Part 8

“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.”

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. ]