways can testify how much handier it is to have nothing to do but unlock the little observatory, and find the telescope ready for work, than to have to carry a heavy instrument out into the open. Plate [IV.] illustrates such a shelter, which has done duty for more than twelve years, covering an 8½-inch With, whose tube and mounting are almost entirely the work of a local smith; and in the Journal of the British Astronomical Association, vol. xiv., p. 283, Mr. Edwin Holmes gives a simple description of a small observatory which was put up at a cost of about £3, and has proved efficient and durable. The telescope-house has also the advantage of protecting the observer and his instrument from the wind, so that observation may often be carried on on nights which would be quite too windy for work in the open.

PLATE IV.

Telescope House and 8½-inch 'with' Reflector.

Should it not be possible to obtain such a luxury, however, undoubtedly the next best is fairly outside. No one who has garden room should ever think of observing from within doors. If the telescope be used at an open window its definition will be impaired by air-currents. The floor of the room will communicate tremors to the instrument, and every movement of the observer will be accompanied by a corresponding movement of the object in the field, with results that are anything but satisfactory. In some cases no other position is available. If this be so, Webb's advice must be followed, the window opened as widely and as long beforehand as possible, and the telescope thrust out as far as is convenient. But these precautions only palliate the evils of indoor observation. The open air is the best, and with a little care in wrapping up the observer need run no risk.

Provide the telescope, if a refractor, with a dew-cap. Without this precaution dew is certain to gather upon the object-glass, with the result of stopping all observation until it is removed, and the accompanying risk of damage to the objective itself. Some instruments are provided by their makers with dew-caps, and all ought to be; but in the absence of this provision a cap may be easily contrived. A tube of tin three or four times as long as the diameter of the object-glass, made so as to slide fairly stiffly over the object end of the tube where the ordinary cap fits, and blackened inside to a dead black, will remove practically all risk. The blackening may be done with lamp-black mixed with spirit varnish. Some makers—Messrs. Cooke, of York, for instance—line both tube and dew-cap with black velvet. This ought to be ideal, and might be tried in the case of the dew-cap by the observer. Finders are rarely fitted with dew-caps, but certainly should be; the addition will often save trouble and inconvenience.

Be careful to cover up the objective or mirror with its proper cap before removing it into the house. If this is not done, dewing at once results, the very proper punishment for carelessness. This may seem a caution so elementary as scarcely to be worth giving; but it is easier to read and remember a hint than to have to learn by experience, which in the case of a reflector will almost certainly mean a deteriorated mirror film. Should the mirror, if you are using a reflector, become dewed in spite of all precautions, do not attempt to touch the film while it is moist, or you will have the pleasure of seeing it scale off under your touch. Bring it into a room of moderate temperature, or stand it in a through draught of dry air until the moisture evaporates; and should any stain be left, make sure that the mirror is absolutely dry before attempting to polish it off. With regard to this matter of polishing, touch the mirror as seldom as possible with the polishing-pad. Frequent polishing does far more harm than good, and the mirror, if kept carefully covered when not in use, does not need it. A fold of cotton-wool between the cap and the mirror will, if occasionally taken out and dried, help greatly to preserve the film.

Next comes a caution which beginners specially need. Almost everyone on getting his first telescope wants to see everything as big as possible, and consequently uses the highest powers. This is an entire mistake. For a telescope of 2½ inches aperture two eye-pieces, or at most three, are amply sufficient. Of these, one may be low in power, say 25 to 40, to take in large fields, and, if necessary, to serve in place of a finder. Such an eye-piece will give many star pictures of surprising beauty. Another may be of medium power, say 80, for general work; and a third may be as high as 120 for exceptionally fine nights and for work on double stars. Nominally a 2½ inch, if of very fine quality, should bear on the finest nights and on stars a power of 100 to the inch, or 250. Practically it will do nothing of the sort, and on most nights the half of this power will be found rather too high. Indeed, the use of high powers is for many reasons undesirable. A certain proportion of light to size must be preserved in the image, or it will appear faint and 'clothy.' Further, increased magnifying power means also increased magnification of every tremor of the atmosphere; and with high powers the object viewed passes through the field so rapidly that constant shifting of the telescope is required, and only a brief glimpse can be obtained before the instrument has to be moved again. It is infinitely more satisfactory to see your object of a moderate size and steady than to see it much larger, but hazy, tremulous, and in rapid motion. 'In inquiring about the quality of some particular instrument,' remarks Sir Howard Grubb, 'a tyro almost invariably asks, "What is the highest power you can use?" An experienced observer will ask, "What is the lowest power with which you can do so and so?"'

Do not be disappointed if your first views of celestial objects do not come up to your expectations. They seldom do, particularly in respect of the size which the planets present in the field. A good deal of the discouragement so often experienced is due to the idea that the illustrations in text-books represent what ought to be seen by anyone who looks through a telescope. It has to be remembered that these pictures are, for one thing, drawn to a large scale, in order to insure clearness in detail, that they are in general the results of observation with the very finest telescopes, and the work of skilled observers making the most of picked nights. No one would expect to rival a trained craftsman in a first attempt at his trade; yet most people seem to think that they ought to be able at their first essay in telescopic work to see and depict as much as men who have spent half a lifetime in an apprenticeship to the delicate art of observation. Given time, patience, and perseverance, and the skill will come. The finest work shown in good drawings represents, not what the beginner may expect to see at his first view, but a standard towards which he must try to work by steady practice both of eye and hand. In this connection it may be suggested that the observer should take advantage of every opportunity of seeing through larger and finer instruments than his own. This will teach him two things at least. First, to respect his own small telescope, as he sees how bravely it stands up to the larger instrument so far as regards the prominent features of the celestial bodies; and, second, to notice how the superior power of the large glass brings out nothing startlingly different from that which is shown by his own small one, but a wealth of delicate detail which must be looked for (compare Plate [XV.] with Fig. [22]). A little occasional practice with a large instrument will be found a great encouragement and a great help to working with a small one, and most possessors of large glasses are more than willing to assist the owners of small ones.

Do not be ashamed to draw what you see, whether it be little or much, and whether you can draw well or ill. At the worst the result will have an interest to yourself which no representation by another hand can ever possess; at the best your drawings may in course of time come to be of real scientific value. There are few observers who cannot make some shape at a representation of what they see, and steady practice often effects an astonishing improvement. But draw only what you see with certainty. Some observers are gifted with abnormal powers of vision, others with abnormal powers of imagination. Strange to say, the results attained by these two classes differ widely in appearance and in value. You may not be endowed with faculties which will enable you to take rank in the former class; but at least you need not descend to the latter. It is after all a matter of conscience.

Do not be too hasty in supposing that everybody is endowed with a zeal for astronomy equal to your own. The average man or woman does not enjoy being called out from a warm fireside on a winter's night, no matter how beautiful the celestial sight to be seen. Your friend may politely express interest, but to tempt him to this is merely to encourage a habit of untruthfulness. The cause of astronomy is not likely to be furthered by being associated in any person's mind with discomfort and a boredom which is not less real because it is veiled under quite inadequate forms of speech. It is better to wait until the other man's own curiosity suggests a visit to the telescope, if you wish to gain a convert to the science.

When observing in the open be sure to wrap up well. A heavy ulster or its equivalent, and some form of covering for the feet which will keep them warm, are absolute essentials. See that you are thoroughly warm before you go out. In all probability you will be cold enough before work is over; but there is no reason why you should make yourself miserable from the beginning, and so spoil your enjoyment of a fine evening.

Having satisfied his craving for a general survey of everything in the heavens that comes within the range of his glass, the beginner is strongly advised to specialize. This is a big word to apply to the using of a 2½- or 3-inch telescope, but it represents the only way in which interest can be kept up. It does no good, either to the observer or to the science of astronomy, for him to take out his glass, have a glance at Jupiter and another at the Orion nebula, satisfy himself that the two stars of Castor are still two, wander over a few bright clusters, and then turn in, to repeat the same dreary process the next fine night. Let him make up his mind to stick to one, or at most two, objects. Lunar work presents an attractive field for a small instrument, and may be followed on useful lines, as will be pointed out later. A spell of steady work upon Jupiter will at least prepare the way and whet the appetite for a glass more adequate to deal with the great planet. Should star work be preferred, a fine field is opened up in connection with the variable stars, the chief requirement of work in this department being patience and regularity, a small telescope being quite competent to deal with a very large number of interesting objects.

The following comments in Smyth's usual pungent style are worth remembering: 'The furor of a green astronomer is to possess himself of all sorts of instruments—to make observations upon everything—and attempt the determination of quantities which have been again and again determined by competent persons, with better means, and more practical acquaintance with the subject. He starts with an enthusiastic admiration of the science, and the anticipation of new discoveries therein; and all the errors consequent upon the momentary impulses of what Bacon terms "affected dispatch" crowd upon him. Under this course—as soon as the more hacknied objects are "seen up"—and he can decide whether some are greenish-blue or bluish-green—the excitement flags, the study palls, and the zeal evaporates in hyper-criticism on the instruments and their manufacturers.'

This is a true sketch of the natural history, or rather, of the decline and fall, of many an amateur observer. But there is no reason why so ignominious an end should ever overtake any man's pursuit of the study if he will only choose one particular line and make it his own, and be thorough in it. Half-study inevitably ends in weariness and disgust; but the man who will persevere never needs to complain of sameness in any branch of astronomical work.

CHAPTER III

THE SUN

From its comparative nearness, its brightness and size, and its supreme importance to ourselves, the sun commands our attention; and in the phenomena which it presents there is found a source of abundant and constantly varying interest. Observation of these phenomena can only be conducted, however, after due precautions have been taken. Few people have any idea of the intense glow of the solar light and heat when concentrated by the object-glass of even a small telescope, and care must be exercised lest irreparable damage be done to the eye. Galileo is said to have finally blinded himself altogether, and Sir William Herschel to have seriously impaired his sight by solar observation. No danger need be feared if one or other of the common precautions be adopted, and some of these will be shortly described; but before we consider these and the means of applying them, let us gather together briefly the main facts about the sun itself.

Our sun, then, is a body of about 866,000 miles in diameter, and situated at a distance of some 92,700,000 miles from us. In bulk it equals 1,300,000 of our world, while it would take about 332,000 earths to weigh it down. Its density, as can be seen from these figures, is very small indeed. Bulk for bulk, it is considerably lighter than the earth; in fact, it is not very much denser than water, and this has very considerable bearing upon our ideas of its constitution.

