THE MOON

Copernicus and the Carpathian Mountains.

THE MOON

A POPULAR TREATISE

By

GARRETT P. SERVISS

D. APPLETON AND COMPANY

NEW YORK

1907

Copyright, 1907, by

D. APPLETON AND COMPANY

Published October, 1907

PREFACE

The reader familiar with astronomical literature will doubtless remark a certain resemblance between the plan on which this book is written and that of Fontenelle’s “Conversations on the Plurality of Worlds,” a French classic of the eighteenth century. The author freely acknowledges that it was the recollection of the pleasure which the reading of Fontenelle’s book gave him, years ago, that led to the adoption of a somewhat similar plan for this description of the moon. But, except that in both cases the conversational method is employed, no great likeness will be found between what is here presented and the work of the witty Frenchman.

Having been invited by the Messrs. Appleton & Co. to prepare a small volume, to be based on a series of lunar photographs representing the moon as it appears on successive evenings during an entire lunation, the author felt that the work should be made as entertaining as possible. He has, therefore, avoided technicalities, while endeavoring to present all the most essential facts known about our satellite. What he has written is intended for the general reader, who desires to learn the results of the great advances in astronomy without being too much troubled with the scientific methods by whose aid those results have been reached.

This is the first time, as far as the author is aware, that a series of lunar photographs, showing our satellite in its varying aspects from New to Old Moon, has been presented in a book, accompanied with a description of the mountains, plains, volcanoes, and other formations shown in each successive photograph. The reader is enabled to place himself, as it were, in an observatory of the first rank, provided with the most powerful apparatus of the astronomer, and, during an entire month, view the moon in her changing phases.

The photographs here reproduced were made at the Yerkes Observatory, and the most grateful acknowledgments are tendered to Prof. Edwin B. Frost, its director, for generously consenting to their use for this purpose. He could only have been induced to do so by his desire to see the fruits of the admirable work accomplished by his associates enjoyed by an ever-widening circle.

The series of photographs representing the moon on successive evenings were taken with the 12-inch telescope of the Yerkes Observatory by Mr. James Wallace, who employed a color filter that he constructed specially for this telescope, which possesses a visual and not a photographic objective. The larger scale photographs, representing certain selected regions on the moon, were taken by Mr. Ritchey, now of the Carnegie Solar Observatory at Mount Wilson, California, with the great 40-inch telescope of the Yerkes Observatory. It is unnecessary to speak of the extraordinary quality of these photographs, which have been admired by astronomers in all lands.

It should, perhaps, be added that while the director of the Yerkes Observatory has shown confidence in the author by intrusting to him the use of these photographs, yet, neither Professor Frost, nor Messrs. Wallace and Ritchey are in any way responsible for the statements made in this book. The author has taken pains to be accurate, but if any errors of fact or opinion have crept in, he alone must be blamed for them.

Garrett P. Serviss.

Château d’Arceau,

Côte d’Or, France, June, 1907.

CONTENTS

LIST OF ILLUSTRATIONS

FULL-PAGE ILLUSTRATIONS

Copernicus and the Carpathian Mountains [Frontispiece]

PHOTOGRAPHS OF THE MOON SHOWING PHASES OF CHANGE

PHOTOGRAPHS OF THE MOON SHOWING SIGNIFICANT FEATURES

DIAGRAMS IN TEXT

INTRODUCTION

INTRODUCTION

ONE serene evening, when the full moon, rising slowly above the tree tops, began to spread over the landscape that peculiar radiance which, by half revealing and half concealing, by softening all outlines, and by imparting a certain mystery to the most familiar objects, fascinates at once the eye and the imagination, I was walking with a friend, a lady of charming intelligence, in a private park adjoining an old mansion in one of the most beautiful districts of central New York. For a long time we both remained silent, admiring the scene before us, so different in every aspect from its appearance in the glare of daylight—each occupied with the thoughts that such a spectacle suggests. Suddenly my friend turned to me and said:

“Tell me—for, like so many thousand others, I am virtually ignorant of these mysteries of the sky—tell me, what is that moon? What do astronomers really know about it?”

“But,” I replied, “you certainly exaggerate your ignorance. You must have read what so many books have told about the moon.”

“Not a word,” was the reply, “or at least, what I have read has made little impression upon my mind. I read few books of science; generally they repel me. But face to face with that marvelous moon, I find it irresistible, and my desire for knowledge concerning it becomes intense. I remember something about eclipses, and something about tides, with which, I believe, the moon is concerned. I recall the statement that the moon has no atmosphere, but does possess great mountains and volcanoes. Yet these things are so jumbled in my memory with technical statements which failed to interest me, that really my ignorance remains profound. But I have heard that many surprising discoveries have been made lately concerning the moon, and that astronomers have succeeded in taking wonderful photographs of scenes in the lunar world. I have, indeed, seen copies of some of these photographs, but beyond awaking curiosity by their bizarre effects of light and shadow, they impressed me little, for lack, I suppose, of information as to their meaning. I beg you, then, to tell me what is really known about the world of the moon. There it is; I see it; I experience the delightful impressions which its light produces—but, after all, what is it, and what should we behold if we could go there? I once read Jules Verne’s romance of a trip to the moon, but unfortunately his adventurers never really got there, and I finished the story with a keen sense of disappointment because, in the end, he told so very little about the moon itself. As for the professional books of the astronomers they are useless to me. Then, please tell me that which, at this moment, with that wonderful orb actually in sight, I so much desire to know.”

It was not possible to resist an appeal so earnestly urged, but I felt compelled to say: “Since you remember so little about the fundamental facts which generations of astronomers have accumulated concerning our nearest neighbor in the sky, I must, for the sake of completeness, and in order to put you au courant with the more captivating things that will come later, begin at the beginning, and the true beginning is not among the mountains of the moon, but here on the earth. We must start from our own globe—as the moon herself did.”

“What do you mean by that?” my friend asked with a tone of surprise.

“Have you not read, somewhere, in the last ten years, that the moon was actually born from the earth?”

“Yes, now that you mention it, I dimly recall something of the kind, but I took it for an extravagant speculation of some savant who possessed more imagination than solid knowledge.”

“The savant who originally demonstrated the earthly origin of the moon,” I replied, “is not one to be easily led into extravagance by his imagination. It is Prof. George Darwin, the son of the famous author of the ‘Origin of Species.’ I shall not mention his mathematics, which are troublesome, but allow me to tell you, in a word, that his investigations have satisfied astronomers that the earth and the moon once composed a single body. How many million years ago that was we can only guess. The causes of the separation which eventually occurred were the plastic condition of the original body while it was yet hot and molten, its swift axial rotation producing an immense centrifugal force at its equator, and the attraction of the sun raising huge tides which affected its entire mass instead of affecting only the waters of the ocean as the tides do at present. At last there came a time when an enormous portion of the swiftly rotating globe was torn loose. That portion included about one-eightieth of the entire mass of the earth. Some astronomers and geologists think that the ‘wound’ left in the side of the earth by this stupendous excision is yet traceable in the basin of the Pacific Ocean.

“The separation being once effected, the material that had escaped gradually assumed a globular form under the influence of the gravitation of its own particles; and, at the same time, by virtue of a curious reaction of the tidal attractions of the two bodies upon each other, the new-born globe was slowly forced away from its mother earth, becoming, in fact, its satellite. Thus, by a process which certainly does seem extravagantly imaginative, but which, nevertheless, is approved by strict mathematical deductions from known physical facts, the moon is believed to have had her birth.”

“Surely,” said my companion, “my imagination would never have dared to form such a picture, even if it had been capable of so extraordinary a flight.”

“No,” I replied, “nor the imagination of the most learned astronomer. You perceive that in things celestial as in things terrestrial fact is far more strange than fiction. We shall have occasion to refer to some of the consequences of the earthly origin of the moon later on, but just now in order that the knowledge you seek may not be too fragmentary, I must tell you some other, more commonly known, facts about our satellite.”

“Judging by myself I doubt if there are many such facts commonly known.”

“Perhaps you are right, but do not judge too severely the authors of astronomical books. Such books are written primarily for those who wish to study, not for those who desire to be intellectually entertained. But let me get through with my preliminaries, and then, under the guidance of science and photography, we shall try to visit the moon. One of the first questions that naturally arise concerning the objects that we see in the heavens relates to their distance from us. The average, or mean, distance of the moon from the earth is 238,840 miles. For the sake of a round number we usually call it 240,000 miles. But the orbit, or path, of the moon in her monthly journey around the earth, is so far from being a true circle that the distance is variable to the extent of 31,000 miles. Even the form of the moon’s path in space is not constant. Owing to the varying effects of the attraction of the earth and the sun, her elliptical orbit becomes now a little more and now a little less eccentric, the consequence being that the moon’s distance from the earth is continually changing. When she is at her greatest possible distance she is 253,000 miles away, but this distance at certain times, may be reduced to only 221,600 miles. As a result of these changes of distance the moon sometimes appears noticeably larger to our eyes than at other times.

“This leads us next to inquire, ‘What is the actual size of the moon?’ When we know the distance of any body from the eye it is not difficult to determine its size. The diameter of the moon is 2,163 miles. The face of the full moon contains 7,300,000 square miles. It is a little larger than the continent of South America. For a reason that we will speak of presently, the moon always keeps the same side toward us no matter in what part of its orbit it may be. Consequently we always see the same features of her surface and, except through inference, we do not know what exists on the other side of the lunar globe. Of the 7,300,000 square miles of surface which the moon presents to us, about 2,900,000 are occupied by those dark gray patches which you see so plainly spotting her face, and which were once supposed to be seas. The remaining 4,400,000 square miles consist of a very rough, broken country, ridged with gigantic mountains and containing hundreds of enormous craters, and mountain-ringed valleys, which are so vast that one hesitates to call them, what many of them seem evidently to be, extinct volcanoes. A single explosion of a volcano of the dimensions of some of these lunar monsters would shake the whole earth to its center!”

“Please stop a moment,” my friend laughingly interrupted. “So many merciless facts, chasing one at the heels of another, are as bad as the books on your science that I have tried to read. Give my imagination time to overtake you.”

“Very well,” I said, “then relieve your attention a little while by regarding the face of the moon. Do you perceive the portrait of the Moon Maiden there?”

“I believe I do, although I never noticed it before. It is in profile, is it not?”

“Yes, and it occupies all the central portion of the western half of the disk. Take the opera glass and you will see it more clearly.”

“Really, I find her quite charming,” said my companion, after gazing for a minute through the glass. “But what a coquette! Look at the magnificent jewel she wears at her throat, and the parure of pearls that binds her hair!”

“Yes,” I replied, “and no terrestrial coquette ever wore gems so unpurchasable as those with which the Moon Maiden has decked herself. That flaming jewel on her breast is a volcano, with a crater more than fifty miles across! Tycho, astronomers call it. Observe with the glass how broad rays shoot out from it in all directions. They are among the greatest mysteries of lunar scenery. And the string of brilliants in her hair consists of a chain of mountains greater than the Alps—the lunar Apennines. They extend more than 450 miles, and have peaks 20,000 feet high, which gleam like polished facets.”

“Truly,” said my companion, smiling, “these gigantesque facts of yours rather tend to dissipate the romantic impression that I had conceived of the Moon Maiden.”

