Chapter IV: Introduction (2)

“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.”

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I

NEW MOON TO FIRST QUARTER

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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]

Footnote 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?”