Natural operations are carried on in this immense globe upon a scale which it is almost impossible for us to realize. A few illustrations gathered from Young's interesting volume, 'The Sun,' may help to make clearer to us the scale of the ruling body of our system. Some conception of the immensity of its distance from us may first be gained from Professor Mendenhall's whimsical illustration. Sensation, according to Helmholtz's experiments, travels at a rate of about 100 feet per second. If, then, an infant were born with an arm long enough to reach to the sun, and if on his birthday he were to exercise this amazing limb by putting his finger upon the solar surface, he would die in blissful ignorance of the fact that he had been burned, for the sensation of burning would take 150 years to travel along that stupendous arm. Were the sun hollowed out like a gigantic indiarubber ball and the earth placed at its centre, the enclosing shell would appear like a far distant sky to us. Our moon would have room to circle within this shell at its present distance of 240,000 miles, and there would still be room for another satellite to move in an orbit exterior to that of the moon at a further distance of more than 190,000 miles. The attractive power of this great body is no less amazing than its bulk. It has been calculated that were the attractive power which keeps our earth coursing in its orbit round the sun to cease, and to be replaced by a material bond consisting of steel wires of a thickness equal to that of the heaviest telegraph-wires, these would require to cover the whole sunward side of our globe in the proportion of nine to each square inch. The force of gravity at the solar surface is such that a man who on the earth weighs 10 stone would, if transported to the sun, weigh nearly 2 tons, and, if he remained of the same strength as on earth, would be crushed by his own weight.

PLATE V.

The Sun, February 3, 1905. Royal Observatory, Greenwich.

The first telescopic view of the sun is apt, it must be confessed, to be a disappointment. The moon is certainly a much more attractive subject for a casual glance. Its craters and mountain ranges catch the eye at once, while the solar disc presents an appearance of almost unbroken uniformity. Soon, however, it will become evident that the uniformity is only apparent. Generally speaking, the surface will quickly be seen to be broken up by one or more dark spots (Plate [V.]), which present an apparently black centre and a sort of grey shading round about this centre. The margin of the disc will be seen to be notably less bright than its central portions; and near the margin, and oftenest, though not invariably, in connection with one of the dark spots, there will be markings of a brilliant white, and often of a fantastically branched shape, which seem elevated above the general surface; while as the eye becomes more used to its work it will be found that even a small telescope brings out a kind of mottled or curdled appearance over the whole disc. This last feature may often be more readily seen by moving the telescope so as to cause the solar image to sweep across the field of view, or by gently tapping the tube so as to cause a slight vibration. Specks of dirt which may have gathered upon the field lens of the eye-piece will also be seen; but these may be distinguished from the spots by moving the telescope a little, when they will shift their place relatively to the other features; and their exhibition may serve to suggest the propriety of keeping eye-pieces as clean as possible.

PLATE VI.

Photograph of Bridged Sunspot (Janssen). Knowledge, February, 1890.

The spots when more closely examined will be found to present endless irregularities in outline and size, as will be seen from the accompanying plates and figures. On the whole, there is comparative fidelity to two main features—a dark central nucleus, known as the umbra, and a lighter border, the penumbra; but sometimes there are umbræ which have no penumbra, and sometimes there are spots which can scarcely be called more than penumbral shadings. The shape of the spot is sometimes fairly symmetrical; at other times the most fantastic forms appear. The umbra appears dark upon the bright disc, but is in reality of dazzling lustre, sending to us, according to Langley, 54 per cent. of the amount of heat received from a corresponding area of the brilliant unspotted surface. Within the umbra a yet darker deep, if it be a deep, has been detected by various observers, but is scarcely likely to be seen with the small optical means which we are contemplating. The penumbra is very much lighter in colour than the umbra, and invariably presents a streaked appearance, the lines all running in towards the umbra, and resembling very much the edge of a thatched roof. It will be seen to be very much lighter in colour on the edge next the umbra, while it shades to a much darker tone on that side which is next to the bright undisturbed part of the surface (Figs. 14 and 15). Frequently a spot will be seen interrupted by a bright projection from the luminous surface surrounding it which may even extend from side to side of the spot, forming a bridge across it (Plate [VI.], and Figs. 16, 17, and 18). These are the outstanding features of the solar spots, and almost any telescope is competent to reveal them. But these appearances have to be interpreted, so far as that is possible, and to have some scale applied to them before their significance can in the least be recognised. The observer will do well to make some attempt at realizing the enormous actual size of the seemingly trifling details which his instrument shows. For example, the spot in Figs. 14 and 15 is identical with that measured by Mr. Denning on the day between the dates of my rough sketches; and its greatest diameter was then 27,143 miles. Spots such as those of 1858, of February, 1892, and February, 1904, have approached or exceeded 140,000 miles in diameter, while others have been frequently recorded, which, though not to be compared to these leviathans, have yet measured from 40,000 to 50,000 miles in diameter, with umbræ of 25,000 to 30,000 miles. Of course, the accurate measurement of the spots demands appliances which are not likely to be in a beginner's hands; but there are various ways of arriving at an approximation which is quite sufficient for the purpose in view—namely, a realization of the scale of any spot as compared with that of the sun or of our own earth.

FIG. 14.—SUN-SPOT, JUNE 18, 1889.

FIG. 15.—SUN-SPOT, JUNE 20, 1889.

Of these methods, the simplest on the whole seems to be that given by Mr. W. F. Denning in his admirable volume, 'Telescopic Work for Starlight Evenings.' Fasten on the diaphragm of an eye-piece (the blackened brass disc with a central hole which lies between the field and eye lenses of the eye-piece) a pair of fine wires at right angles to one another. Bring the edge of the sun up to the vertical wire, the eye-piece being so adjusted that the sun's motion is along the line of the horizontal wire. This can easily be accomplished by turning the eye-piece round until the solar motion follows the line of the wire. Then note the number of seconds which the whole disc of the sun takes to cross the vertical wire. Note, in the second place, the time which the spot to be measured takes to cross the vertical wire; and, having these two numbers, a simple rule of three sum enables the diameter of the spot to be roughly ascertained. For the sun's diameter, 866,000 miles, is known, and the proportion which it bears to the number of seconds which it takes to cross the wire will be the same as that borne by the spot to its time of transit. Thus, to take Mr. Denning's example, if the sun takes 133 seconds to cross the wire, and the spot takes 6·5, then 133 : 866,000 : : 6·5 : 42,323, which latter number will be, roughly speaking, the diameter of the spot in miles. This, method is only a very rough approximation; still, it at least enables the observer to form some conception of the scale of what is being seen. It will answer best when the sun is almost south, and is, of course, less and less accurate as the spot in question is removed from the centre of the disc; for the sun being a sphere, and not a flat surface, foreshortening comes largely and increasingly into play as spots near the edge (or limb) of the disc.

Continued observation will speedily lead to the detection of the exceedingly rapid changes which often affect the spots and their neighbourhood. There are instances in which a spot passes across the disc without any other apparent changes save those which are due to perspective; and the same spot may even accomplish a complete rotation and appear again with but little change. But, generally speaking, it will be noticed that the average spot changes very considerably during the course of a single rotation. Often, indeed, the changes are so rapid as to be apparent within the course of a few hours. Figs. 14 and 15 represent a spot which was seen on June 18 and 20, 1889, and sketched by means of a 2½-inch refractor with a power of 80. A certain proportion of the change noticeable is due to perspective, but there are also changes of considerable importance in the structure of the spot which are actual, and due to motion of its parts. Mr. Denning's drawing ('Telescopic Work,' p. 95) shows the spot on the day between these two representations, and exhibits an intermediate stage of the change. The late Professor Langley has stated that when he was making the exquisite drawing of a typical sun-spot which has become so familiar to all readers of astronomical text-books and periodicals, a portion of the spot equal in area to the continent of South America changed visibly during the time occupied in the execution of the drawing; and this is only one out of many records of similar tenor. Indeed, no one who has paid any attention to solar observation can fail to have had frequent instances of change on a very large scale brought under his notice; and when the reality of such change has been actually witnessed, it brings home to the mind, as no amount of mere statement can, the extraordinary mobility of the solar surface, and the fact that we are here dealing with a body where the conditions are radically different from those with which we are familiar on our own globe. Changes which involve the complete alteration in appearance of areas of many thousand square miles have to be taken into consideration as things of common occurrence upon the sun, and must vitally affect our ideas of his constitution and structure (Figs. 16, 17, 18).

FIG. 16.—SUN-SPOT SEEN IN 1870.

Little more can be done by ordinary observation with regard to the spots and the general surface. Common instruments are not likely to have much chance with the curious structure into which the coarse mottling of the disc breaks up when viewed under favourable circumstances. This structure, compared by Nasmyth to willow-leaves, and by others to rice-grains, is beautifully seen in a number of the photographs taken by Janssen and others; but it is seldom that it can be seen to full advantage.

FIG. 17.—ANOTHER PHASE OF SPOT (FIG. 16).

FIG. 18.—PHASE OF SPOT (FIGS. 16 AND 17).

On the other hand, the spots afford a ready means by which the observer may for himself determine approximately the rotation period of the sun. A spot will generally appear to travel across the solar disc in about 13 days 14½ hours, and to reappear at the eastern limb after a similar lapse of time, thus making the apparent rotation-period 27 days 5 hours. This has to be corrected, as the earth's motion round the sun causes an apparent slackening in the rate of the spots, and a deduction of about 2 days has to be made for this reason, the resulting period being about 25 days 7 hours. It will quickly be found that no single spot can be relied upon to give anything like a precise determination, as many have motions of their own independent of that due to the sun's rotation; and, in addition, there has been shown to be a gradual lengthening of the period in high latitudes. Thus, spots near the equator yield a period of 25·09 days, those in latitude 15° N. or S. one of 25·44, and those in latitude 30° one of 26·53.

This law of increase, first established by Carrington, has been confirmed by the spectroscopic measures of Dunér at Upsala. His periods, while uniformly in excess of those derived from ordinary observations, show the same progression. For 0° his period is 25·46 days, for 15° 26·35, and for 30° 27·57. Continuing his researches up to 15° from the solar pole, Dunér has found that at that point the period of rotation is protracted to 38.5 days.

Reference has already been made to the bright and fantastically branched features which diversify the solar surface, generally appearing in connection with the spots, and best seen near the limb, though existing over the whole disc. These 'faculæ,' as they are called, will be readily seen with a small instrument—I have seen them easily with a 2-inch finder and a power of 30. They suggest at once to the eye the idea that they are elevations above the general surface, and look almost like waves thrown up by the convulsions which produce the spots. The rotation-period given by them has also been ascertained, and the result is shorter than that given by the spots. In latitude 0° it is 24·66 days, at 15° it is 25·26, at 30° 25·48. These varieties of rotation show irresistibly that the sun cannot in any sense of the term be called a rigid body. As Professor Holden remarks: 'It is more like a vast whirlpool, where the velocities of rotation depend on the situation of the rotating masses, not only as to latitude, but also as to depth beneath the rotating surface.' Plate [VII.], from a photograph of the sun taken by Mr. Hale, in which the surface is portrayed by the light of one single calcium ray of the solar spectrum, presents a view of the mottled appearance of the disc, together with several bright forms which the author of the photograph considers to be faculæ. M. Deslandres, of the Meudon Observatory, who has also been very successful in this new branch of solar photography, considers, however, that these forms are not faculæ, but distinct phenomena, to which he proposes to assign the name 'faculides'; and for various reasons his view appears to be the more probable. They are, however, in any case, in close relation with the faculæ, and, as Miss Clerke observes, 'symptoms of the same disturbance.'