“No doubt,” I replied. “It is only distance that lends her enchantment. But we must not disregard the facts. Her hair, you perceive, is formed by some of the vast gray plains of which I spoke a few minutes ago. She is like a face in the clouds—approach her, or change the point of view and she disappears or dissolves into something else.

“Now, to return to my preliminaries, upon which I must insist. Knowing the distance and the size of the moon, the next question relates to her motions. You are aware that she travels around the earth about once every month. There are two ways in which we measure the length of time that the moon takes for each revolution. First, regarding the face of the sky as a great dial, with the stars for marks upon it, we notice the time that elapses between two successive conjunctions of the moon with the same star. In the interval she has gone completely around the earth and come back to the starting point. This is called the moon’s sidereal revolution, and it occupies, on the average, twenty-seven days, seven hours, forty-three minutes, twelve seconds. Every twenty-four hours the moon advances among the stars, from west to east, about 13° 11´.

“But there is another, more usual way of measuring the orbital period of the moon. This way is connected with her phases, or changes of shape, from the sickle of the New Moon to the round disk of the Full Moon, and back again to the reversed sickle of the waning moon. It is the time that elapses from one New Moon to the next, or from one Full Moon to the next which now concerns us, and it amounts, on the average, to twenty-nine days, twelve hours, forty-four minutes. This is called the moon’s synodic revolution, and it is equivalent to the ordinary lunar month. It is variable to the amount of about thirteen hours. The reason why the synodic revolution is more than two days longer than the sidereal revolution is because the continual advance of the earth in its orbit around the sun causes the latter to move eastward among the stars, and before the moon’s monthly phases, which depend upon her position with regard to the sun, can recommence, she must overtake the sun.”

“What a hopeless task to try to remember all that!”

“At any rate, if you cannot remember these things my conscience will be clear, for I am simply doing my duty in telling you of them. If you forget, go to the books on astronomy and refresh your memory. But do not persuade yourself that the preliminaries are now finished. You are going to think that my story of the moon resembles Walter Scott’s novels in the length of its introduction; but if, in the end, I can interest you half as much as he finally interests his readers I shall thank the stars for my good fortune.

“The next thing that I must try to explain,” I continued, “is the cause of the moon’s phases, or her continual changes of form. You know that the New Moon is shaped like a thin crescent, and always appears in the west immediately after sundown, with the convex side facing the setting sun. The moon at First Quarter is a half circle and is visible in the southern part of the sky just after sunset. The Full Moon, which we have at present, is a complete round disk, and is always seen directly opposite to the place of the sun, so that she rises when the sun sets. The moon at last quarter is again a half circle, and appears on the meridian in the south at sunrise. The waning moon is like the new moon, crescent-shaped, but the convexity of the bow faces the rising sun, and she is visible only in the morning sky just as dawn begins. To explain the reasons for these changes of shape, which the moon regularly undergoes every month, I must ask you to go indoors and examine a little diagram which I have made.”

“Oh!” said my companion, “it is too bad to abandon this charming spectacle, illuminated by rays so fascinating, for the sake of looking at mathematical lines drawn on paper! But I suppose that this is one of the sacrifices demanded by your inexorable science, and must be made.”

“Yes,” I said, “but if science sometimes demands sacrifices, at least she always rewards them most generously.”

When we had returned to the house I placed upon the drawing-room table this diagram.

Phases and Rotation of the Moon.

As I spread it out, my companion, after a regretful glance through the open door at the silvery lawn, on which the moon, having cleared the obstructing branches of the bordering trees, was now pouring down the full splendor of her rays, put her elbows on the table to follow my explanation.

“The globe, half bright and half black, in the center,” I said, “represents the earth. The large circle surrounding the earth we will call the moon’s orbit, which she traverses once every month. The smaller globe, also half white and half black, shown in four successive positions in the orbit, is the moon. Suppose the sun to be away off here on the left. It illuminates the earth and the moon only on the side turned toward it. The opposite side of both is buried in night. Now, let us begin with the moon at the point A. She is then between the earth and the sun, the bright side being necessarily toward the sun and the dark side toward the earth. In that position we do not see the moon at all from the earth, unless she happens to come so exactly in a line with the sun as to cover the latter, in which event we have an eclipse of the sun. Now, suppose the moon to move in her orbit toward B. In a little more than seven days she will arrive at B. In the meantime, while moving away from the position of the sun, she begins to present a part of her illuminated hemisphere toward the earth. This part appears in the form of a sickle, or crescent, which grows gradually broader, until, at B, it has grown to a half circle. In other words, when the moon is in the position B we on the earth see one half of her illuminated surface. This phase is called First Quarter. The narrow crescent, which appears as soon as the moon begins to move from A toward B, is the New Moon. As the moon continues on from B toward C, more and more of her illuminated half is visible from the earth, and when she arrives at C, just opposite to the position of the sun, she becomes a Full Moon. We then see, as occurs to-night, the whole of that face of the moon which is presented sunward. The upper half of the diagram shows how the moon moves from the position of Full Moon back again to New Moon, or conjunction with the sun. During this latter part of her course the moon rises later and later every night, until, when she assumes the form of a waning crescent, she is visible only in the morning sky just before sunrise.[^[1]]

“Now, there is another interesting thing shown by this diagram,” I continued—but my companion, who had followed my explanations thus far with flattering attention, here suddenly ran to the door exclaiming:

“For mercy’s sake, what is happening to the moon?”

[1].

The Moon’s Path with Respect to the Sun and the Earth.

It may be well to add to what is said in the text about the orbit of the moon, that, while the moon does perform a revolution around the earth once a month, yet her orbit is drawn out, by the common motion of both earth and moon around the sun, into a long curve, whose radius is continually changing, but which is always concave toward the sun. This is illustrated in the accompanying diagram. Suppose we start with the earth at A. The moon is then between the sun and the earth, or in the phase of New Moon. The earth’s orbit at this point is more curved than the moon’s, and the earth is moving relatively faster than the moon. At B (First Quarter) the earth is directly ahead of the moon. But now the moon’s orbit becomes more curved than the earth’s and it begins to overtake the earth. At C (Full Moon) the moon has come up even with the earth, but on the opposite side from the sun. From that point to D (Last Quarter) the moon gains upon the earth until she is directly ahead of it. Then, from D to E (New Moon, once more) the earth gains until the two bodies are in the same relative positions which they occupied at A. Throughout the entire lunation, however, notwithstanding the changes which the shape of the moon’s orbit undergoes, the latter is constantly concave toward the sun. This shows that the sun’s attraction is really the governing force, and that the attraction of the earth simply serves to vary the form of the moon’s path, and cause it to move in a virtual ellipse with the earth for its focus.

I glanced over her shoulder, and saw a smudgy scallop in the moon’s edge.

“Really,” I said, “I am ashamed of myself. There is an eclipse of the moon to-night, and I had positively forgotten it! What you see is the shadow of the earth, which has the form of a long cone stretching away more than eight hundred thousand miles into space, and whenever our satellite at the time of Full Moon gets nearly in a direct line with the earth and the sun, it passes through that shadow and undergoes an eclipse. That is what is happening at the present moment.”

“And the shadow has a round form because the earth is round, I suppose.”

“Certainly; the shadow of a globe must have a circular outline. But the shadow of the earth, although it finally diminishes to a point, is, at the moon’s distance, still about 5,700 miles in diameter, or more than two and a half times the diameter of the moon. In consequence of the motion of the earth in its orbit around the sun, its shadow constantly moves eastward, like a great pencil of darkness sweeping straight across the heavens, but invisible to us except when the moon, traveling eastward faster than the shadow, overtakes and passes through it. This does not by any means happen at every full moon, because, for a reason which I shall explain presently, the moon usually passes either above or below the shadow of the earth, and thus escapes an eclipse. When an eclipse does occur it lasts a long time because the shadow is moving in the same direction as the moon. The moon must pass entirely through it before the eclipse ends. On this occasion the moon will be in the shadow more than three hours, and during an hour and a half she will be totally immersed. We shall have plenty of time, then, to observe the phenomenon, and after you have satisfied your curiosity a little by watching the slow advance of the shadow movement across the moon, we can return to our diagram and finish its explanation before the eclipse becomes total.”

Accordingly, after having watched the progress of the eclipse for half an hour, during which time the shadow began perceptibly to diminish the moonlight in the park, we returned to the lamplight and the diagram on the table.

“I was saying,” I resumed, “that another interesting thing in addition to the cause of the moon’s changing phases is represented here. You observe that a little cross stands on each of the four circles representing the moon, and that, in every case, the cross is in the center of that side of the moon which faces the earth. In fact the position of the cross upon the moon is fixed and invariable, and it always points toward the earth because the moon makes exactly one rotation on her axis in the course of one revolution around her orbit, or, as it is often called, one lunation. We know that this is so because we always see the same features of the lunar surface, no matter where the moon may be situated. This is true although, in consequence of the phases, we cannot see the whole face of the moon except when she is full. But whether it is the New Moon, or First Quarter, or Full Moon, or Last Quarter, or Old Moon, that we look at, the mountains and plains visible are identically the same. If the moon did not turn once on her axis in going once around the earth we would see all of her sides in succession, although only at Full Moon could we see an entire hemisphere illuminated by the sun. At Old and New Moon the side presented to the earth would be just the opposite to that presented at Full Moon. At Last Quarter the side facing the earth would be the opposite to that facing the earth at First Quarter.”

“But, tell me,” said my friend, “how did the moon ever come to so humiliating a pass that she must be forever turning on her heel to face the earth?”

“That,” I replied, “is a result of the same forces which originally separated her from the earth and gradually pushed her off to her present distance. In a word it is due to ‘tidal friction.’ Before the moon had solidified, the attraction of the earth raised huge tides in her molten mass. These tides acted on the rotating moon like brakes on a wheel, and at length they slowed down her rotation until its period became identical with that of her revolution around the earth. For the mathematical calculations on which all this is based you must go to Professor Darwin’s book on ‘The Tides,’ or some similar technical treatise; but I imagine you will never do that.”

“Not just at present, I assure you. I do not know what unexpected ambition for the acquirement of scientific knowledge may arise after I have seen those wonders that you have promised to show me in the moon, but, for the moment, I am content to accept your statement of the simple fact.”

“Good!” I replied. “And now, perhaps, you will have the patience to listen to an explanation of a very important relation which exists between the moon and the earth. We are led to it by what I have just said concerning tides. You know, of course that the tides in the oceans are due principally to the attraction of the moon. The sun also raises tides in the seas, but the moon, being so much nearer than the sun, is the chief agent in producing them. Sometimes the moon and the sun act together; at other times they pull in different directions. At Full Moon and at New Moon they pull together, because then they are either on opposite sides of the earth, or both on the same side. At such times we have the highest tides in all our seaports. That occurs about once every fortnight. But when the moon is at either First or Last Quarter, as you will perceive by looking at the diagram, her position, as seen from the earth, is at a right angle with a line drawn to the sun. Then the sun raises tides in one direction and the moon in another direction. The result is that at such periods the tides are lowest. An exact knowledge of these things is very important for mariners because there are harbors whose channels can be navigated by large ships only when the tides are high. Tables predicting the times and heights of the tides have been prepared for all the principal seaports of the world. In truth, the moon renders important services to the inhabitants of the earth, not merely in supplying them with a certain amount of light in the absence of the sun, but also in enabling them to navigate waters which are too shallow for ships except when deepened by the tide. The tides also, in many cases, serve to scour out channels and keep them open.”