PLATE VII.

Solar Surface with Faculæ. Yerkes Observatory.

The question of the nature of the sun spots is one that at once suggests itself; but it must be confessed that no very satisfactory answer can yet be given to it. None of the many theories put forward have covered all the observed facts, and an adequate solution seems almost as far off as ever. No one can fail to be struck with the resemblance which the spots present to cavities in the solar surface. Instinctively the mind seems to regard the umbra of the spot as being the centre of a great hollow of which the penumbra represents the sloping sides; and for long it was generally held that Wilson's theory, which assumed this appearance to correspond to an actual fact, was correct. Wilson found by observation of certain spots that when the spot was nearest to one limb the penumbra disappeared, either altogether or in part, on the side towards the centre, and that this process was reversed as the spot approached the opposite limb, the portion of the penumbra nearest the centre of the disc being always the narrowest.

This is the order of appearances which would naturally follow if the spot in question were a cavity; and if it were invariable there could scarcely be any doubt as to its significance. But while the Wilsonian theory has been recognised in all the text-books for many years, there has always been a suspicion that it was by no means adequately established, and that it was too wide an inference from the number of cases observed; and of late years it has been falling more and more into discredit. Howlett, for example, an observer of great experience, has asserted that the appearances on which the theory is based are not the rule, but the exception, and that therefore it must be given up. Numbers of spots seem to present the appearance of elevations rather than of depressions, and altogether it seems as though no category has yet been attained which will embrace all the varieties of spot-form. On this point further observation is very much needed, and the work that has to be done is well within the reach of even moderate instruments.

The fact that sun-spots wax and wane in numbers in a certain definite period was first ascertained by the amateur observer Schwabe of Dessau, whose work is a notable example of what may be accomplished by steadfast devotion to one particular branch of research. Without any great instrumental equipment, Schwabe effected the discovery of this most important fact—a discovery second to none made in the astronomical field during the last century—simply by the patient recording of the state of the sun's face for a period of over thirty years, during which he succeeded in securing an observation, on the average, on about 300 days out of every year. The period now accepted differs slightly from that assigned by him, and amounts to 11·11 years. Beginning with a minimum, when few spots or none may be visible for some time, the spots will be found to increase gradually in number, until, about four and a half years from the minimum, a maximum is reached; and from this point diminution sets in, and results, in about 6·6 years, in a second minimum. The period is not one of absolute regularity—a maximum or a minimum may sometimes lag considerably behind its proper time, owing to causes as yet unexplained. Still, on the whole, the agreement is satisfactory.

This variation is also accompanied by a variation in the latitude of the spots. Generally they follow certain definite zones, mostly lying between 10° and 35° on either side of the solar equator. As a minimum approaches, they tend to appear nearer to the equator than usual; and when the minimum has passed the reappearance of the spots takes the opposite course, beginning in high latitudes.

It has further been ascertained that a close connection exists between the activity which results in the formation of sun-spots, and the electrical phenomena of our earth. Instances of this connection have been so repeatedly observed as to leave no doubt of its reality, though the explanation of it has still to be found. It has been suggested by Young that there may be immediate and direct action in this respect between the sun and the earth, an action perhaps kindred with that solar repulsive force which seems to drive off the material of a comet's tail. As yet not satisfactorily accounted for is the fact that it does not always follow that the appearance of a great sun-spot is answered by a magnetic storm on the earth. On the average the connection is established; but there are many individual instances of sun-spots occurring without any answering magnetic thrill from the earth. To meet this difficulty, Mr. E. W. Maunder has proposed a view of the sun's electrical influence upon our earth, which, whether it be proved or disproved in the future, seems at present the most living attempt to account for the observed facts. Briefly, he considers it indubitably proved—

1. That our magnetic disturbances are connected with the sun.

2. That the sun's action, of whatever nature, is not from the sun as a whole, but from restricted areas.

3. That the sun's action is not radiated, but restricted in direction.

On his view, the great coronal rays or streamers seen in total eclipses (Plate [VIII.]) are lines of force, and similarly the magnetic influence which the sun exerts upon the earth acts along definite and restricted lines. Thus a disturbance of great magnitude upon the sun would only be followed by a corresponding disturbance on the earth if the latter happened to be at or near the point where it would fall within the sweep of the line of magnetic force emanating from the sun. In proportion as the line of magnetic force approached to falling perpendicularly on the earth, the magnetic disturbance would be large: in proportion as it departed from the perpendicular it would diminish until it vanished finally altogether. The suggestion seems an inviting one, and has at least revived very considerably the interest in these phenomena.

Such, then, are the solar features which offer themselves to direct observation by means of a small telescope. The spots, apart from their own intrinsic interest, are seen to furnish a fairly accurate method by which the observer can determine for himself the sun's rotation period. Their size may be approximately measured, thus conveying to the mind some idea of the enormous magnitude of the convulsions which take place upon this vast globe. The spot zones may be noted, together with the gradual shift in latitude as the period approaches or recedes from minimum; while observations of individual spots may be conducted with a view to gathering evidence which shall help either to confirm or to confute the Wilsonian theory. In this latter department of observation the main requisite is that the work should be done systematically. Irregular observation is of little or no value; but steady work may yield results of high importance. While, however, systematic observation is desirable, it is not everyone who has the time or the opportunity to give this; and to many of us daily solar observation may represent an unattainable ideal. Even if this be the case, there still remains an inexhaustible fund of beauty and interest in the sun-spots. It does not take regular observation to enable one to be interested in the most wonderful intricacy and beauty of the solar detail, in its constant changes, and in the ideas which even casual work cannot fail to suggest as to the nature and mystery of that great orb which is of such infinite importance to ourselves.

A small instrument, used in the infrequent intervals which may be all that can be snatched from the claims of other work, will give the user a far more intelligent interest in the sun, and a far better appreciation of its features, than can be gained by the most careful study of books. In this, and in all other departments of astronomy, there is nothing like a little practical work to give life to the subject.

In the conduct of observation, however, regard must be paid to the caution given at the beginning of this chapter. Various methods have been adopted for minimizing the intense glare and heat. For small telescopes—up to 2½ inches or so—the common device of the interposition of a coloured glass between the eye-piece and the eye will generally be found sufficient on the score of safety, though other arrangements may be found preferable. Such glasses are usually supplied with small instruments, mounted in brass caps which screw or slide on to the ends of the various eye-pieces. Neutral tint is the best, though a combination of green and red also does well. Red transmits too much heat for comfort. Should dark glasses not be supplied, it is easy to make them by smoking a piece of glass to the required depth, protecting it from rubbing by fastening over it a covering glass which rests at each end on a narrow strip of cardboard.

With anything larger than 2½ inches, dark glass is never quite safe. A 3-inch refractor will be found quite capable of cracking and destroying even a fairly thick glass if observation be long continued. The contrivance known as a polarizing eye-piece was formerly pretty much beyond the reach of the average amateur by reason of its costliness. Such eye-pieces are now becoming much cheaper, and certainly afford a most safe and pleasant way of viewing the sun. They are so arranged that the amount of light and heat transmitted can be reduced at will, so as to render the use of a dark glass unnecessary, thus enabling the observer to see all details in their natural colouring. The ordinary solar diagonal, in which the bulk of the rays is rejected, leaving only a small portion to reach the eye, is cheaper and satisfactory, though a light screen-glass is still required with it. But unquestionably the best general method of observing, and also the least costly, is that of projecting the sun's image through the telescope upon a prepared white surface, which may be of paper, or anything else that may be found suitable.

To accomplish this a light framework may be constructed in the shape of a truncated cone. At its narrow end it slips or screws on to the eye-end of the telescope, and it may be made of any length required, in proportion to the size of solar disc which it is desired to obtain. It should be covered with black cloth, and its base may be a board with white paper stretched on it to receive the image, which is viewed through a small door in the side. In place of the board with white paper, other expedients may be tried. Noble recommends a surface of plaster of Paris, smoothed while wet on plate glass, and this is very good if you can get the plaster smooth enough. I have found white paint, laid pretty thickly on glass and then rubbed down to a smooth matt surface by means of cuttle-fish bone, give very satisfactory results. Should it be desired to exhibit the sun's image to several people at once, this can easily be done by projecting it upon a sheet of paper fastened on a drawing-board, and supported at right angles to the telescope by an easel. The framework, or whatever takes its place, being in position, the telescope is pointed at the sun by means of its shadow; when this is perfectly round, or when the shadow of the framework perfectly corresponds to the shape of its larger end, the sun's image should be in the field of view.

CHAPTER IV

THE SUN'S SURROUNDINGS

We have now reached the point beyond which mere telescopic power will not carry us, a point as definite for the largest instrument as for the smallest. We have traced what can be seen on the visible sun, but beyond the familiar disc, and invisible at ordinary seasons or with purely telescopic means, there lie several solar features of the utmost interest and beauty, the study of which very considerably modifies our conception of the structure of our system's ruler. These features are only revealed in all their glory and wonder during the fleeting moments in which a total eclipse is central to any particular portion of the earth's surface.

A solar eclipse is caused by the fact that the moon, in her revolution round the earth, comes at certain periods between us and the sun, and obscures the light of the latter body either partially or totally. Owing to the fact that the plane of the orbit in which the moon revolves round the earth does not coincide with that in which the earth revolves round the sun, the eclipse is generally only partial, the moon not occupying the exact line between the centres of the sun and the earth. The dark body of the moon then appears to cut off a certain portion, larger or smaller, of the sun's light; but none of the extraordinary phenomena to be presently described are witnessed. Even during a partial eclipse, however, the observer may find considerable interest in watching the outline of the dark moon, as projected upon the bright background of the sun. It is frequently jagged or serrated, the projections indicating the existence, on the margin of the lunar globe, of lofty mountain ranges.

FIG. 19.—ECLIPSES OF THE SUN AND MOON.

Occasionally the conditions are such that the moon comes centrally between the earth and the sun (Fig. 19), and then an eclipse occurs which may be either total or annular. The proportion between the respective distances from us of the sun and the moon is such that these two bodies, so vastly different in real bulk, are sensibly the same in apparent diameter, so that a very slight modification of the moon's distance is sufficient to reduce her diameter below that of the sun. The lunar orbit is not quite circular, but has a small eccentricity. It may therefore happen that an eclipse occurs when the moon is nearest the earth, at which point she will cover the sun's disc with a little to spare; or the eclipse may occur when she is furthest away from the earth, in which case the lunar diameter will appear less than that of the sun, and the eclipse will be only an annular one, and a bright ring or 'annulus' of sunlight will be seen surrounding the dark body of the moon at the time when the eclipse is central.