“Really, I am quite interested, and the more so because I find the moon, like a dutiful daughter, trying to be of some use to her mother. But have I not heard that the tides occur on both sides of the earth at once, and not simply on the side where the moon happens to be at the time? Please tell me how that can be so?”

“A complete reply to your question would carry us into the realm of mathematical physics, but perhaps I can throw a little light upon the matter with the aid of this second diagram.

The Moon and the Tides.

“The eclipse is not yet total,” I continued, glancing out of the door, “and we can finish our explanation before it becomes so. Have the kindness, then, to look at the diagram. Suppose E to be the center of the earth, and M the center of the moon. The protuberant portions of the earth C A D and D B C represent the waters of the ocean pulled away from the surface of the earth, if I may so describe it, by the moon’s attraction. You are probably aware that the attraction of gravitation varies with the distance of the attracting body. The distance from the center of the earth to the center of the moon is about 239,000 miles. But the earth being nearly 8,000 miles in diameter, the surface of the ocean at A is about 4,000 miles nearer to the moon than is the center of the earth E. It follows that the force of the moon’s attraction is greater at A than at E. If the water of the ocean were a fixed, solid part of the earth there would be no perceptible effect resulting from this difference in the amount of the moon’s attraction. But since the water is free to move, to a certain extent, it yields to the attraction, and is drawn up a little toward the moon. At the same time it is, in effect, drawn away from C and D. The consequence is the production of a tide on the side facing the moon.

“Now, for the other tide, produced at the same time on that side of the earth which is turned away from the moon. The point B is about 4,000 miles farther from the moon than E; consequently the moon’s attractive force is less at B than at E. From this it results that the body of the earth is more forcibly attracted by the moon than is the water at B. The earth therefore tends to move away from the water at that point, and another tidal protuberance is produced, with its highest part at B. I should add that while the water of the ocean is, to a certain degree, free to respond to these differences of attraction, the earth itself, being solid, can only move as a single body, and, mathematically, we may regard it as if its entire mass were concentrated at the center E. Please remember, however, that this explanation is only elementary, only intended as a graphic representation of the tides, and not as a mathematical demonstration of the way they are produced. Such a demonstration would only be suited to one of the technical books that you have not found as interesting as—some other branches of literature.

“There is just one other thing to which I must ask your attention, and then we shall return to the moon herself and the phenomena of the eclipse now in progress. You will notice in the diagram, that two arrows show the direction in which the earth is continually rotating on its axis, and that a dotted curve, terminating with an arrow point, indicates the course of the moon in her orbit surrounding the earth. The rotation of the earth is so much more rapid than the motion of the moon that the points A and B are carried out of the line drawn from the center of the moon to the center of the earth, in the direction of the arrows. From this it follows that the tides are never directly under the moon, or exactly opposite to her, but sweep in great waves round the globe. The tides produced by the attraction of the sun are only about two fifths as high as those caused by the moon. As I have already explained they are sometimes superposed upon the lunar tides—at New and at Full Moon—and sometimes they are situated at right angles to the lunar tides—at First and Last Quarters.”

“But the eclipse!” interrupted my friend, whose attention had evidently begun to wander. “I think the totality of which you spoke must be at hand, for notice how dark the park has become, and the fireflies are so brilliant under the trees.”

The total phase of the eclipse was, indeed, beginning, and we stepped out on the lawn before the door to watch it. The moon had now passed entirely within the earth’s shadow, but although her light was almost completely obscured as far as its power to illuminate the landscape was concerned, still the face of the moon was dimly visible, as if concealed behind a thick veil. Certain parts of it had a coppery color, producing a very weird effect.

“Dear me!” exclaimed my companion, “I did not think it would look like that! I naïvely supposed that one could not see the eclipsed moon at all, but that she either disappeared or was turned into a kind of black circle in the heavens. And what a strange color she has! Positively it fills me with awe.”

“It is very rare,” I said, “for the moon to become invisible during an eclipse. That can only occur when the earth is enveloped in clouds.”

“Indeed, and what have the clouds to do with it? If the solid body of the earth cannot cast a shadow dense enough to hide the moon, I should not expect things so evanescent as clouds to be more effective.”

“It is all owing to the earth’s atmosphere,” I replied. “If our globe were not surrounded with a shell of air the moon would always be totally invisible when eclipsed. But the atmosphere acts like a lens of glass inclosing the earth; that is to say, it refracts, or bends the rays of sunlight around the edge of the earth on all sides, and throws a portion of them even into the middle of the shadow, at the moon’s distance. It is these refracted rays which cause the singular illumination that you perceive on the moon. But when, as occurs only occasionally, all that part of the atmosphere which surrounds the earth along the edge visible from the moon is filled with clouds, the air can no longer transmit the refracted rays, and then, no light being sent into the shadow, a ‘dark eclipse,’ as astronomers call it, results. An eclipse of the sun is a very different thing. That is caused not by a shadow but by the opaque globe of the moon passing between the earth and the solar orb. When this occurs the sun is completely hidden behind the moon, and only its corona, which projects beyond the moon on all sides, is visible.”

“Indeed! I supposed that all eclipses were very much the same thing.”

“By no means. An eclipse of the sun is an event of extreme importance to astronomers, while an eclipse of the moon possesses comparatively little scientific interest.”

“I do not see why that should be so.”

“It is so, for the reason that when the sun is eclipsed, as I have just told you, the solar corona, which cannot be seen at any other time owing to the overpowering brilliance of the solar orb, becomes plainly visible, and by studying the form and other particulars of the corona astronomers are able to draw most important conclusions concerning the constitution of the sun, the mechanism of its radiation, and its effects upon the earth. During an eclipse of the moon, on the other hand, practically nothing new is revealed, and, accordingly, astronomers pay comparatively little attention to such phenomena. Lunar eclipses, however, possess a certain importance, because by predicting the times of their occurrence and then comparing the predictions with the events, something is learned about the motions of the moon. I should add that recently eclipses of the moon have been carefully watched by a few astronomers, notably by Prof. William H. Pickering, because of peculiar effects which seem to be produced at certain points on the moon by the chill which the shadow casts upon her surface. There are also interesting observations to be made concerning the reflection of heat from the moon during an eclipse. But, upon the whole, a lunar eclipse is mainly interesting as a curious spectacle, and as a test of the correctness of astronomical calculations of the motions of the heavenly bodies.

“I may add, however, that eclipses of the moon have been of some use to historians in fixing the dates of important occurrences thousands of years ago. This is possible because astronomers can by calculation ascertain the times of eclipses in the past as well as in the future. Perhaps the most interesting of all instances of this kind is that which relates to the date of the beginning of the Christian era. This has been fixed by means of an eclipse of the moon mentioned by the ancients as having happened the night before the death of Herod, king of the Jews.”

“It seems to me,” said my friend, “that the faint light on the moon’s face is continually changing. It does not appear constantly to have the same tint. While we have been standing here, I have noticed that some parts have grown darker and others lighter, and that the red color on the right has become a little more intense.”

“Yes, and that, too, is no doubt caused by the earth’s atmosphere. While the eclipse lasts, the earth is rapidly rotating, and consequently new parts of the atmosphere are continually brought to the edge where their refractive effects come into play. If the atmosphere at the edge of the earth is a little more or a little less dense its refraction varies proportionally. Then, changes in the relative clearness or cloudiness of the air are taking place all the time, and these are reflected in the illumination on the moon.”

“It seems to me, then, that the earth would present a very remarkable spectacle if we were now on the moon looking at it.”

“Surely it would. Seen from the moon the earth appears several times larger than the sun. For the people of the moon, if we imagine them to exist, an eclipse of the sun is now in progress. For them the earth now occupies the same relative position which the moon occupies for us just before it appears in the west as New Moon. They cannot see it except in silhouette as it passes over the sun. More than an hour ago, if they were watching (and if they exist, and are intelligent beings we may be sure that they were on the alert), they suddenly perceived a black round-edged notch in the side of the sun. Instead of being more or less cloudlike and indefinite in outline, like the shadow of the earth on the moon, this notch appeared to them perfectly black and smooth. At a glance, they saw that the body producing it was much larger than the sun. As the sun’s disk was gradually hidden behind the earth the shadow of the latter fell over them, until the sun was wholly concealed. Then—and this is true at the present moment—they perceived that the huge disk of the earth was ringed with light, probably of a reddish tinge. This light, as I have already indicated, is that which the atmosphere refracts around the edge of the earth.”

“It must be truly a magnificent sight,” said my companion.

“Yes, and it is doubtless rendered far more magnificent by the other phenomena which our people at the moon have before their eyes. In consequence of the virtual absence of air there, an observer on the moon would see all the stars, even in full daylight, blazing in a jet black sky. The brilliance of the stars and of the Milky Way would hardly be increased by the hiding of the sun, but probably the long silvery streamers of the solar corona would glow perceptibly brighter when seen projecting out on each side of the enormous disk of the earth.”

“But is it true that the moon has no air?”

“Very, very little, and what little she has is probably different in composition from our atmosphere. Some observations seem to indicate that there is a very rare atmosphere on the moon, but to us it would seem a perfect vacuum. We could not breathe there at all.”

“How then do those intelligent inhabitants, whom you have pictured for me watching the earth at this moment, manage to survive?”

“Ah, I did not say that there actually are inhabitants in the moon. I only imagined them to exist for the sake of showing how this eclipse would appear seen from the moon. Still, we cannot be absolutely sure that there are no inhabitants on the moon. Even without air like ours it is conceivable that beings of some kind, and intelligent beings, too, might exist there. However, astronomers have never yet been able to discover evidence of their presence. Lately, indications have been found of the probable existence of vegetation on the moon, but I shall speak of that later, when with the aid of the series of lunar pictures made at the Yerkes observatory we try to make a ‘photographic journey’ in the moon.”

“But tell me, has the moon always been so airless?”

“That is another unsettled question. Some astronomers have thought that formerly, ages ago, the moon possessed a much more dense atmosphere than she has at present. Having separated from the earth, in the way I have described, it is natural to suppose that at first she may have had an atmosphere very like ours. The explanation of its disappearance which was once generally accepted was that it had been absorbed into the lunar rocks, as the globe of the moon cooled off. But recent progress in our knowledge of the nature of the gases composing the atmosphere has led to a different explanation. This assumes that nearly all of the moon’s atmosphere has flown away from her because the lunar globe does not possess sufficient gravitating force or attraction to retain it. If the mass of the earth were no greater than that of the moon, our atmosphere also would probably have escaped by flying off into space.”

“But how, and why, do these gases fly away?”

“They do it by virtue of what physicists call their molecular velocity. A gas, of whatever kind, is a mass of molecules which are in continual vibration, moving in all directions among one another with very great velocities. These velocities have been measured, and it has been found that the molecules of nitrogen, one of the components of the air, move at the rate of two miles in a second. The velocity of the molecules of oxygen is a little less; that of the molecules of hydrogen is very great, nearly seven and a half miles in a second! Now, it is also known that the attraction of the earth is sufficient to retain permanently upon its surface all moving particles or molecules which have a velocity less than seven miles in a second, while the attraction of the moon only suffices to retain those whose velocities fall under a mile and a half in a second. So you perceive that all of the gases I have named would soon escape from the moon, even if they were present upon it at the beginning of its history.

“I must also remind you that there is no water upon the moon, at least not in the form of rivers, oceans, lakes, ponds, or even of clouds. But Professor Pickering has recently noted certain appearances which may be due to the formation of a kind of hoar frost. If there were once oceans upon the moon, as the great plains, called maria, or seas, in the lunar charts, seem to indicate, they, too, have escaped by evaporation. The velocity of the molecules of water vapor is two and a half miles per second, a mile greater than the ‘critical velocity’ which the attraction of the moon would be able to control.”

“But,” interrupted my companion, “I am puzzled to understand how you know so much about the power of the moon to hold things.”

“It is really quite simple,” I replied. “The attraction of gravitation, which is a property belonging to all known bodies, is measured by the mass, or amount of matter, in a body. It also varies with the distance between the attracting and attracted bodies. We know, by means which I shall not attempt to describe here, the mass both of the earth and of the moon. We also know the size of both of these bodies. They attract objects as if their entire masses were concentrated at their centers. A body of a certain kind and size at the surface of the earth weighs just one pound. If the earth were reduced to half its actual diameter, while retaining the same mass or amount of matter, more closely packed together, the body which now weighs one pound would then weigh four pounds, because it would be twice as near to the center of the earth as before, and the attraction of gravitation varies according to the square of the distance from the center. As the distance diminishes the force increases. The square of two is four, therefore the body would be attracted with four times the force which it experiences at present. Now, the moon is not only much smaller than the earth, but its average density, or the closeness with which the molecules of its rocks are packed together, is less. It results from these facts that the ratio of the entire mass of the moon is to that of the earth as one to eighty-one. Hence the inherent power of the moon to attract bodies is less than one-eightieth as great as the earth’s. If the diameter of the moon were the same as that of the earth, a body weighing one pound on the earth would weigh only one eighty-oneth part of a pound on the moon. But the diameter of the moon is less than one quarter as great as that of the earth. It follows that bodies on the moon are almost four times (more accurately about 3.66 times) nearer to the center of attraction. This fact must be taken into account in calculating the force of gravity on the moon’s surface. As far as the mass of the moon is concerned, bodies on her surface experience less than one-eightieth of the attractive force which the earth exercises upon bodies on its surface, but this is so far counterbalanced by their greater nearness to the center, that the actual attraction upon them is about one sixth of that which they would experience on the earth.”

“Thank you,” said my companion dryly, “your explanation appears to me to be very scientific.”

“Not by any means as scientific as it might be, or as it ought to be,” I replied, laughing. “But, really, if you wish to understand these things you should not be too much afraid of the bugbear ‘science.’ Science makes the world go nowadays, and everybody ought to know a little about it, just as everybody with any pretensions to education a hundred years ago had to learn more or less Greek and Latin. But let me continue a little farther. Since the force of attraction on the moon is only one sixth as great as it is on the earth, the weight of all bodies is in the same proportion. Pardon me if I guess at your weight; it is, perhaps, 120 pounds. Very well, translated to the moon you would weigh only 20 pounds.”

“Dear me, then skipping the rope may be the favorite pastime of middle-aged ladies on the moon.”

“And throwing somersaults that of gray-haired lunar gentlemen. Let me tell you of one very interesting consequence of the small force of the moon’s gravity, which affects not merely the weight of bodies but the flight of projectiles, and, indeed, all motions of every kind. You will see, when we come to the photographs, that some of the lunar volcanoes are of a magnitude almost incredible. This is doubtless due to the fact that the ejections from volcanic craters there were able, with no greater expenditure of explosive force, to attain an elevation six times that which they would attain if thrown from a volcano on the earth. During the eruption of Vesuvius in April, 1906, the column of smoke, steam, and cinders from its crater reached, according to the measures of Professor Matteucci, a maximum height of about eight miles. On the moon the same force would have blown these things almost fifty miles high! It is not difficult, in view of such facts, to see how the giant volcanic craters and mountain rings of the moon were formed.”

In the meantime the eclipse continued, and, having tired of watching it, we returned to the drawing-room.

“When shall we see these famous photographs and begin our imaginary journey in the moon?” my companion asked.

“To-morrow,” I replied. “But I shall have to demand one more brief exercise of your patience this evening, while I finish with this subject of eclipses.”

“Then we are not through yet?”

“Not quite, for I have not yet told you why the moon is not eclipsed every time she approaches the earth’s shadow, and why she does not eclipse the sun once every month at the time of New Moon.”

“Well, tell me then, and I promise to be as interested as possible; only please don’t talk any more mathematics than is absolutely necessary.”

“Very well, I’ll spare your attention as much as possible. To begin with the eclipses of the moon: The reason why they are not of regular monthly occurrence is simply because the orbit of the moon is a little inclined, about 5¼°, to the orbit of the earth. Even then there would be an eclipse once every month if the orbit of the moon were fixed in space, and if the point where that orbit crosses the plane of the earth’s orbit were always directly opposite to the sun. But instead of being fixed in position the orbit of the moon has a curious motion of revolution of its own. This causes the two opposite points, where it crosses the plane of the earth’s orbit, and which are called the moon’s ‘nodes,’ to move continually onward in a direction opposite to that in which the moon revolves, but much more slowly. A period of about nineteen years is required for the moon’s nodes to complete a revolution. The consequence is that the nodes are not always in line with the earth and the sun, and except when they are nearly in line no eclipse can occur. To enter into a complete explanation of this would require more ‘mathematics’ than you would like, but what I have said may at least serve to give you an idea of the reason why eclipses are comparatively of rare occurrence.”

“I think I understand the reason sufficiently. But what a complicated affair you astronomers make of what, it seems to me, should really be a very simple thing.”

“It is like a sewing machine,” I replied, “which seems very simple when you see it running smoothly, and do not trouble yourself about all the various parts of its mechanism. But if you undertake to explain to yourself, or to make clear to another person, exactly how the machine works, you find that your attention is rather severely taxed, and that the apparent simplicity is based upon no little complexity of construction and interaction of parts. You will have understood from what I have said, that the reason why the moon does not eclipse the sun once every month is based upon the same fact, namely, the inclination of the moon’s orbit to the plane of the orbit of the earth; and that when she does eclipse the sun her nodes must be somewhere near a line drawn from the earth to the sun. There is one broad difference between an eclipse of the moon and an eclipse of the sun which I have not yet mentioned. This arises from the fact that the moon being so much smaller than the earth, her shadow, when she hides the sun, does not cover the entire earth, as the earth’s shadow covers the whole moon, but comes almost to a point before reaching the earth. The average length of the moon’s shadow is only 232,150 miles, 6,690 miles less than the average distance between the moon and the earth. But since, in consequence of the eccentricity of her orbit, the moon’s distance is continually varying, the length of her shadow also varies to the extent of about 4,000 miles each way. Thus it may be as short as 228,300 miles, or as long as 236,050 miles. When the greatest length of the moon’s shadow coincides with her least distance from the earth (221,600 miles), her shadow extends more than 18,000 miles beyond the earth. Under such circumstances its diameter at the surface of the earth is about 167 miles. That is the greatest diameter that the shadow of the moon can have at its intersection with the earth. Ordinarily, when it reaches the earth at all, its diameter is less than 100 miles, and often very much less. If the earth and the moon were motionless during an eclipse, her shadow would form a round, dark spot on the earth, and all observers within the circumference of that spot would behold the sun totally eclipsed. But, in consequence both of the motion of the moon in her orbit, and the rotation of the earth on its axis, the shadow spot moves swiftly in an easterly direction over the earth’s surface, forming what is called the path of the eclipse. The astronomer calculates beforehand across what parts of the earth the path will lie, and selects his points of observation accordingly.

“When the length of the shadow is too small to reach the earth, the moon appears projected against the sun as a round black disk, hiding the center of the solar orb, but leaving a brilliant ring all around. Such phenomena are called annular eclipses. There are about three annular eclipses for every two total ones. When the moon, as often occurs, does not traverse the center of the sun’s disk, as seen from any part of the earth, a partial eclipse is the result. This means that only a portion of the sun is hidden by the moon. Even a total eclipse appears as a partial one to observers who are not placed within the limits of the shadow path.”

“But it seems to me,” said my friend, “you have hedged round your eclipses with so many difficulties, what with the tip of the moon’s orbit, and what with the shortness of her shadow, that they must be very few in number. Yet I often hear of an eclipse, although I have never seen one before to-night.”

“They are not so rare as you might suppose,” I replied. “It is not necessary, in order that an eclipse, either partial, or total, or annular, may occur, that the moon’s nodes be in a direct line with the centers of the sun and the earth. The moon may be a few degrees out of line, and yet either pass into the earth’s shadow or be seen crossing the sun from one point or another on the earth. There are about 70 eclipses in every eighteen years, 41 of the sun and 29 of the moon, but the number varies a little. Generally there can be no more than two eclipses of the moon in any one year, but it is possible for three to occur. The greatest number of solar eclipses in a year is five, but this is very rare, the usual number being two. In fact, there must be at least two solar eclipses in a year, but there are many years which have no eclipses of the moon at all. And now, I think I have said all that is necessary about eclipses, and we arrive very opportunely at the end of the discourse, for behold the moon is passing out of the shadow, and her light begins once more to glow in the park.”

This was indeed the case. Going to the door, we saw the earth’s shadow slowly withdrawing from the face of the moon, while the landscape was brightening under her returning rays. For a few minutes we watched, in silence, the brilliant spectacle. Then my companion turned to me.

“Would you know my whole thought?” she asked. “I fear that I cannot recall many of the scientific facts you have just been telling me, but for them I can go back, at need, to the books. Yet one thing I feel that I have certainly gained. It is a sense of friendly, companionable interest in the moon. Henceforth she will be more to me than she ever was before. I shall always be conscious, when looking at her face, that she is the offspring of the earth, and that there exists between these two bodies an intimacy that I had never imagined possible. For me your tides and your eclipses seem an inarticulate language, a caressing exchange of communications between these two celestial beings of one blood. To my mind they are, in a certain sense, personalities, and, as a creature of the earth, I feel now my relationship to the moon.”

“Very good,” I replied. “All science and all forms of knowledge are rooted in the imagination. To-morrow we shall begin with the photographs, and many most interesting things that I have not yet mentioned will then naturally present themselves before us.”

“Good night then,” said my companion, “and to-morrow I shall count upon the delights of a photographic journey in the moon.”

I

NEW MOON TO FIRST QUARTER

I
NEW MOON TO FIRST QUARTER

AT breakfast the next morning I asked my friend if she still had sufficient curiosity concerning the moon to induce her to undertake the contemplated journey amid lunar scenes.

“Yes, surely,” she replied. “My dreams last night were filled with wonderful spectacles; great cones of shadow flitted continually through the heavens, eclipsing, in turn, moon, sun, and stars; and I stared, as it seemed, for hours at strange faces veiled behind a maze of mathematical diagrams covering the moon. I am not sure that your discourses have made me scientifically much wiser, but I feel that my imagination is sufficiently aroused to enable me to enjoy the photographic excursion that you have proposed, and I am quite ready to start at once.”

“Excellent!” I said, producing my portfolio. “Here then are the photographs which I trust will enable us, in imagination, to spend an interesting month upon the moon. These photographs were made at the Yerkes observatory and they represent the moon, as you will perceive, in all of her principal phases, beginning with the narrow crescent of the New Moon, and ending with the similar, but reversed, sickle of the Old Moon.”