All conditions being favourable, however—that is to say, the eclipse being central, and the moon at such a position in her orbit as to present a diameter equal to, or slightly greater than, that of the sun—a picture of extraordinary beauty and wonder reveals itself the moment that totality has been established. The centre of the view is the black disc of the moon. From behind it on every side there streams out a wonderful halo of silvery light which in some of its furthest streamers may sometimes extend to a distance of several million miles. In the Indian Eclipse of 1898, for example, one streamer was photographed by Mrs. Maunder, which extended to nearly six diameters from the limb of the eclipsed sun (Plate [VIII.]). The structure of this silvery halo is of the most remarkable complexity, and appears to be subject to continual variations, which have already been ascertained to be to some extent periodical and in sympathy with the sun-spot period. At its inner margin this halo rests upon a ring of crimson fire which extends completely round the sun, and throws up here and there great jets or waves, which frequently assume the most fantastic forms and rise to heights varying from 20,000 to 100,000 miles, or in extreme instances to a still greater height. To these appearances astronomers have given the names of the Corona, the Chromosphere, and the Prominences. The halo of silvery light is the Corona, the ring of crimson fire the Chromosphere, and the jets or waves are the Prominences.

PLATE VIII.

Coronal Streamers: Eclipse of 1898. From Photographs by Mrs. Maunder.

The Corona is perhaps the most mysterious of all the sun's surroundings. As yet its nature remains undetermined, though the observations which have been made at every eclipse since attention was first directed to it have been gradually suggesting and strengthening the idea that there exists a very close analogy between the coronal streamers and the Aurora or the tails of comets. The extreme rarity of its substance is conclusively proved by the fact that such insubstantial things as comets pass through it apparently unresisted and undelayed. Its structure presents variations in different latitudes. Near the poles it exhibits the appearance of brushes of light, the rays shooting out from the sun towards each summit of his axis, while the equatorial rays curve over, presenting a sort of fish-tail appearance. These variations are modified, as already mentioned, by some cause which is at all events coincident with the sun-spot period. At minimum the corona presents itself with polar brushes of light and fish-tail equatorial rays, the latter being sometimes of the most extraordinary length, as in the case of the eclipse of July 29, 1878, when a pair of these wonderful streamers extended east and west of the eclipsed sun to a distance of at least 10,000,000 miles.

When an eclipse occurs at a spot-maximum, the distribution of the coronal features is found to have entirely changed. Instead of being sharply divided into polar brushes and equatorial wings, the streamers are distributed fairly evenly around the whole solar margin, in a manner suggesting the rays from a star, or a compass-card ornament. The existence of this periodic change has been repeatedly confirmed, and there can be no doubt that the corona reflects in its structure the system of variation which prevails upon the sun. 'The form of the corona,' says M. Deslandres, 'undergoes periodical variations, which follow the simultaneous periodical variations already ascertained for spots, faculæ, prominences, and terrestrial magnetism.' Certainty as to its composition has not yet been attained; nor is this to be wondered at, for the corona is only to be seen in the all too brief moments during which a total eclipse is central, and then only over narrow tracts of country, and all attempts to secure photographs of it at other times have hitherto failed. When examined with the spectroscope, it yields evidence that its light is derived from three sources—from the incandescence of solid or liquid particles, from reflected sunshine, and from gaseous emissions. The characteristic feature of the coronal spectrum is a bright green line belonging to an unknown element which has been named 'coronium.'

The Chromosphere and the Prominences, unlike the elusive corona, may now be studied continuously by means of the spectroscope, and instruments are now made at a comparatively moderate price, which, in conjunction with a small telescope—3 inches will suffice—will enable the observer to secure most interesting and instructive views of both. The chromosphere is, to use Miss Clerke's expression, 'a solar envelope, but not a solar atmosphere.' It surrounds the whole globe of the sun to a depth of probably from 3,000 to 4,000 miles, and has been compared to an ocean of fire, but seems rather to present the appearance of a close bristling covering of flames which rise above the surface of the visible sun like the blades of grass upon a lawn. Any one of these innumerable flames may be elevated into unusual proportions in obedience to the vast and mysterious forces which are at work beneath, and then becomes a prominence. On the whole the constitution of the chromosphere is the same as that of the prominences. Professor Young has found that its normal constituents are hydrogen, helium, coronium, and calcium. But whenever there is any disturbance of its surface, the lines which indicate the presence of these substances are at once reinforced by numbers of metallic lines, indicating the presence of iron, sodium, magnesium, and other substances.

The scale to which these upheavals attain in the prominences is very remarkable. For example, Young records the observation of a prominence on October 7, 1880. When first seen, at about 10.30 a.m., it was about 40,000 miles in height and attracted no special attention. Half an hour later it had doubled its height. During the next hour it continued to soar upwards until it reached the enormous altitude of 350,000 miles, and then broke into filaments which gradually faded away, until by 12.30 there was nothing left of it. On another occasion he recorded one which darted upwards in half an hour from a moderate elevation to a height of 200,000 miles, and in which clouds of hydrogen must have been hurled aloft with a speed of at least 200 miles per second. (Plate [IX.] gives a representation of the chromosphere and prominences from a photograph by M. Deslandres.) Between the chromosphere and the actual glowing surface of the sun which we see lies what is known as the 'reversing layer,' from the fact that owing to its presence the dark lines of the solar spectrum are reversed in the most beautiful way during the second at the beginning and end of totality in an eclipse. Young, who was the first to observe this phenomenon (December 22, 1870), remarks of it that as soon as the sun has been hidden by the advancing moon, 'through the whole length of the spectrum, in the red, the green, the violet, the bright lines flash out by hundreds and thousands, almost startlingly; as suddenly as stars from a bursting rocket-head, and as evanescent, for the whole thing is over within two or three seconds.'

PLATE IX.

The Chromosphere and Prominences, April 11, 1894. Photographed by M. H. Deslandres.

The spectrum of the reversing layer has since been photographed on several occasions—first by Shackleton, at Novaya Zemlya, on August 9, 1896—and its bright lines have been found to be true reversals of the dark lines of the normal solar spectrum. This layer may be described as a thin mantle, perhaps 500 miles deep, of glowing metallic vapours, surrounding the whole body of the sun, and normally, strange to say, in a state of profound quiescence. Its presence was of course an integral part of Kirchhoff's theory of the mode in which the dark lines of the solar spectrum were produced. Such a covering was necessary to stop the rays whose absence makes the dark lines; and it was assumed that the rays so stopped would be seen bright, if only the splendour of the solar light could be cut off. These assumptions have therefore been verified in the most satisfactory manner.

Thus, then, the structure of the sun as now known is very different from the conception of it which would be given by mere naked-eye, or even telescopic, observation. We have first the visible bright surface, or photosphere, with its spots, faculæ, and mottling, and surrounded by a kind of atmosphere which absorbs much of its light, as is evidenced by the fact that the solar limb is much darker than the centre of the disc (Plate [V.]); next the reversing layer, consisting of an envelope of incandescent vapours, which by their absorption of the solar rays corresponding to themselves give rise to the dark lines in the spectrum. Beyond these again lies the chromosphere, rising into gigantic eruptive or cloud-like forms in the prominences; and yet further out the strange enigmatic corona.

It must be confessed that the reversing layer, the chromosphere, and the corona lie somewhat beyond the bounds and purpose of this volume; but without mention of them any account of the sun is hopelessly incomplete, and it is not at all improbable that a few years may see the spectroscope so brought within the reach of ordinary observers as to enable them in great measure to realize for themselves the facts connected with the complex structure of the sun. In any case, the mere recital of these facts is fitted to convey to the mind a sense of the utter inadequacy of our ordinary conceptions of that great body which governs the motions of our earth, and supplies to it and to the other planets of our system life and heat, light and guidance. With the unaided eye we view the sun as a small tranquil white disc; the telescope reveals to us that it is a vast globe convulsed by storms which involve the upheaval or submersion, within a few hours, of areas far greater than our own world; the spectroscope or the total eclipse adds to this revelation the further conception of a sweltering ocean of flame surrounding the whole solar surface, and rising in great jets of fire which would dissolve our whole earth as a drop of wax is melted in the flame of a candle; while beyond that again the mysterious corona stretches through unknown millions of miles its streamers of silvery light—the great enigma of solar physics. Other bodies in the universe present us with pictures of beautiful symmetry and vast size: some even within our own system suggest by their appearance the presence within their frame of tremendous forces which are still actively moulding them; but the sun gives us the most stupendous demonstration of living force that the mind of man can apprehend. Of course there are many stars which are known to be suns on which processes similar to those we have been considering are being carried on on a yet vaster scale; but the nearness of our sun brings the tremendous energy of these processes home to us in a way that impresses the mind with a sense almost of fear.

'Is it possible,' says Professor Newcomb, 'to convey to the mind any adequate conception of the scale on which natural operations are here carried on? If we call the chromosphere an ocean of fire, we must remember that it is an ocean hotter than the fiercest furnace, and as deep as the Atlantic is broad. If we call its movements hurricanes, we must remember that our hurricanes blow only about 100 miles an hour, while those of the chromosphere blow as far in a single second. They are such hurricanes as, coming down upon us from the north, would, in thirty seconds after they had crossed the St. Lawrence, be in the Gulf of Mexico, carrying with them the whole surface of the continent in a mass not simply of ruin, but of glowing vapour.... When we speak of eruptions, we call to mind Vesuvius burying the surrounding cities in lava; but the solar eruptions, thrown 50,000 miles high, would engulf the whole earth, and dissolve every organized being on its surface in a moment. When the mediæval poets sang, "Dies iræ, dies illa, solvet sæclum in favilla," they gave rein to their wildest imagination without reaching any conception of the magnitude or fierceness of the flames around the sun.'

The subject of the maintenance of the sun's light and heat is one that scarcely falls within our scope, and only a few words can be devoted to it. It is practically impossible for us to attain to any adequate conception of the enormous amount of both which is continually being radiated into space. Our own earth intercepts less than the two thousand millionth part of the solar energy. It has been estimated that if a column of ice 2¼ miles in diameter could be erected to span the huge interval of 92,700,000 miles between the earth and the sun, and if the sun could concentrate the whole of his heat upon it, this gigantic pillar of ice would be dissolved in a single second; in seven more it would be vaporized. The amount of heat developed on each square foot of solar surface is 'equivalent to the continuous evolution of about 10,000 horse-power'; or, as otherwise stated, is equal to that which would be produced by the hourly burning of nine-tenths of a ton of anthracite coal on the same area of 1 square foot.