“Let us take them out into the park under the trees,” my friend suggested.

The shafts of morning sunshine, falling through the branches and illuminating the broad lawns and brilliant flower-beds, offered the greatest possible contrast with the strange scenes of the preceding night. We chose the shadow of a huge elm, and had a table placed there for our accommodation. On this I spread the photographs, and my companion began to examine them with many expressions of interest.

“It is not often,” I said, “that science finds so flattering an audience.”

“And I hope, surely, never so small a one,” she responded, laughing. “But you must admit that science very seldom presents herself in so attractive a form as that of these pictures.”

“They are indeed of the highest excellence,” I replied. “It is the very moon herself that you see there.”

“But are you certain that they have not been embellished? Has not the hand of an artist retouched and improved them—particularly these large ones that seem to contain a thousand curious things which I can hardly believe really exist on the moon?”

“No,” I said, “there is nothing fictitious or imaginary in what you see. The only art displayed here is that of the astronomer-photographer, whose greatest ambition is to make his pictures absolutely true to nature. A defect in one of his plates, producing the appearance of a speck of light or shadow which does not actually exist, causes him as much distress of mind as you would experience upon hearing a false note from your piano. Indeed, the astronomer is so desirous of having nothing but the truth represented in his pictures that he often prefers, for his own study, the original negatives alone, because every time that they are reversed to make a ‘positive’ copy something is sure to be lost, and some slight defect is certain to be introduced. Let us begin, if you please, with the series of smaller pictures showing the various phases, and the gradual advance of daylight across the moon’s surface. Take first the photograph which I have labeled No. 1. It shows the New Moon when it is between three and four days old. You must often have seen it in that form in the western sky soon after sunset. Photographs of the New Moon have been made when the crescent is still narrower than that here shown, but there is no such photograph in this series, and it would possess little interest for you because almost no details of mountains, craters, and plains would be visible. It is hardly possible to make a good photograph of the moon when it is only one or two days from the sun in its monthly journey, on account both of the glare of the solar light in our atmosphere and of the nearness of the moon to the horizon, where the air lacks transparency and steadiness. In the photograph before us you will observe a great number of strange forms and shadings. I shall tell you what these are presently, but first let me call your attention to the fact that the picture does not exhibit a phenomenon which you would behold if you were actually looking at the moon in the phase here represented. You see here the New Moon very clearly, but not the Old Moon in her arms.”

“Indeed! It is a pity that the photograph does not show so interesting a sight.”

“Yes, it is a pity. The cause lies in the defect of light from what I have called the ‘Old Moon.’ The part that we see in the photograph is illuminated with sunshine, while the remainder of the moon reflects only the earthshine, which is too faint to be photographed (at least with the amount of exposure required to make a good picture of the brightly lighted crescent); although, as I have said, you would see it clearly if you were looking at the New Moon herself.”

No. 1. February 19, 1904; Moon’s Age 3.85 Days.

“But,” interrupted my companion, “do you mean to tell me that the earth illuminates the moon?”

“Surely it does. Why not?” I replied, smiling. “You must remember that the earth is simply a huge moon to our imagined inhabitants of the lunar world. Our globe sends to the moon about fourteen times as much reflected sunlight as the moon sends to the earth. The consequence is than an earthlit night on the moon is far more brilliant than a moonlit night on the earth.”

“Then why do we not always see the moon shining with light from the earth?”

“It is a question of contrast. You cannot see a faint light in the immediate presence of an overpoweringly brighter light. The part of the moon that the sun illuminates is in the full glare of day, and this is so much more brilliant than the reflected earthlight that that portion of the moon which enjoys only the latter is not visible to us, except for a few days after New Moon, when the amount of light from the crescent is not yet great enough to dazzle our eyes and hide the rest from sight. I should advise you when the next New Moon occurs—you can find the date in any almanac—to look at it in the western sky. You will see in addition to the bright crescent the full round orb of the moon, shining faintly, with a dull, rather copperish, tint, and you will find it interesting, then, to remember that that light is reflected from our earth.

“And now,” I continued, “let us examine our photograph more closely. There is one remark that I had expected which you have not made; it concerns the position of the crescent. You observe that it is bowed toward the left. If you saw it with the naked eye in the sky it would be bowed toward the right, or toward the place of sunset. The reason is that the photograph presents the moon as seen with a telescope, which reverses objects, turning them top for bottom. In this picture, and in all the others that we shall examine, the southern part of the moon is at the top and the northern part at the bottom, the western part at the left and the eastern part at the right. The first thing that you probably notice in the photograph is a conspicuous oval plain, somewhat below the center of the crescent.”

“Yes, and I see clearly why you call it a plain, for it is perfectly flat and smooth.”

“Not quite so flat and smooth as you suppose. This object is one of the most celebrated on the moon. It is the so-called Mare Crisium, or Sea of Crises, as we may translate the name given to it by the astronomers of a couple of centuries ago, many of whom knew more Latin than science. Owing to its apparent smoothness of surface, as well as to its form and general aspect, they took it for a great lake or sea.”

“To tell you the truth,” said my friend, “if I were an astronomer and had discovered this curious place on the moon, I should certainly believe just what your Latin-loving predecessors believed, but I doubt if I should have been capable of inventing so singular a name for it.”

“In the singularity of the names they chose for objects on the moon,” I replied, “their invention is unrivaled. We shall see some remarkable examples. Of course they are not at all to be blamed for thinking that this oval spot, and other similar ones of much greater magnitude, were seas and oceans. They simply judged by appearance and by analogy. Finding mountains on the moon, they saw no improbability in supposing that there were bodies of water also. They had not the means of knowing, as we know to-day, that there is no water on the moon. Yet, perhaps, they were not so far wrong after all. The Mare Crisium certainly has the look of an empty sea bed, and I should not be willing to assert that ages ago it was not filled with water.”

“Like the Great Salt Lake, dried up,” suggested my companion.

“Not exactly, for the Great Salt Lake dried up would probably present a surface as white as snow, whereas the Mare Crisium is very dark. It must be admitted, however, that gradually the white deposit would grow darker, and there may be much significance in the fact, which some observers have noticed, that, at times, parts of the dark plains on the moon seem to glitter with minute points of light. Your imagination is at liberty to see deposits of salt there.”

“In that case,” said my companion, laughing, “I should prefer to regard the Mare Crisium as resembling that wonderful valley discovered by Sindbad the Sailor, whose floor was sprinkled with diamonds.”

“Well,” I replied, “science certainly cannot deny the possibility of diamonds on the moon, for she is par excellence the world of volcanoes, and one of the most striking discoveries of recent years is that of the intimate association existing between ancient volcanic vents and deposits of diamonds. The diamonds of South Africa are found in lava rocks that cooled off ages ago.”

“Then I hope that no future Columbus will find a way to the moon, for we should have too many diamonds, and they would lose all their charm.”

“That is true, but suppose that not only diamonds but even more beautiful gems should be discovered in the lunar world? You surely would not object to a transethereal traffic bringing them to our doors. However, there is not the slightest prospect that we shall ever be able to go from the earth to the moon. Let us resume our examination of the photograph, and concentrate our attention on the known facts.”

I then proceeded to tell my friend, whose interest I was delighted to find had not yet begun to flag even in the face of comparatively matter-of-fact statements, that the Mare Crisium is a profound depression, about 350 miles in length by 280 in breadth. Exactly how far it lies below the general level of the lunar surface we do not know; but, at any rate, if it was ever filled with water it formed a deep, navigable sea. Its encircling mountains, which appear generally bright in the photograph, especially along the eastern border, where the sunlight strikes directly against their slopes, are in many places steep and abrupt. At one place, on the southwestern side, there is a mountainous promontory 11,000 feet in height. There are a number of small craters on the floor of the Mare Crisium, but the scale of this photograph is not large enough to show them clearly.

“You will notice,” I continued, “that there is a kind of bay on the eastern side, which runs back into the mountains, and is bordered with high, steep cliffs. Near this point, on that part of the moon over which the sun has not yet risen, there is a very remarkable mountain which we shall see in a later photograph. But let us finish with this one. Look at the comparatively small oval adjoining the Mare Crisium below (toward the north). It is one of the great crater rings of the moon, and is named Cleomedes. It is much larger than it looks, being nearly 80 miles in its greatest diameter, and there is a peak on its surrounding wall 10,000 feet in height. Still farther toward the north you will observe two or three other smaller craters or rings, which are very interesting when studied with the telescope.

“Now, please turn your attention to the photograph bearing the number 2. You see again the Mare Crisium, and nearly in the center of the crescent, and just on the border line between day and night, a perfect oval ring with a central peak. It is called Langrenus. It is even larger than Cleomedes, being about 90 miles across. It has the form of an oval, as we see it, but that is an effect of perspective, since it is so far round the side of the lunar globe. In reality it is a nearly circular circumvallation, or rather an almost perfect hexagon, composed of gigantic mountains including a valley, in the center of which rises a cluster of peaks 3,000 feet in height.”

“This second photograph,” interrupted my friend, “was taken later than the first, I suppose, since it shows more of the moon’s surface.”

No. 2. September 24, 1903; Moon’s Age 3.87 Days.

“I should have told you that,” I replied. “Yes, it does represent the moon at a time when more of its surface, visible to us, is illuminated by the sun. In fact, we may regard it as a picture of the moon made about a day later than the other. But I must now tell you that these photographs were not all taken in regular succession, a day apart, or even two days apart. That was impracticable for reasons that I need not explain. Some of them were made at one season of the year and some at another. Yet taken together they form a sufficiently continuous series to enable us, with their aid, to follow the changing aspects of the moon during more than three weeks, or all that part of a lunation in which the moon is a conspicuous object in the sky.”[^[2]]

[2]. In addition to what is said in the text concerning the photographs the reader should be informed that, in consequence of her “librations,” the moon does not, all the time, present exactly the same surface toward the earth. If she did we should never see more than one half of her surface. In fact, however, at one time or another, we see, in all (but never at the same time), about fifty-nine per cent of her surface, leaving forty-one per cent which is forever invisible because never turned in our direction. The librations, or “balancings,” of the moon, which bring now one and now another portion of the usually invisible hemisphere into view, are of three kinds: First, the libration in latitude, arising from the combined effects of the inclination of the moon’s orbit to the plane of the earth’s orbit, and the inclination of her axis of rotation to the plane of her own orbit. When added together these two inclinations make the axis of the moon lean one way or the other with respect to the earth about 6½°. But, since the inclination of the moon’s orbit to that of the earth is continually varying to a small extent, the amount of this libration is also variable. Its effect is to cause now the North and now the South Pole of the moon to incline slightly toward the observer on the earth, so that he can see alternately a little way round the northern and the southern edges of the moon’s disk.

Second, the libration in longitude, which arises from the eccentricity of the moon’s orbit, causing her to move a little faster when she is nearer the earth, or in perigee, and a little slower when she is farther from the earth, or in apogee. In consequence of this, she gets alternately about 6° ahead of, or behind, the position which she would have if her orbit were a perfect circle and her motion perfectly uniform. But, inasmuch as her rotation on her axis is never either faster or slower, she shows a little of her usually invisible hemisphere on the western side when she is between perigee and apogee, and a little on the eastern side when she is between apogee and perigee. The accompanying diagram is designed to aid the reader in understanding these effects.

Effect of Moon, Varying Velocity in Orbit Producing Libration in Longitude.

Third, the diurnal libration, which arises from the fact that the diameter of the earth bears a considerable proportion to the distance of the moon. If the observer were at the center of the earth there would be no effect of this kind, but being situated about 4,000 miles from the center, there is a parallactic effect in consequence of which we see a little around the western side of the moon when she is rising and a little around the eastern side when she is setting. The maximum diurnal libration is a little more than one degree. The maximum libration in latitude is 6° 44´, and that in longitude 7° 45´. An illustration of the results of libration will be found by comparing photographs Nos. 1 and 2. They were both taken at nearly the same “age of the moon,” about three days, twenty hours, but under different librations, so that in No. 2 more of the western edge of the moon is visible, and the crescent appears broader. Even more remarkable examples of the results of libration are seen in Nos. 6 and 7, and 8 and 9. In No. 6, the moon is actually “older” by about half a day than in No. 7, yet, owing to libration, the “terminator,” or line between day and night on the moon, is considerably farther toward the east in the latter than in the former. A similar effect is seen in comparing Nos. 8 and 9. The exact dates and ages of the moon corresponding to these photographs are given in the Appendix.

“If you will follow the curve of the terminator toward the south (upward in the photograph), you will perceive that there is a long line of ovals, more or less resembling Langrenus. The first of these, darker in appearance than Langrenus, is named Vendelinus.”

“What extraordinary names!” exclaimed my companion, “and how unpicturesque!”

“Yes, it is true that the invention of the old astronomers who supplied these names seems to have failed a little at times. They did exceedingly well in naming the ‘seas’ and similar objects, but for the mountains, craters, and ring plains they could think of no better plan than that of attaching to them their own names, and the names of other savants, or supposed savants of their time, or of preceding centuries. And in Latinizing these names they gave them a kind of uniformity, which is hardly pleasing to our taste to-day. But let me continue. Vendelinus is an extremely beautiful sight when the sunlight strikes its broken walls in such a manner as to bring into prominence, by contrast with the deep shadows, the rugged peaks, precipices, and ridges of which its very irregular ring is composed. You should see it with a powerful telescope, especially under the rays of the setting sun. Then the bottom of the valley within has been described by Mr. Eiger, an English student of lunar phenomena, as appearing punctured like a sieve with holes.”

“And what are they?”

“Volcanic craters, probably, long since extinct.”

“So many volcanoes in one place?”

“Oh, yes. You have been at Naples and have seen Vesuvius. But probably you have not visited the Phlægrean Fields which lie northwest of Naples. If you had had a passion for geology when you were in Italy you would have explored that region, and there you would have found something not altogether unlike the valley of Vendelinus in the moon. There is a great number of extinct volcanic craters near Naples, and they show how similar in many ways the moon is, or has been, to the earth.”

“But, dear me,” my friend exclaimed, “are we going to see nothing but burned-out craters and wild, ragged mountains on the moon? I am sure that I should never have thought of visiting Naples for the sake of looking at its Phlægrean Fields.”

“Still,” I replied, “you must certainly know that Pompeii and Herculaneum and the memories of their tragic fate are the most vivid attraction of Naples to-day, although the Pompeiians have all been dead for almost 2,000 years. So in looking at these spectacles in the moon we cannot but be interested by the reflection that they are reminders and relics of a wonderful history, whatever its precise character may have been. The moon seems to me to stand for the most affecting of all tragedies—the passing of a world. When I survey its extraordinary landscapes, it is like looking upon a long-abandoned stage, whose actors are in their graves, whose scenery is moldering under a gaping roof, whose machinery is broken, whose very traditions are forgotten, but which yet retains a semblance of its former brilliance. I do not have to imagine inhabitants in the moon at the present day in order to find it interesting. The possibility that it may once have had inhabitants is enough, remembering its nearness to the earth and the manner of its origin, to make it the most fascinating thing that the heavens contain.”

“Indeed, I had never thought of the moon quite in that way,” was the reply. “If you can read a history for me in these craters and ring plains I believe I shall find them more interesting than I expected.”

“I cannot promise you a history as full of romantic details as that of Herodotus,” I said, “but it may contain nearly as many actual facts. However, we shall see about that as we go along. Let us now return to the inspection of the photograph. Be kind enough to look a little above Vendelinus. You observe there another still larger ring plain, or walled valley, with a conspicuous mountain in the center. This is Petavius. It belongs to the chain of similar formations which includes Langrenus and Vendelinus, but it is more wonderful than either of them. It is nearly a hundred miles long from north to south. For some reason, as with Vendelinus, its ruggedness and complexity of structure are more conspicuous in the lunar afternoon than in the lunar morning. It is a question of the direction in which the light falls across it. A curious thing about Petavius is the convexity of its vast floor. The center is about 800 feet higher than the edges along the feet of the surrounding mountains.”

“How do you know that?”

“The shadows tell the story. The height of objects on the moon is measured by observing the length of their shadows under a known inclination of the sun’s rays. When I stand this book upright on the table, allowing the sunlight to strike it on one side, it casts a shadow on the table. If I did not know the height of the book, and could not measure it directly, I could find it out by measuring the length of its shadow, other simple trigonometrical data, easily ascertained, being known. There is an enormous cleft not clearly visible in the photograph, extending from the central mountains of Petavius to the southwestern wall of the valley. Still farther south, above Petavius, you will notice another conspicuous oval plain and several smaller ones near it. The largest of these is named Furnerius. They all lay in the morning sunshine, not far from the terminator, when this photograph was taken.”

“Tell me, please, about the ‘terminator’ of which you have spoken several times. As I understand you it is the line between day and night on the moon.”

“Yes, and a very wonderful line it is, too. There is nothing just like it on the earth. Owing to the effects of our atmosphere in dispersing the light, day and night do not stand face to face with one another on the earth in the same way that they do on the moon. Here we have twilight in the evening and dawn in the morning, and night neither comes nor goes for us with the startling suddenness that characterizes it on the moon. For an hour or two after sunset and before sunrise, we receive rays of reflected and refracted light from the atmosphere above us, which spread a soft, pleasing illumination over the landscape, and render all objects more or less distinctly visible. But if you were on the moon in certain situations, the passage from day to night or from night to-day would be as rapid as the falling or rising of a curtain. Imagine yourself standing on the western wall of Vendelinus or Petavius at the time when this photograph was taken. You would be in a blaze of pitiless, untempered sunshine, but glancing down the precipice at your feet you would seem to be looking into a gulf of blackness. But for the light reflected back from the eastern cliff, and that coming from the earth, there would be scarcely a ray of illumination on the rocks below you. You would look down into inky darkness, and would scarcely dare to make a step from fear of falling over the edge of a bottomless pit. At the same time, as I told you last night, you would see the stars all about you in the sky, even close to the sun.

“This is the reason,” I continued, “why the march of day across the moon, always keeping sharp on the heels of night, is a spectacle so imposing and unparalleled. It is this wonderful march that we are going to follow with the aid of the photographs. I shall now ask you to give your attention to [photograph No. 3]. It was made more than a day and a half later than the others, measured by the age of the moon, which, in this case, was about five days and a half. You notice how in the interval the sunlight has swept eastward over the moon’s surface. The Mare Crisium is recognizable in the lowest or most northerly, of three large, dark plains. The small white oval a considerable distance above it is our old acquaintance Langrenus, whose floor and walls are now very brilliant in the full sunshine, which falls upon them at a high angle. Vendelinus and Petavius are less conspicuous. The broad, dark plain which has come into view eastward from Langrenus is the Mare Fœcunditatis, which we may translate ‘Sea of Fecundity’! You certainly cannot aver that on this occasion the invention of the old astronomers failed in the matter of romantic suggestiveness. The name calls up pictures of a great body of tranquil water, fanned by gentle, stimulating breezes, filled with fish of every variety, dotted with vine- and flower-garlanded islets, and bordered by well-watered shores, rich with vegetation, and supporting a numerous and happy population. Some such idea of the Mare Fœcunditatis may have been in the minds of its sponsors a couple of centuries ago. But telescopes have become too powerful in our day to permit us to be any longer deceived as to the actual nature of this singular lunar region. Like the Mare Crisium, it may have been the bed of a sea many years ago, but at the present time it contains no water, and its shores present an endless succession of fire-scarred cliffs, peaks, and volcanoes. The only ‘islands’ in it are extinct craters.”

“But,” said my companion, smiling, “where then is its history?”

“Ah!” I replied, “is not this old sea itself history enough? When it has receded sufficiently into the past, all history loses its details, and presents only its setting and its grand primary elements. Suppose that, some ages in the future, you should be an inhabitant of a distant planet, surveying with a telescope the dried-up basin of the Atlantic Ocean. Provided only that you were convinced, in your own mind, that it had once been an ocean, with fertile, inhabited shores, and with ships sailing upon it, you would be singularly lacking in imagination if you could not reconstruct its history for yourself. The details could safely be left to your invention and you could change them from time to time to suit your varying moods. Terrestrial historians have sometimes done that.”

No. 3. July 29, 1903; Moon’s Age 5.54 Days.

“But do you believe that the Mare Fœcunditatis was ever such a sea, and the scene of such events?”

“That is certainly a very pointed question. Questions of that kind are always in order when one is treating of ascertained verifiable facts, but just now, you know, we have wandered a little aside from the straight path of scientific exactitude. Still, I will be frank with you and say that I really possess no settled opinion concerning the former condition of the moon, except so far as what we may call its ‘geological’ history is revealed by its present state. I am sure that the moon was once the seat of tremendous volcanic action, and I think it not improbable that its great depressed plains were once occupied by water, but as to inhabitants, I know no more about them than you do. Still, I am disposed to think that, as we go on, you, at least, will reach the conclusion that all life has not yet disappeared from the moon. We are going to learn some very suggestive and significant things before we are through.

“Farther toward the south and closer toward the terminator you will see in the photograph a third dark plain with five sides, the northern one convex and ill-defined. At its upper corner is an incomplete ring plain. This region bears a still more curious name than the Mare Fœcunditatis. It is the Mare Nectaris or ‘Sea of Nectar.’”

“Apparently your astronomers of old took the moon for an abode of the gods.”

“Yes, or for their wine cellar. But we shall get a better look at the surroundings of this Sea of Nectar in a later photograph, and then I shall have more to tell you about it. In the meantime let us return to the Mare Crisium. To the east (right-hand side) of the Mare Crisium you will observe a diamond-shaped district, not very dark, with a bright point at the corner which faces the Mare. You could never guess its name. It is called the Palus Somnii, which may be translated ‘Marsh of a Dream.’ It is a very singular place, and, seen with the telescope, possesses a color which is unique upon the moon, a kind of light brown, quite unlike the hue of any of the other plains or mountain regions. It is covered all over with short, low ridges, as if its surface had been broken up in a most irregular manner with a giant plow. What the person who named it saw there to lead him to connect it in his mind with dreams I have never been able to imagine. The bright point on its western edge is a remarkable crater mountain, named Proclus. What that mountain is made of nobody knows, but it gleams with extraordinary brilliance when the sun strikes it.”

“Why may it not be snow-covered?”

“That is a suggestion which has often been made, but one great objection to it is that we have reason for believing that snow, at least in such a situation, cannot exist on the moon. Another objection is that only a few of the lunar mountains are comparable in brightness with Proclus, and they are not the loftiest ones. Upon the whole it is much more probable that the reflecting power of Proclus is due to the composition of its rocks, perhaps to broad crystalline surfaces exposed in the sunshine.”