It is evident, therefore, that mere burning cannot be the source of supply. Lord Kelvin has shown that the sun, if composed of solid coal, would burn itself out in about 6,000 years.

Another source of heat may be sought in the downfall of meteoric bodies upon the solar surface; and it has been calculated that the inrush of all the planets of our system would suffice to maintain the present energy for 45,604 years. But to suppose the existence near the sun of anything like the amount of meteoric matter necessary to account, on this theory, for the annual emission of heat involves consequences which are quite at variance with observed facts, though it is possible, or even practically certain, that a small proportion of the solar energy is derived from this source.

We are therefore driven back upon the source afforded by the slow contraction of the sun. If this contraction happens, as it must, an enormous amount of heat must be developed by the process, so much so that Helmholtz has shown that an annual contraction of 250 feet would account for the total present emission. This contraction is so slow that about 9,500 years would need to elapse before it became measurable with anything like certainty. In the meantime, then, we may assume as a working hypothesis that the light and heat of the central body of our system are maintained, speaking generally, by his steady contraction. Of course this process cannot have gone on, and cannot go on, indefinitely; but as the best authorities have hitherto regarded the date when the sun shall have shrunk so far as to be no longer able to support life on the earth as distant from us by some ten million years, and as the latest investigations on the subject, those of Dr. See, point in the direction of a very large extension of this limit, we may have reasonable comfort in the conviction that the sun will last our time.

CHAPTER V

MERCURY

The planet nearest to the sun is not one which has proved itself particularly attractive to observers in the past; and the reasons for its comparative unattractiveness are sufficiently obvious. Owing to the narrow limits of his orbit, he never departs further from the sun either East or West than between 27° and 28°, and the longest period for which he can be seen before sunrise or after sunset is two hours. It follows that, when seen, he is never very far from the horizon, and is therefore enveloped in the denser layers of our atmosphere, and presents the appearance sadly familiar to astronomers under the name of 'boiling,' the outlines of the planet being tremulous and confused. Of course, observers who have powerful instruments provided with graduated circles can find and follow him during the day, and it is in daylight that nearly all the best observations have been secured. But with humbler appliances observation is much restricted; and, in fact, probably many observers have never seen the planet at all.

Views of Mercury, however, such as they are, are by no means so difficult to secure as is sometimes supposed. Denning remarks that he has seen the planet on about sixty-five occasions with the naked eye—that in May, 1876, he saw it on thirteen different evenings, and on ten occasions between April 22 and May 11, 1890; and he states it as his opinion that anyone who will make it a practice to obtain naked-eye views should succeed from about twelve to fifteen times in the year. During the spring of 1905, to take a recent example, Mercury was quite a conspicuous object for some time in the Western sky, close to the horizon, and there was no difficulty whatever in obtaining several views of him both with the telescope and with the naked eye, though the disc was too much disturbed by atmospheric tremors for anything to be made of it telescopically. In his little book, 'Half-hours with the Telescope,' Proctor gives a method of finding the planet which would no doubt prove quite satisfactory in practice, but is somewhat needlessly elaborate. Anyone who takes the pains to note those dates when Mercury is most favourably placed for observation—dates easily ascertained from Whitaker or any other good almanac—and to carefully scan the sky near the horizon after sunset either with the naked eye, or, better, with a good binocular, will scarcely fail to detect the little planet which an old English writer more graphically than gracefully calls 'a squinting lacquey of the sun.'

Mercury is about 3,000 miles in diameter, and circles round the sun at a mean distance of 36,000,000 miles. His orbit is very eccentric, so that when nearest to the sun this distance is reduced to 28,500,000, while when furthest away from him it rises to 43,500,000. The proportion of sunlight which falls upon the planet must therefore vary considerably at different points of his orbit. In fact, when he is nearest to the sun he receives nine times as much light and heat as would be received by an equal area of the earth; but when the conditions are reversed, only four times the same amount. The bulk of the planet is about one-nineteenth that of the earth, but its weight is only one-thirtieth, so that its materials are proportionately less dense than those of our own globe. It is about 3½ times as dense as water, the corresponding figure for the earth being rather more than 5½.

Further, it is apparent that the materials of which Mercury's globe is composed reflect light very feebly. It has been calculated that the planet reflects only 17 per cent. of the light which falls upon it, 83 per cent. being absorbed; and this fact obviously carries with it the conclusion that the atmosphere of this little world cannot be of any great density. For clouds in full sunlight are almost as brilliantly white as new-fallen snow, and if Mercury were surrounded with a heavily cloud-laden atmosphere, he would reflect nearly five times the amount of light which he at present sends out into space.

As his orbit falls entirely within that of our own earth, Mercury, like his neighbour Venus, exhibits phases. When nearest to us the planet is 'new,' when furthest from us it is 'full,' while at the stages intermediate between these points it presents an aspect like that of the moon at its first and third quarters. It may thus be seen going through the complete series from a thin crescent up to a completely rounded disc. The smallness of its apparent diameter, and the poor conditions under which it is generally seen, make the observation of these phases by no means so easy as in the case of Venus; yet a small instrument will show them fairly well. Observers seem generally to agree that the surface has a dull rosy tint, and a few faint markings have, by patient observation, been detected upon it (Fig. 20); but these are far beyond the power of small telescopes. Careful attention to them and to the rate of their apparent motion across the disc has led to the remarkable conclusion that Mercury takes as long to rotate upon his axis as he does to complete his annual revolution in his orbit; in other words, that his day and his year are of the same length—namely, eighty-eight of our days. This conclusion, when announced in 1882 by the well-known Italian observer Schiaparelli, was received with considerable hesitation. It has, however, been confirmed by many observers, notably by Lowell at Flagstaff Observatory, Arizona, and is now generally received, though some eminent astronomers still maintain that really nothing is certainly known as to the period of rotation.

FIG. 20.—MERCURY AS A MORNING STAR. W. F. DENNING, 10-INCH REFLECTOR.

If the long period be accepted, it follows that Mercury must always turn the same face to the sun—that one of his hemispheres must always be scorching under intense heat, and the other held in the grasp of an unrelenting cold of which we can have no conception. 'The effects of these arrangements upon climate,' says Miss Agnes Clerke, 'must be exceedingly peculiar.... Except in a few favoured localities, the existence of liquid water must be impossible in either hemisphere. Mercurian oceans, could they ever have been formed, should long ago have been boiled off from the hot side, and condensed in "thick-ribbed ice" on the cold side.'

From what has been said it will be apparent that Mercury is scarcely so interesting a telescopic object as some of the other planets. Small instruments are practically ruled out of the field by the diminutive size of the disc which has to be dealt with, and the average observer is apt to be somewhat lacking in the patience without which satisfactory observations of an object so elusive cannot be secured. At the same time there is a certain amount of satisfaction and interest in the mere detection of the little sparkling dot of light in the Western sky after the sun has set, or in the Eastern before it has risen; and the revelation of the planet's phase, should the telescope prove competent to accomplish it, gives better demonstration than any diagram can convey of the interior position of this little world. It is consoling to think that even great telescopes have made very little indeed of the surface of Mercury. Schiaparelli detected a number of brownish stripes and streaks, which seemed to him sufficiently permanent to be made the groundwork of a chart, and Lowell has made a remarkable series of observations which reveal a globe seamed and scarred with long narrow markings; but many observers question the reality of these features altogether.

It is perhaps just within the range of possibility that, even with a small instrument, there may be detected that blunting of the South horn of the crescent planet which has been noticed by several reliable observers. But caution should be exercised in concluding that such a phenomenon has been seen, or that, if seen, it has been more than an optical illusion. Those who have viewed Mercury under ordinary conditions of observation will be well aware how extremely difficult it is to affirm positively that any markings on the surface or any deformations of the outline of the disc are real and actual facts, and not due to the atmospheric tremors which affect the little image.

Interesting, though of somewhat rare occurrence, are the transits of Mercury, when the planet comes between us and the sun, and passes as a black circular dot across the bright solar surface. The first occasion on which such a phenomenon was observed was November 7, 1631. The occurrence of this transit was predicted by Kepler four years in advance; and the transit itself was duly observed by Gassendi, though five hours later than Kepler's predicted time. It gives some idea of the uncertainty which attended astronomical calculations in those early days to learn that Gassendi considered it necessary to begin his observations two days in advance of the time fixed by Kepler. If, however, the time of a transit can now be predicted with almost absolute accuracy, it need not be forgotten that this result is largely due to the labours of men who, like Kepler, by patient effort and with most imperfect means, laid the foundations of the most accurate of all sciences.

The next transit of Mercury available for observation will take place on November 14, 1907. It may be noted that during transits certain curious appearances have been observed. The planet, for example, instead of appearing as a black circular dot, has been seen surrounded with a luminous halo, and marked by a bright spot upon its dark surface. No satisfactory explanation of these appearances has been offered, and they are now regarded as being of the nature of optical illusions, caused by defects in the instruments employed, or by fatigue of the eye. It might, however, be worth the while of any who have the opportunity of observing the transit of 1907 to take notice whether these features do or do not present themselves. For their convenience it may be noted that the transit will begin about eleven o'clock on the forenoon of November 14, and end about 12.45.

CHAPTER VI

VENUS

Next in order to Mercury, proceeding outwards from the sun, comes the planet Venus, the twin-sister, so to speak, of the earth, and familiar more or less to everybody as the Morning and Evening Star. The diameter of Venus, according to Barnard's measures with the 36-inch telescope of the Lick Observatory, is 7,826 miles; she is therefore a little smaller than our own world. Her distance from the sun is a trifle more than 67,000,000 miles, and her orbit, in strong contrast with that of Mercury, departs very slightly from the circular. Her density is a little less than that of the earth.

There is no doubt that, to the unaided eye, Venus is by far the most beautiful of all the planets, and that none of the fixed stars can for a moment vie with her in brilliancy. In this respect she is handicapped by her position as an inferior planet, for she never travels further away from the sun than 48°, and, even under the most favourable circumstances, cannot be seen for much more than four hours after sunset. Thus we never have the opportunity of seeing her, as Mars and Jupiter can be seen, high in the South at midnight, and far above the mists of the horizon. Were it possible to see her under such conditions, she would indeed be a most glorious object. Even as it is, with all the disadvantages of a comparatively low position and a denser stratum of atmosphere, her brilliancy is extremely striking, having been estimated, when at its greatest, at about nine times that of Sirius, which is the brightest of all the fixed stars, and five times that of Jupiter when the giant planet is seen to the best advantage. It is, in fact, so great that, when approaching its maximum, the shadows cast by the planet's light are readily seen, more especially if the object casting the shadow have a sharply defined edge, and the shadow be received upon a white surface—of snow, for instance. This extreme brilliance points to the fact that the surface of Venus reflects a very large proportion of the sunlight which falls upon it—a proportion estimated as being at least 65 per cent., or not very much less than that reflected by newly fallen snow. Such reflective power at once suggests an atmosphere very dense and heavily cloud-laden; and other observations point in the same direction. So that in the very first two planets of the system we are at once confronted with that diversity in details which coexists throughout with a broad general likeness as to figure, shape of orbit, and other matters. Mercury's reflective power is very small, that of Venus is exceedingly great; Mercury's atmosphere seems to be very attenuated, that of Venus, to all appearance, is much denser than that of our own earth.

Periodically, when Venus appears in all her splendour in the Western sky, one meets with the suggestion that we are having a re-appearance of the Star of Bethlehem; and it seems to be a perpetual puzzle to some people to understand how the same body can be both the Morning and the Evening Star. Those who have paid even the smallest attention to the starry heavens are not, however, in the least likely to make any mistake about the sparkling silver radiance of Venus; and it would seem as though the smallest application of common-sense to the question of the apparent motion of a body travelling round an almost circular orbit which is viewed practically edgewise would solve for ever the question of the planet's alternate appearance on either side of the sun. Such an orbit must appear practically as a straight line, with the sun at its middle point, and along this line the planet will appear to travel like a bead on a wire, appearing now on one side of the sun, now on another. If the reader will draw for himself a diagram of a circle (sufficiently accurate in the circumstances), with the sun in the centre, and divide it into two halves by a line supposed to pass from his eye through the sun, he will see at once that when this circle is viewed edgewise, and so becomes a straight line, a planet travelling round it is bound to appear to move back and forward along one half of it, and then to repeat the same movement along the other half, passing the sun in the process.

Like Mercury, and for the same reason of a position interior to our orbit, Venus exhibits phases to us, appearing as a fully illuminated disc when she is furthest from the earth, as a half-moon at the two intermediate points of her orbit, and as a new moon when she is nearest to us. The actual proof of the existence of these phases was one of the first-fruits which Galileo gathered by means of his newly invented telescope. It is said that Copernicus predicted their discovery, and they certainly formed one of the conclusive proofs of the correctness of his theory of the celestial system. It was the somewhat childish custom of the day for men of science to put forth the statement of their discoveries in the form of an anagram, over which their fellow-workers might rack their brains; probably this was done somewhat for the same reason which nowadays makes an inventor take out a patent, lest someone should rob the discoverer of the credit of his discovery before he might find it convenient to make it definitely public. Galileo's anagram, somewhat more poetically conceived than the barbarous alphabetic jumble in which Huygens announced his discovery of the nature of Saturn's ring, read as follows: 'Hæc immatura a me jam frustra leguntur o. y.' This, when transposed into its proper order, conveyed in poetic form the substance of the discovery: 'Cynthiæ figuras æmulatur Mater Amorum' (The Mother of the Loves [Venus] imitates the phases of Cynthia). It is true that two letters hang over the end of the original sentence, but too much is not to be expected of an anagram.

As a telescopic object, Venus is apt to be a little disappointing. Not that her main features are difficult to see, or are not beautiful. A 2-inch telescope will reveal her phases with the greatest ease, and there are few more exquisite sights than that presented by the silvery crescent as she approaches inferior conjunction. It is a picture which in its way is quite unique, and always attractive even to the most hardened telescopist.

Still, what the observer wants is not merely confirmation of the statement that Venus exhibits phases. The physical features of a planet are always the most interesting, and here Venus disappoints. That very brilliant lustre which makes her so beautiful an object to the naked eye, and which is even so exquisite in the telescopic view, is a bar to any great progress in the detection of the planet's actual features. For it means that what we are seeing is not really the surface of Venus, but only the sunward side of a dense atmosphere—the 'silver lining' of heavy clouds which interpose between us and the true surface of the planet, and render it highly improbable that anything like satisfactory knowledge of her features will ever be attained. Newcomb, indeed, roundly asserts that all markings hitherto seen have been only temporary clouds and not genuine surface markings at all; though this seems a somewhat absolute verdict in view of the number of skilled observers who have specially studied the planet and assert the objective reality of the markings they have detected. The blunting of the South horn of the planet, visible in Mr. MacEwen's fine drawing (Plate [X.]), is a feature which has been noted by so many observers that its reality must be conceded. On the other hand, some of the earlier observations recording considerable irregularities of the terminator (margin of the planet between light and darkness), and detached points of light at one of the horns, must seemingly be given up. Denning, one of the most careful of observers, gives the following opinion: 'There is strong negative evidence among modern observations as to the existence of abnormal features, so that the presence of very elevated mountains must be regarded as extremely doubtful.... The detached point at the South horn shown in Schröter's telescope was probably a false appearance due to atmospheric disturbances or instrumental defects.' It will be seen, therefore, that the observer should be very cautious in inferring the actual existence of any abnormal features which may be shown by a small telescope; and the more remarkable the features shown, the more sceptical he may reasonably be as to their reality. The chances are somewhat heavily in favour of their disappearance under more favourable conditions of seeing.

PLATE X.

Venus. H. MacEwen. 5-inch Refractor.

The same remark applies, with some modifications, to the dark markings which have been detected on the planet by all sorts of observers with all sorts of telescopes. There is no doubt that faint grey markings, such as those shown in Plate [X.], are to be seen; the observations of many skilled observers put this beyond all question. Even Denning, who says that personally he has sometimes regarded the very existence of these markings as doubtful, admits that 'the evidence affirming their reality is too weighty and too numerously attested to allow them to be set aside'; and Barnard, observing with the Lick telescope, says that he has repeatedly seen markings, but always so 'vague and ill-defined that nothing definite could be made of them.'

The observations of Lowell and Douglass at Flagstaff, Arizona, record quite a different class of markings, consisting of straight, dark, well-defined lines; as yet, however, confirmation of these remarkable features is scanty, and it will be well for the beginner who, with a small telescope and in ordinary conditions of observing, imagines he has detected such markings to be rather more than less doubtful about their reality. The faint grey areas, which are real features, at least of the atmospheric envelope, if not of the actual surface, are beyond the reach of small instruments. Mr. MacEwen's drawings, which accompany this chapter, were made with a 5-inch Wray refractor, and represent very well the extreme delicacy of these markings. I have suspected their existence when observing with an 8½-inch With reflector in good air, but could never satisfy myself that they were really seen.

Up till the year 1890 the rotation period of Venus was usually stated at twenty-three hours twenty-one minutes, or thereby, though this figure was only accepted with some hesitation, as in order to arrive at it there had to be some gentle squeezing of inconvenient observations. But in that year Schiaparelli announced that his observations were only consistent with a long period of rotation, which could not be less than six months, and was not greater than nine. The announcement naturally excited much discussion. Schiaparelli's views were strongly controverted, and for a time the astronomical world seemed to be almost equally divided in opinion. Gradually, however, the conclusion has come to be more and more accepted that Venus, like Mercury, rotates upon her axis in the same time as she takes to make her journey round the sun—in other words, that her day and her year are of the same length, amounting to about 225 of our days. In 1900 the controversy was to some extent reopened by the statement of the Russian astronomer Bélopolsky that his spectroscopic investigations pointed to a much more rapid rotation—to a period, indeed, considerably shorter than twenty-four hours. It is difficult, however, to reconcile this with the absence of polar flattening in the globe of Venus. Lowell's spectroscopic observations are stated by him to point to a period in accordance with his telescopic results—namely, 225 days. The matter can scarcely be regarded as settled in the meantime, but the balance of evidence seems in favour of the longer period.

Another curious and unexplained feature in connection with the planet is what is frequently termed the 'phosphorescence' of the dark side. This is an appearance precisely similar to that seen in the case of the moon, and known as 'the old moon in the young moon's arms.' The rest of the disc appears within the bright crescent, shining with a dull rusty light. In the case of Venus, however, an explanation is not so easily arrived at as in that of the moon, where, of course, earth-light accounts for the visibility of the dark portion. Had the planet been possessed of a satellite, the explanation might have lain there; but Venus has no moon, and therefore no moonlight to brighten her unilluminated portion; and our world is too far distant for earth-shine to afford an explanation. It has been suggested that electrical discharges similar to the aurora may be at the bottom of the mystery; but this seems a little far-fetched, as does also the attribution of the phenomenon to real phosphorescence of the oceans of Venus. Professor Newcomb cuts the Gordian knot by observing: 'It is more likely due to an optical illusion.... To whatever we might attribute the light, it ought to be seen far better after the end of twilight in the evening than during the daytime. The fact that it is not seen then seems to be conclusive against its reality.' But the appearance cannot be disposed of quite so easily as this, for it is not accurate to say that it is only seen in the daytime, and against Professor Newcomb's dictum may be set the judgment of the great majority of the observers who have made a special study of the planet.

We may, however, safely assign to the limbo of exploded ideas that of the existence of a satellite of Venus. For long this object was one of the most persistent of astronomical ghosts, and refused to be laid. Observations of a companion to the planet, much smaller, and exhibiting a similar phase, were frequent during the eighteenth century; but no such object has presented itself to the far finer instruments of modern times, and it may be concluded that the moon of Venus has no real existence.

Venus, like Mercury, transits the sun's disc, but at much longer intervals which render her transits among the rarest of astronomical events. Formerly they were also among the most important, as they were believed to furnish the most reliable means for determining the sun's distance; and most of the estimates of that quantity, up to within the last twenty-five years, were based on transit of Venus observations. Now, however, other methods, more reliable and more readily applicable, are coming into use, and the transit has lost somewhat of its former importance. The interest and beauty of the spectacle still remain; but it is a spectacle not likely to be seen by any reader of these pages, for the next transit of Venus will not take place until June, 2004.

As already indicated, Venus presents few opportunities for useful observation to the amateur. The best time for observing, as in the case of Mercury, is in broad daylight; and for this, unless in exceptional circumstances, graduated circles and a fairly powerful telescope are required. Practically the most that can be done by the possessor of a small instrument is to convince himself of the reality of the phases, and of the non-existence of a satellite of any size, and to enjoy the exquisite and varying beauty of the spectacle which the planet presents. Should his telescope be one of the small instruments which show hard and definite markings on the surface, he may also consider that he has learned a useful lesson as to the possibility of optical illusion, and, incidentally, that he may be well advised to procure a better glass when the opportunity of doing so presents itself. The 'phosphorescence' of the dark side may be looked for, and it may be noted whether it is not seen after dark, or whether it persists and grows stronger. Generally speaking, observations should be made as early in the evening as the planet can be seen in order that the light of the sky may diminish as much as possible the glare which is so evident when Venus is viewed against a dark background.