“It is a surprise to me, then, that that ‘earthly godfather’ of lunar wonders, who had a sufficiently vivid fancy to invent the ‘Marsh of a Dream’ close by, did not name this mountain for some jewel, real or imaginary.”

“It would have been more poetic, indeed, but as I have already told you, the mountains and volcanoes of the moon nearly all bear very prosaic designations, while a wealth of fancy has been lavished in naming the ‘seas’ and plains. The astronomer Riccioli is responsible for most of the commonplace nomenclature that we find in lunar charts. If you will now glance at the northern (lower) ‘horn’ of the moon in the photograph you will notice, near the terminator, about two thirds of the way from the Mare Crisium to the end of the horn, a pair of ring plains, or crater rings, apparently almost touching one another. They are Atlas and Hercules, the latter being the smaller one on the right. A darker oval below them near the bright edge of the moon is Endymion.”

“That, at least,” exclaimed my companion, delighted, “is a romantic and appropriate name! I am enchanted to think that Endymion has not been separated by your cold-hearted science from her who loved him so well.”

“But if you should look at Endymion with a telescope you would wonder what the moon could find in him to admire. He has been turned into a huge, broken-walled ring plain. You will observe that the other, the southern or upper horn of the moon in the photograph, appears extraordinarily roughened. It is completely pitted with craters and rings. There are so many of them, and they are so entangled, that I shall not undertake to indicate them by their individual names, especially as there is none among them of the very first importance. If, however, you will bring your attention back to the Mare Nectaris I shall be able to point out to you a very extraordinary object, which lies just on the border between day and night here, but will be seen in the next photograph that we examine, in full morning light. The object that I mean is a ring on the right-hand edge of the Mare Nectaris. Its eastern wall and the top of its central peak are brightly illuminated by the rays of the rising sun; while beyond it, to the eastward, everything, with the exception of the tips of one or two high peaks, is steeped in night. This is one of the mightiest volcanic formations that the moon contains. Its name is Theophilus. To see it and certain gigantic neighbors that it has, fully displayed, we shall turn, after this glance at its first appearance, to photograph No. 4.

“In this photograph the sunrise line on the moon has advanced so much farther eastward that the Mare Nectaris lies well within the illuminated part of the disk, and Theophilus has become the most conspicuous object of the kind in view. You now observe that it does not stand alone, but is linked, so to speak, with another similar ring on its southeastern side, while still farther southward is a third less regular ring which seems to belong to the same group.”

“Oh, yes,” cried my companion, “they certainly do seem to be connected. They look like three links of an enormous broken chain dropped upon the moon.”

“The ring nearest to Theophilus,” I continued, “and whose northwestern side has been destroyed to give room for the full circle of the wall of Theophilus, is named Cyrillus. The other more distant one is Catharina. If you wish to become a little learned in the geography of the moon it is necessary that you should remember these names. As to the objects that the names designate, they are far too wonderful ever to be forgotten, and it is impossible to confuse them with any other features of the lunar world. There is a great deal of ‘history’ connected with these three enormous volcanic formations, but I am going to reserve that for a while, because by and by we shall examine a larger photograph of these same objects in which you will see their marvelous details displayed. Now let me direct your attention to the first chain of mountains that we have found upon the moon. Above Catharina you will notice a thin, crinkled line of light passing through a comparatively level district and ending at another ring. It is a range of peaks and cliffs named the Altai Mountains. They are of no great height, and cannot be compared in magnificence with the lunar Alps and the lunar Apennines which we shall see in the photographs taken a few days later, but they are nevertheless very interesting. The ring mountain at which the Altai range terminates is named Piccolomini. It is another marvelous object for telescopic study. The incomplete ring, with a dark interior, which forms the southern corner of the Mare Nectaris, resembling a semicircular bay, is Fracastorius. It is a very curious object because close inspection reveals that the missing part of its ring has been submerged, but is still faintly visible through the surface of the Mare.”

No. 4. November 24, 1903; Moon’s Age 5.74 Days.

“I suppose it cannot be water that has covered it, since you have so often assured me that there is no water on the moon.”

“No, it is not water, but rock or sand or solidified lava, or some kind of solid matter. It looks as though the whole bed of the Mare Nectaris had welled up in one mighty convulsive outpouring of liquid lava, which broke down the wall of Fracastorius, inundated the interior, and then hardened like a floor of cement. The probability that a catastrophe of the kind I have described has occurred here is heightened by the fact that the bed of the Mare Nectaris is concave, sunken in the center, as if it had broken and settled down ‘like ice upon a pond.’ Scattered more or less all over its surface and particularly near its shores, there are indications of this breaking down, and of something that has been covered up.”

“To me it seems very mysterious,” said my friend, “and very terrible also.”

“It is more or less mysterious to the astronomer likewise. Still, geology shows that there have been somewhat similar occurrences on the earth. If you will now direct your eyes to the lower (northern) part of the photograph you will notice some additional things that have come into view with the advance of the sunlight. You observe that a vast somber region occupies the inner portion of the crescent below the center. This consists of two immense plains, one of which sends a large ‘bay’ as far south as the ring of Theophilus, where it is connected by a narrow ‘strait’ with the Mare Nectaris.

“Turning to [photograph No. 5] we see the two plains to which I have referred more fully displayed. The sun has now risen over their entire surface. The upper one is the Mare Tranquillitatis, ‘Sea of Tranquillity’; and the lower one the Mare Serenitatis, ‘Sea of Serenity.’”

“I have always thought that astronomers must be happy persons,” said my companion, with a smile, “and these names are convincing.”

No. 5. July 1, 1903; Moon’s Age 6.24 Days.

“Yes, perhaps, but then in bestowing the names they may have been transferring to the moon ideals of tranquillity and serenity which they did not find realized upon the earth. I am not going to talk about these two ‘seas’ at present because they are better represented upon one of the large photographs which we shall examine later. I prefer to direct your attention just now to some other things. In the first place look once more at Theophilus and its companion rings, and observe how they maintain their preëminence. The entire surface of the moon to the eastward and southward is broken and heaped up with mountains, craters, and rings, but nowhere do we see anything comparable with Theophilus except, perhaps, far toward the south, where near the inner border appear two still larger, but less regular, rings lying in line at a right angle to the terminator. The one on the left is Maurolycus, and the other, still half obscured by night, is Stöfler.”

“The names of old astronomers, I suppose.”

“Yes, astronomers sufficiently famous in their day, but who would be virtually forgotten at the present time if their friend Riccioli had not thus immortalized them. You see it is a great piece of good fortune to have your name in the moon. It is a kind of revenge for the neglect of future generations at home.”

“And it seems to me an equal good fortune to have had an admirer willing to set your name up in the moon.”

“Surely. But Riccioli’s own name is there also. Afterwards I shall show you his lunar monument, a truly magnificent one. Permit me now to tell you that Maurolycus is much greater in extent than any of the rings that we have yet seen. Not by any means so perfect in form as Theophilus, it covers a vast extent of surface, as much as 150 miles across, with an amazing mass of broken rings, walls, ramparts, ridges and chasms. Some of its peaks are 14,000 or 15,000 feet in height. It has a very lofty central mountain, visible in the photograph, and whose peak comes into view when the sun is rising long before the surroundings have been illuminated, so that it resembles a star glowing amid the blackest night. The neighbor of Maurolycus, Stöfler, is equally extensive and almost equally wild and magnificent when the sunlight is leaping across it from pinnacle to pinnacle and ridge to ridge. In this photograph, however, it is too near the terminator to be well seen. We shall presently pass to photograph No. 6, where Stöfler appears in full light, but before doing so let us glance at the northern part of the moon as here pictured. Close to the terminator, below the grand oval form of the Mare Serenitatis, you will perceive two rings, one above the other. They seem to be the complement of the other pair, Atlas and Hercules, which we looked at when the sun had recently risen upon them in another photograph, and which now appear far off toward the west. You observe that Atlas and Hercules lie upon an east and west line, and the others upon a north and south line. The northernmost one is named Aristoteles, and the other Eudoxus. They are situated near the edge of a plain called the Mare Frigoris, ‘Sea of Cold,’ thus named, I suppose, because it lies so far north. Aristoteles is about 60 miles in diameter, and its immense wall is very high and splendidly terraced. Eudoxus, equally deep, is only 40 miles in diameter.

“Turning to photograph No. 6, taken when the moon was more than a day older than it was when No. 5 was made, we have a striking example of the effect of libration in presenting the moon at perceptibly different angles to our line of sight at corresponding phases. We have now arrived at First Quarter, and behold all the western half of the moon illuminated by the sun. You will perceive that we now have in view, simultaneously, six of the great plains called ‘seas,’ namely, the Mare Crisium, the Mare Fœcunditatis, the Mare Nectaris, the Mare Tranquillitatis, the Mare Serenitatis, and the Mare Frigoris, while others are beginning to emerge out of night on the east. Maurolycus and Stöfler, the pair of giant rings in the south, are better seen than before because daylight has advanced farther across them. In fact Stöfler now appears more imposing than its great neighbor, and a smaller ring breaking the continuity of its wall on the western side is visible. Above these, in the direction of the south pole of the moon, and around the pole itself, the surface is marvelously rough and broken. It looks as if it would be impossible to find a level acre of ground in all that region. The rings and craters are veritably innumerable. It is the existence of these irregularities which causes the terminator to appear so crooked and broken. At some places you perceive small bright points within the edge of the night half of the moon. These, of course, are the summits of peaks, which have just been touched by the sunlight while the surface all around them is still covered with darkness.

“Below Stöfler, all along the terminator, as far as the middle of the moon, an irregular row of rings appears. Three of these bear some resemblance to the great group of which Theophilus is the chief member. They are, counting from south toward north, Aliacensis, Werner, and Blanchinus. Below them two other much larger ones are conspicuous, Albategnius, the more southerly, and Hipparchus. These two are full of moon history. Albategnius, the smaller, is very deep and comparatively perfect in condition, while Hipparchus, more than 90 miles across, has been vividly described as a ‘wreck and ruin,’ its walls, once possibly of great height, being now low and broken, and traversed with gaps and valleys, while a great cleft exists crossing a part of the broad, irregular floor. It is probable that Hipparchus is an older formation than Albategnius.”

No. 6. November 26, 1903; Moon’s Age 7.75 Days.

“Pardon me,” interrupted my companion, “but I must cry for mercy. Really, these strange names escape from my mind as fast as you mention them. Is there not something a little more romantic in the moon—something to relieve the strain of all this nomenclature of words terminating in ‘us,’ and this frightful lunar geology?”

“Yes,” I said, “I believe that on the other half of the moon, which has not yet seen the sun rise, we shall find something better to your taste. But do not be too impatient. Reflect that these names represent very wonderful things visible to us in another world than ours, things the knowledge of which has cost the lifelong labors of many gifted men, and that will be remembered, studied, talked, and written about centuries after we are dead. Fortunately for your powers of attention the eastern half of the moon, upon which day will be seen gradually dawning in the next set of photographs, has a general character quite different from that of the western half. It contains the greatest ranges of lunar mountains, yet upon the whole it is more level, being covered to a great extent with broad plains, in the midst and along the borders of which stand the most remarkable and interesting of all the lunar formations. In and around some of them we shall search for the evidences which some astronomers think that they have found of life upon the moon.”