CHAPTER VII

THE MOON

Our attention is next engaged by the body which is our nearest neighbour in space and our most faithful attendant and useful servant. The moon is an orb of 2,163 miles in diameter, which revolves round our earth in a slightly elliptical orbit, at a mean distance of about 240,000 miles. The face which she turns to us is a trifle greater in area than the Russian Empire, while her total surface is almost exactly equal to the areas of North and South America, islands excluded. Her volume is about ²⁄99 of that of the earth; her materials are, however, much less dense than those of which our world is composed, so that it would take about eighty-one moons to balance the earth. One result of these relations is that the force of gravity at the lunar surface is only about one-sixth of that at the surface of the earth, so that a twelve-stone man, if transported to the moon, would weigh only two stone, and would be capable of gigantic feats in the way of leaping and lifting weights. The fact of the diminished force of gravity is of importance in the consideration of the question of lunar surfacing.

FIG. 21.—THE TIDES.

A, Spring Tide (New Moon); B, Neap Tide.

The most conspicuous service which our satellite performs for us is that of raising the tides. The complete statement of the manner in which she does this would be too long for our pages; but the general outline of it will be seen from the accompanying rough diagram (Fig. 21), which, it must be remembered, makes no attempt at representing the scale either of the bodies concerned or of their distances from one another, but simply pictures their relations to one another at the times of spring and neap tides. The moon (M in Fig. 21, A) attracts the whole earth towards it. Its attraction is greatest at the point nearest to it, and therefore the water on the moonward side is drawn up, as it were, into a heap, making high tide on that side of the earth. But there is also high tide at the opposite side, the reason being that the solid body of the earth, which is nearer to the moon than the water on the further side, is more strongly attracted, and so leaves the water behind it. Thus there are high tides at the two opposite sides of the earth which lie in a straight line with the moon, and corresponding low tides at the intermediate positions. Tides are also produced by the attraction of the sun, but his vastly greater distance causes his tide-producing power to be much less than that of the moon. His influence is seen in the difference between spring and neap tides. Spring tides occur at new or full moon (Fig. 21, A, case of new moon). At these two periods the sun, moon, and earth, are all in one straight line, and the pull of the sun is therefore added to that of the moon to produce a spring tide. At the first and third quarters the sun and moon are at right angles to one another; their respective pulls therefore, to some extent, neutralize each other, and in consequence we have neap tide at these seasons.

PLATE XI.

The Moon, April 5, 1900. Paris Observatory.

No one can fail to notice the beautiful set of phases through which the moon passes every month. A little after the almanac has announced 'new moon,' she begins to appear as a thin crescent low down in the West, and setting shortly after the sun. Night by night we can watch her moving eastward among the stars, and showing more and more of her illuminated surface, until at first quarter half of her disc is bright. The reader must distinguish this real eastward movement from the apparent east to west movement due to the daily rotation of the earth. Its reality can readily be seen by noting the position of the moon relatively to any bright star. It will be observed that if she is a little west of the star on one night, she will have moved to a position a little east of it by the next. Still moving farther East, she reaches full, and is opposite to the sun, rising when he sets, and setting when he rises. After full, her light begins to wane, till at third quarter the opposite half of her disc is bright, and she is seen high in the heavens in the early morning, a pale ghost of her evening glories. Gradually she draws nearer to the sun, thinning down to the crescent shape again until she is lost once more in his radiance, only to re-emerge and begin again the same cycle of change.

The time which the moon actually takes to complete her journey round the earth is twenty-seven days, seven hours, and forty-three minutes; and if the earth were fixed in space, this period, which is called the sidereal month, would be the actual time from new moon to new moon. While the moon has been making her revolution, however, the earth has also been moving onwards in its journey round the sun, so that the moon has a little further to travel in order to reach the 'new moon' position again, and the time between two new moons amounts to twenty-nine days, twelve hours, forty-four minutes. This period is called a lunar month, and is also the synodic period of our satellite, a term which signifies generally the period occupied by any planet or satellite in getting back to the same position with respect to the sun, as observed from the earth.

The fact that the moon shows phases signifies that she shines only by reflected light; and it is surprising to notice how little of the light that falls upon her is really reflected by her. On an ordinarily clear night most people would probably say that the moon is much brighter than any terrestrial object viewed in the daytime, when it also is lit by the sun, as the moon is. Yet a very simple comparison will show that this is not so. If the moon be compared during the daytime with the clouds floating around her, she will be seen to be certainly not brighter than they, generally much less bright; indeed, even an ordinary surface of sandstone will look as bright as her disc. In fact, the reason of her great apparent brightness at night is merely the contrast between her and the dark background against which she is seen; a fragment of our own world, put in her place, would shine quite as brightly, perhaps even more so. It is possibly rather difficult at first to realize that our earth is shining to the moon and to the other planets as they do to us, but anyone who watches the moon for a few days after new will find convincing evidence of the fact. Within the arms of the thin crescent can be seen the whole body of the lunar globe, shining with a dingy coppery kind of light—'the ashen light,' as it is called. People talk of this as 'the old moon in the young moon's arms,' and weather-wise (or foolish) individuals pronounce it to be a sign of bad weather. It is, of course, nothing of the sort, for it can be seen every month when the sky is reasonably clear; but it is the sign that our world shines to the other worlds of space as they do to her; for this dim light upon the part of the moon unlit by the sun is simply the light which our own world reflects from her surface to the moon. In amount it is thirteen times more than that which the moon gives to us, as the earth presents to her satellite a disc thirteen times as large as that exhibited by the latter.

The moon's function in causing eclipses of the sun has already been briefly alluded to. In turn she is herself eclipsed, by passing behind the earth and into the long cone of shadow which our world casts behind it into space (Fig. 19). It is obvious that such eclipses can only happen when the moon is full. A total eclipse of the moon, though by no means so important as a solar eclipse, is yet a very interesting and beautiful sight. The faint shadow or penumbra is often scarcely perceptible as the moon passes through it; but the passage of the dark umbra over the various lunar formations can be readily traced, and is most impressive. Cases of 'black eclipses' have been sometimes recorded, in which the moon at totality has seemed actually to disappear as though blotted out of the heavens; but in general this is not the case. The lunar disc still remains visible, shining with a dull coppery light, something like the ashen light, but of a redder tone. This is due to the fact that our earth is not, like its satellite, a next to airless globe, but is possessed of a pretty extensive atmosphere. By this atmosphere those rays of the sun which would otherwise have just passed the edge of the world are caught and refracted so that they are directed upon the face of the eclipsed moon, lighting it up feebly. The redness of the light is due to that same atmospheric absorption of the green and blue rays which causes the body of the setting sun to seem red when viewed through the dense layer of vapours near the horizon. When the moon appears totally eclipsed to us, the sun must appear totally eclipsed to an observer stationed on the moon. A total solar eclipse seen from the moon must present features of interest differing to some extent from those which the similar phenomenon exhibits to us. The duration of totality will be much longer, and, in addition to the usual display of prominences and corona, there will be the strange and weird effect of the black globe of our world becoming gradually bordered with a rim of ruddy light as our atmosphere catches and bends the solar rays inwards upon the lunar surface.

In nine cases out of ten the moon will be the first object to which the beginner turns his telescope, and he will find in our satellite a never-failing source of interest, and a sphere in which, by patient observation and the practice of steadily recording what is seen, he may not only amuse and instruct himself, but actually do work that may become genuinely useful in the furtherance of the science. The possession of powerful instrumental means is not an absolute essential here, for the comparative nearness of the object brings it well within the reach of moderate glasses. The writer well remembers the keen feeling of delight with which he first discovered that a very humble and commonplace telescope—nothing more, in fact, than a small ordinary spy-glass with an object-glass of about 1 inch in aperture—was able to reveal many of the more prominent features of lunar scenery; and the possessor of any telescope, no matter whether its powers be great or small, may be assured that there is enough work awaiting him on the moon to occupy the spare time of many years with one of the most enthralling of studies. The view that is given by even the smallest instrument is one of infinite variety and beauty; and its interest is accentuated by the fact that the moon is a sphere where practically every detail is new and strange.

If the moon be crescent, or near one or other of her quarters at the time of observation, the eye will at once be caught by a multitude of circular, or nearly circular depressions, more clearly marked the nearer they are to the line of division between the illuminated and unilluminated portions of the disc. (This line is known as the Terminator, the circular outline, fully illuminated, being called the Limb). The margins of some of these depressions will be seen actually to project like rings of light into the darkness, while their interiors are filled with black shadow (Plates [XI.], [XIII.], [XV.], and [XVI.]). At one or two points long bright ridges will be seen, extending for many miles across the surface, and marking the line of one or other of the prominent ranges of lunar mountains (Plates [XI.], [XIII.], [XVI.], [XVII.]); while the whole disc is mottled over with patches of varied colour, ranging from dark grey up to a brilliant yellow which, in some instances, nearly approaches to white.

If observation be conducted at or near the full, the conditions will be found to have entirely changed. There are now very few ruggednesses visible on the edge of the disc, which now presents an almost smooth circular outline, nor are there any shadows traceable on the surface. The circular depressions, formerly so conspicuous, have now almost entirely vanished, though the positions and outlines of a few of them may still be traced by their contrast in colour with the surrounding regions. The observer's attention is now claimed by the extraordinary brilliance and variety of the tones which diversify the sphere, and particularly by the curious systems of bright streaks radiating from certain well-marked centres, one of which, the system originating near Tycho, a prominent crater not very far from the South Pole, is so conspicuous as to give the full moon very much the appearance of a badly-peeled orange (Plate [XII.]).

PLATE XII.

The Moon, November 13, 1902. Paris Observatory.

As soon as the moon has passed the full, the ruggedness of its margin begins once more to become apparent, but this time on the opposite side; and the observer, if he have the patience to work late at night or early in the morning, has the opportunity of seeing again all the features which he saw on the waxing moon, but this time with the shadows thrown the reverse way—under evening instead of under morning illumination. In fact the character of any formation cannot be truly appreciated until it has been carefully studied under the setting as well as under the rising and meridian sun.

We must now turn our attention to the various types of formation which are to be found upon the moon. These may be roughly summarized as follows: (1) The great grey plains, commonly known as Maria, or seas; (2) the circular or approximately circular formations, known generally as the lunar craters, but divided by astronomers into a number of classes to which reference will be made later; (3) the mountain ranges, corresponding with more or less closeness to similar features on our own globe; (4) the clefts or rills; (5) the systems of bright rays, to which allusion has already been made.

1. The Great Grey Plains.—These are, of course, the most conspicuous features of the lunar surface. A number of them can be easily seen with the naked eye; and, so viewed, they unite with the brighter portions to form that resemblance to a human face—'the man in the moon'—with which everyone is familiar. A field-glass or small telescope brings out their boundaries with distinctness, and suggests a likeness to our own terrestrial oceans and seas. Hence the name Maria, which was applied to them by the earlier astronomers, whose telescopes were not of sufficient power to reveal more than their broader outlines. But a comparatively small aperture is sufficient to dispel the idea that these plains have any right to the title of 'seas.' The smoothness which at first suggests water proves to be only relative. They are smooth compared with the brighter regions of the moon, which are rugged beyond all terrestrial precedent; but they would probably be considered no smoother than the average of our own non-mountainous land surfaces. A 2 or 2½-inch telescope will reveal the fact that they are dotted over with numerous irregularities, some of them very considerable. It is indeed not common to find a crater of the largest size associated with them; but, at the same time, craters which on our earth would be considered huge are by no means uncommon upon their surface, and every increase of telescopic power reveals a corresponding increase in the number of these objects (Plates [XIII.], [XV.], [XVII.]).

PLATE XIII.

The Moon, September 12, 1903. Paris Observatory.

Further, the grey plains are characterized by features of which instances may be seen with a very small instrument, though the more delicate specimens require considerable power—namely, the long winding ridges which either run concentrically with the margins of the plains, or cross their surface from side to side. Of these the most notable is the great serpentine ridge which traverses the Mare Serenitatis in the north-west quadrant of the moon. As it runs, approximately, in a north and south direction, it is well placed for observation, and even a low power will bring out a good deal of remarkable detail in connection with it. It rises in some places to a height of 700 or 800 feet (Neison), and is well shown on many of the fine lunar photographs now so common. Another point of interest in connection with the Maria is the existence on their borders of a number of large crater formations which present the appearance of having had their walls breached and ruined on the side next the mare by the action of some obscure agency. From consideration of these ruined craters, and of the 'ghost craters,' not uncommon on the plains, which present merely a faint outline, as though almost entirely submerged, it has been suggested, by Elger and others, that the Maria, as we see them represent, not the beds of ancient seas, but the consolidated crust of some fluid or viscous substance such as lava, which has welled forth from vents connected with the interior of the moon, overflowing many of the smaller formations, and partially destroying the walls of these larger craters. Notable instances of these half-ruined formations will be found in Fracastorius (Plate [XIX.], No. 78, and Plate [XI.]), and Pitatus (Plate [XIX.], No. 63, and Plate [XV.]). The grey plains vary in size from the vast Oceanus Procellarum, nearly 2,000,000 square miles in area, down to the Mare Humboldtianum, whose area of 42,000 square miles is less than that of England.

2. The Circular, or Approximately Circular Formations.—These, the great distinguishing feature of lunar scenery, have been classified according to the characteristics, more or less marked, which distinguish them from one another, as walled-plains, mountain-rings, ring-plains, craters, crater-cones, craterlets, crater-pits, and depressions. For general purposes we may content ourselves with the single title craters, using the more specific titles in outstanding instances.

PLATE XIV.

Region of Maginus: Overlapping Craters. Paris Observatory.

To these strange formations we have scarcely the faintest analogy on earth. Their multitude will at once strike even the most casual observer. Galileo compared them to the 'eyes' in a peacock's tail, and the comparison is not inapt, especially when the moon is viewed with a small telescope and low powers. In the Southern Hemisphere particularly, they simply swarm to such an extent that the district near the terminator presents much the appearance of a honeycomb with very irregular cells, or a piece of very porous pumice (Plate [XIV.]). Their vast size is not less remarkable than their number. One of the most conspicuous, for example, is the great walled-plain Ptolemäus, which is well-placed for observation near the centre of the visible hemisphere. It measures 115 miles from side to side of its great rampart, which, in at least one peak, towers more than 9,000 feet above the floor of the plain within. The area of this enormous enclosure is about equal to the combined areas of Yorkshire, Lancashire, and Westmorland—an extent so vast that an observer stationed at its centre would see no trace of the mountain-wall which bounds it, save at one point towards the West, where the upper part of the great 9,000-feet peak already referred to would break the line of the horizon (Plate [XIX.], No. 111; Plate [XIII.]).

Nor is Ptolemäus by any means the largest of these objects. Clavius, lying towards the South Pole, measures no less than 142 miles from wall to wall, and includes within its tremendous rampart an area of at least 16,000 square miles. The great wall which encloses this space, itself no mean range of mountains, stands some 12,000 feet above the surface of the plain within, while in one peak it rises to a height of 17,000 feet. Clavius is remarkable also for the number of smaller craters associated with it. There are two conspicuous ones, one on the north, one on the south side of its wall, each about twenty-five miles in diameter, while the floor is broken by a chain of four large craters and a considerable number of smaller ones.

Though unfavourably placed for observation, there is no lunar feature which can compare in grandeur with Clavius when viewed either at sunrise or sunset. At sunrise the great plain appears first as a huge bay of black shadow, so large as distinctly to blunt the southern horn of the moon to the naked eye. As the sun climbs higher, a few bright points appear within this bay of darkness—the summits of the walls of the larger craters—these bright islands gradually forming fine rings of light in the shadow which still covers the floor of the great plain. In the East some star-like points mark where the peaks of the eastern wall are beginning to catch the dawn. Then delicate streaks of light begin to stream across the floor, and the dark mass of shadow divides itself into long pointed shafts, which stretch across the plain like the spires of some great cathedral. The whole spectacle is so magnificent and strange that no words can do justice to it; and once seen it will not readily be forgotten. Even a small telescope will enable the student to detect and draw the more important features of this great formation; and for those whose instruments are more powerful there is practically no limit to the work that may be done on Clavius, which has never been studied with the minuteness that so great and interesting an object deserves. (Clavius is No. 13, Plate [XIX.] See also Plates [XIII.] and [XV.], and Fig. 22, the latter a rough sketch with a 2⅝-inch refractor.)

From such gigantic forms as these, the craters range downwards in an unbroken sequence through striking objects such as Tycho and the grand Copernicus, both distinguished for their systems of bright rays, as well as for their massive and regular ramparts, to tiny pits of black shadow, a few hundred feet across, and with no visible walls, which tax the powers of the very finest instruments. Schmidt's great map lays down nearly 33,000 craters, and it is quite certain that these are not nearly all which can be seen even with a moderate-sized telescope.

PLATE XV.

Clavius, Tycho, and Mare Nubium. Yerkes Observatory.

As to the cause which has resulted in this multitude of circular forms, there is no definite consensus of opinion. Volcanic action is the agency generally invoked; but, even allowing for the diminished force of gravity upon the moon, it is difficult to conceive of volcanic action of such intensity as to have produced some of the great walled-plains. Indeed, Neison remarks that such formations are much more akin to the smaller Maria, and bear but little resemblance to true products of volcanic action. But it seems difficult to tell where a division is to be made, with any pretence to accuracy, between such forms as might certainly be thus produced and those next above them in size. The various classes of formation shade one into the other by almost imperceptible degrees.

FIG. 22.

Clavius, June 7, 1889, 10 p.m., 2⅝ inch.

3. The Mountain Ranges.—These are comparatively few in number, and are never of such magnitude as to put them, like the craters, beyond terrestrial standards of comparison. The most conspicuous range is that known as the Lunar Apennines, which runs in a north-west and south-east direction for a distance of upwards of 400 miles along the border of the Mare Imbrium, from which its mass rises in a steep escarpment, towering in one instance (Mount Huygens) to a height of more than 18,000 feet. On the western side the range slopes gradually away in a gentle declivity. The spectacle presented by the Apennines about first quarter is one of indescribable grandeur. The shadows of the great peaks are cast for many miles over the surface of the Mare Imbrium, magnificently contrasting with the wild tract of hill-country behind, in which rugged summits and winding valleys are mingled in a scene of confusion which baffles all attempt at delineation. Two other important ranges—the Caucasus and the Alps—lie in close proximity to the Apennines; the latter of the two notable for the curious Alpine Valley which runs through it in a straight line for upwards of eighty miles. This wonderful chasm varies in breadth from about two miles, at its narrowest neck, to about six at its widest point. It is closely bordered, for a considerable portion of its length, by almost vertical cliffs thousands of feet in height, and under low magnifying powers appears so regular as to suggest nothing so much as the mark of a gigantic chisel, driven by main force through the midst of the mountain mass. The Alpine Valley is an easy object, and a power of 50 on a 2-inch telescope will show its main outlines quite clearly. Indeed, the whole neighbourhood is one which will well repay the student, some of the finest of the lunar craters, such as Plato, Archimedes, Autolycus, and Aristillus, lying in the immediate vicinity (Plates [XIII.] and [XVII.]).

PLATE XVI.

Region of Theophilus and Altai Mountains. Yerkes Observatory.

Among the other mountain-ranges may be mentioned the Altai Mountains, in the south-west quadrant (Plate [XVI.]), the Carpathians, close to the great crater Copernicus, and the beautiful semicircle of hills which borders the Sinus Iridum, or Bay of Rainbows, to the east of the Alpine range. This bay forms one of the loveliest of lunar landscapes, and under certain conditions of illumination its eastern cape, the Heraclides Promontory, presents a curious resemblance, which I have only seen once or twice, to the head of a girl with long floating hair—'the moon-maiden.' The Leibnitz and Doerfel Mountains, with other ranges whose summits appear on the edge of the moon, are seldom to be seen to great advantage, though they are sometimes very noticeably projected upon the bright disc of the sun during the progress of an eclipse.[*] They embrace some of the loftiest lunar peaks reaching 26,000 feet in one of or two instances, according to Schröter and Mädler.

FIG. 23.

Aristarchus and Herodotus, February 20, 1891, 6.15 p.m., 3⅞ inch.

4. The Clefts or Rills.—In these, and in the ray-systems, we again meet with features to which a terrestrial parallel is absolutely lacking. Schröter of Lilienthal was the first observer to detect the existence of these strange chasms, and since his time the number known has been constantly increasing, till at present it runs to upwards of a thousand. These objects range from comparatively coarse features, such as the Herodotus Valley (Fig. 23), and the well-known Ariadæus and Hyginus clefts, down to the most delicate threads, only to be seen under very favourable conditions, and taxing the powers of the finest instruments. They present all the appearance of cracks in a shrinking surface, and this is the explanation of their existence which at present seems to find most favour. In some cases, such as that of the great Sirsalis cleft, they extend to a length of 300 miles; their breadth varies from half a mile, or less, to two miles; their depth is very variously estimated, Nasmyth putting it at ten miles, while Elger only allows 100 to 400 yards. In a number of instances they appear either to originate from a small crater, or to pass through one or more craters in their course. The student will quickly find out for himself that they frequently affect the neighbourhood of one or other of the mountain ranges (as, for example, under the eastern face of the Apennines, Plate [XVII.]), or of some great crater, such as Archimedes. They are also frequently found traversing the floor of a great walled-plain, and at least forty have been detected in the interior of Gassendi (Plate [XIX.], No. 90). Smaller instruments are, of course, incompetent to reveal more than a few of the larger and coarser of these strange features. The Serpentine Valley of Herodotus, the cleft crossing the floor of Petavius, and the Ariadæus and Hyginus rills are among the most conspicuous, and may all be seen with a 2½-inch telescope and a power of 100.