“Oh, that indeed will be interesting!” exclaimed my friend with reviving animation.

“But,” I added, “do not place your expectations too high. Keep your imagination under control, try always to be just a little ‘scientific’ in your way of looking at things, and then I believe you will not be disappointed.”

“Oh, please do not think that I have been disappointed,” she said deprecatingly. “But positively you must admit that ‘Albategnius,’ ‘Aliacensis,’ ‘Blanchinus,’ and ‘Maurolycus,’ are not precisely captivating. Remember that I have read little except poetry and romance, and those histories that are full of stories.”

“You will find a deep vein of poetry and romance in the moon,” I replied, “before we have finished, and after you have reflected upon what we have seen and what we have been saying.”

Leaving the remaining photographs to be examined after lunch, we now entered the house.

II

FIRST QUARTER TO FULL MOON

II
FIRST QUARTER TO FULL MOON

NOTWITHSTANDING the signs of impatience which my friend had manifested when we were passing, in our review of the photographs, from one lunar ring mountain to another, all more or less similar in appearance and characteristics, I was gratified to see that her mind was still attracted to the subject of the moon, and during the lunch she, of her own accord, began to talk of it.

“You have said so much about volcanic occurrences on the moon,” she remarked, “that I wonder why you do not call those immense mountains ‘volcanoes.’ I observe that you always speak of them as ‘rings,’ or ‘mountain rings,’ or ‘ring plains’; while to me, although to be sure I am no geologist and have perhaps no right to an opinion, they seem plainly to be just huge volcanoes and nothing else.”

“Your observation is quite correct,” I replied, “as far as superficial appearance goes, and I may add that these great rings are often called volcanoes. If we apply the proper adjective and name them ‘lunar volcanoes,’ perhaps there can be no objection to the term. But they are certainly widely different from our terrestrial volcanoes. The difference is not in size alone, although in that regard it is enormous. There is a far more significant difference, which you could hardly be expected to notice in a simple inspection of the photographs, although it is evident when once pointed out. I refer to the fact that what seem to be the craters of lunar volcanoes are not situated on the tops of mountains. They are immense plains, more or less irregular in surface, and often having a peak or a group of peaks in the center, while around these plains always extends a mountain ring, steep on the inner side, and having a gradual slope without. But most significant fact of all, the plains, or floors inside the ring, are almost invariably situated thousands of feet below the general level of the moon. If the terrestrial volcanoes were formed on the plan of the lunar ones, when we visit Vesuvius, instead of climbing up a mountain rising out of the midst of a plain and capped with a cone, having a funnel-shaped crater in the center, we should find before us a relatively low, circular elevation, on surmounting which there would appear on the inside of the circle a great basinlike hollow, far below the level of the surrounding country. In the center of this, distant from the lofty encircling walls, would be seen a conical hill with smoke and vapor issuing from a vent at its summit. The top of this crater hill would be lower than the rim of the basin-shaped hollow, so that the whole volcano with its immediate surroundings would be inclosed and shut off from the environing upper world by the sides of the basin. While you finish your coffee I will make a sketch which may render this difference between lunar and terrestrial volcanoes evident at a glance.”

Lunar Volcano, in Section.

Terrestrial Volcano, in Section.

Accordingly, after a few minutes, I presented to her these two diagrams, remarking that it should be borne in mind that the two sketches were not made on the same relative scale. “I was compelled,” I said, “to change the true proportions in the section of the lunar volcano, for if I had drawn them as they are in fact, the width of the basin would have been enormous in proportion to its depth. You will recall that I told you that such rings as Albategnius and Maurolycus are a hundred miles and even more in diameter, while their depth does not exceed two or three miles. It results from this necessary falsification of proportions in the sketch that the terrestrial volcano, although so widely different in form, appears comparable in magnitude with the lunar one. But the fact is that you could take a dozen of the largest volcanic mountains on the earth and throw them into one of the great lunar rings without filling it.”

“I am the more astonished by what you say,” remarked my friend, “because you have already told me that the moon is so much smaller than the earth. How does it happen, then, that her volcanoes are so much larger? I should think that in a little world all things would be small in proportion.”

“It is quite natural to think so,” I replied, “until you reflect upon the consequences of the smaller force of gravitation on a small world. I told you last evening that gravitation on the moon, is only one sixth as powerful as it is on the earth, and you will recall that one consequence which I pointed out was that you would weigh only twenty pounds if you were on the moon. Since the same reasoning applies to all objects in the lunar world, it is clear that a similar force exerted there would be able to produce enormously greater effects, as for instance in the formation of vast hollows or depressions, by violent explosions, the products of which would be thrown to immense distances. Some selenographers, which is a term applied to those who study the features of the lunar world, have suggested that in this cause alone is to be found the explanation of the giant lunar ring mountains. At some remote period of the past, according to them, the volcanic forces of the moon reached a maximum of activity and energy. The lava, cinders, ashes, and other products of ejection, were hurled to a height of scores of miles, and when this fell back at a great distance from the centers of eruption these were piled up in huge rings, fifty, eighty, or a hundred miles in diameter, while the surface of the moon within the rings sank in consequence of the withdrawal of the material thus ejected. To account for the existence of the central mountains so often found in the middle of the rings, it has been suggested that at a much later period, when the volcanic energy had become comparatively insignificant, as a result of the cooling of the interior of the moon, less violent explosions, not greater than many that have occurred on the earth, took place, and by these the central peaks were formed.”

“You are going to think me too romantic, or too imaginative, again,” said my friend, with a smile, “but I cannot prevent myself from wondering what the inhabitants of the moon did and thought while all those marvelous things were happening.”

“I have not said that there were inhabitants of the moon.”

“No, but you have confessed that there might have been inhabitants, some time, and I should like to know whether they were there when those terrible volcanoes were formed.”

“If they were,” I replied, “they could not have survived such a universal upheaval as the surface of the moon has undergone. You have seen in the photographs that the great rings and smaller craters are scattered thickly over the moon. It is true that comparatively few are found in the level expanses called ‘seas,’ but if those regions were covered with water they could only have been inhabited by beings provided with gills and fins.”

“How long ago did these explosions occur?”

“I cannot tell you, except that it must have been many ages in the past; so long ago, indeed, that the whole course of human history seems but a day in comparison.”

“Then,” said my friend with animation, “there has been time enough since that dreadful period for inhabitants to develop upon the moon, has there not?”

“Yes, time enough, perhaps, provided that sufficient water and air and other vital requisites remained after the exhaustion of the volcanic energies.”

“Oh, let us say that they did remain. I am eager to believe that the moon has not always been so desolate as she appears at present.”

“Very well, you are at liberty to believe that if you like. No astronomer is likely positively to contradict you, although he may smile a little incredulously. Besides, as I have already told you, there are certain rather inconclusive indications of some kind of life, and of some kind of activity, still on the moon.”

“Please show them to me, then, or tell me about them. Perhaps I shall find them less inconclusive than you do.”

“Everything in its turn,” I replied. “We shall come to the indications that I have spoken of after we resume the inspection of the photographs.”

“Then I am ready to resume at once.”

Accordingly we returned to the table and the photographs under the pleasant shade of the elm. Taking up the photograph numbered 7, I remarked that it exhibited the moon as it appears a little after First Quarter; that is to say, a trifle more than half the face turned toward the earth is in the sunlight. I called attention once more to the six “seas,” which we had already remarked, and to the continued conspicuousness of Theophilus and its companions, a little above the middle of the visible hemisphere.

“You observe now,” I continued, “how the rotundity of the lunar globe begins to manifest itself as the sunlight sweeps farther eastward. The crescent shape is gone and the line between day and night begins to be bowed outward, convexly. The Mare Crisium is particularly well defined, and also the diamond-shaped region called the Palus Somnii. With the sun so nearly vertical above it, the remarkable peak of Proclus, between the Palus Somnii and the Mare Crisium, has become very brilliant. In a telescope you would see it glowing almost like a star. You observe also that several long, straight, bright rays proceed from it in several directions.”

“All the more reason, it seems to me,” said my friend, “why your unimaginative astronomer, Riccioli, should have named it for some brilliant gem instead of attaching to so dazzling an object the prosaic designation of ‘Proclus.’”

“After all,” I replied, “what’s in a name?” Now that you are familiar with the appearance of Proclus, its name will henceforth call up to your mind an image as brilliant as if it had been named ‘Mount Diamond’ or ‘Mount Amethyst.’”

No. 7. July 2, 1903; Moon’s Age 7.24 Days.

“Pardon me,” said my friend, “but it was not of names like those that I was thinking. Observe how he who named the neighboring Palus Somnii, ‘Marsh of a Dream,’ exhibited an exquisite delicacy of fancy. It suggests something indefinitely strange, romantic, imaginative. That unknown astronomer, unknown at least to me, put a little of himself, a little of his inmost mind, into the name, and I thank him for it. I shall never forget the ‘Marsh of a Dream’ in the moon. It will haunt my own dreams. I shall be all my life seeking and never finding its meaning.”

“Since you are in so poetic a mood,” I responded, “I rejoice that besides its bald facts, its fireless volcanoes, and its dried-up plains, the moon possesses many things that can stir the imagination of the most sentimental observer. But, in order that we may not wander too far from the paths of science, let me recall your attention to the photograph. We have been going over ground already trodden by returning to the neighborhood of the Mare Crisium. I shall now lead you back to the terminator, where we shall find a little that is new. Still nearly hidden in night we perceive many great rings on which the sun is beginning to rise, and four of the most important ranges of mountains are coming into view. One of these, on the southern border of the Mare Serenitatis, is visible throughout its entire extent. It forms a portion of the coquettish ornaments with which the Moon Maiden has decorated her hair, as we shall see clearly in the next photograph. This range is named the Hæmus mountains. Near its center, quite at the edge of the ‘sea,’ is a bright crater ring, one of the most conspicuous on the moon. It is called Menelaus.”

“Menelaus?” exclaimed my friend. “Ah, then Riccioli did not confine his favoritism to the astronomers and philosophers in putting their names in the moon. Menelaus, if I remember my classical reading correctly, was the husband of Helen of Troy.”

“Yes, the brother of Agamemnon himself. You must admit that Riccioli occasionally felt his imagination a little awakened. He was not altogether destitute of the spirit of poetry.”

“But did he also put Helen in the moon?”

“I am sorry to say that he did not. It would have been a very suitable abode for her. However, if you like, you may recognize Helen in the Moon Maiden herself.”

“Thank you, that will be, indeed, an unexpected pleasure.”

“Meanwhile allow me to point out to you that there is a curious light streak, very faintly shown in the photograph, which crosses the Mare Serenitatis from Menelaus to the opposite shore, and reappears more distinctly, on the lighter-colored plain toward the north. This streak comes all the way from a great ring mountain named Tycho in the southern part of the moon. It is more than 2,000 miles long, and is one of the greatest mysteries of the lunar world. Tycho, which lies just on the sunrise line, is not well seen in this photograph. It has a great number of these strange streaks or rays proceeding from it in all directions. We shall study them in one of the photographs which are to come. One word in regard to the plain north of the Mare Serenitatis of which I have just spoken. It, too, has a name that is calculated to appeal to your lively imagination. It is called the Lacus Somniorum, which if my knowledge of Latin is correct, means ‘Lake of the Sleepers.’”