Part 15

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III

FULL MOON TO OLD MOON

AFTER dinner, in the brilliantly lighted drawing-room, we once more spread out the photographs on a table.

“This time,” I said, taking up No. 14, “we are going to watch the advance of night over the moon. Before, it was the march of sunrise that we followed. Both begin at the same place, the western edge or limb of the moon. Comparing this photograph, which was taken when the moon was about fifteen and two-third days old, with No. 13, taken when the moon’s age was more than a day less, you perceive, at a glance, wherein the chief difference lies. In No. 13 sunrise is just reaching the eastern limb; in No. 14 sunset has begun at the western limb. Having watched day sweep across the lunar world, we shall now see night following on its track. West of the Mare Crisium and the Mare Fœcunditatis, which I expect you to recognize on sight by this time, darkness has already fallen, and the edge of the moon in that direction is invisible. The long, cold night of a fortnight’s duration has begun its reign there. The setting sun illuminates the western wall of the ring mountain Langrenus, which you will remember was one of the first notable formations of the kind that we saw emerging in the lunar morning. But then it was its eastern wall that was most conspicuous in the increasing sunlight. For the selenographer the difference of aspect presented by the various objects of the lunar world when seen first under morning and then under evening illumination is extremely interesting and important. Many details not readily seen, or not visible at all, in the one case become conspicuous in the other. But it is only close along the line where night is advancing that notable changes are to be seen. Over the general surface of the moon there is not yet any perceptible change, because the sunshine still falls nearly vertical upon it. Tycho’s rays are as conspicuous as ever. Aristarchus, away over on the eastern side, is, if possible, brighter than before, and the three small dark ovals, Endymion a little west of the north (or lower) point, Plato at the edge of the Mare Imbrium, and Grimaldi near the bright eastern limb, are all conspicuous.”

NO. 14. AUGUST 26, 1904; MOON’S AGE 15.65 DAYS. ]

“But look!” exclaimed my friend, putting her finger upon the photograph. “Here is something that you have not mentioned at all. I believe that I have made a discovery, although you probably will not accept it as a scientific one. I see here a dark woman in the moon.”

“I confess,” I replied, “that I am not acquainted with her, and do not even see her. Please point her out to me.”

“She appears in profile, like the brilliant Moon Maiden, but is not so much of a beauty. In fact I begin to suspect that she is the ‘Old Woman in the Moon,’ that I have often heard of.”

“Positively I do not see her.”

“Then I will try to recall some of the names that you have been telling me in order to indicate where you should. She faces west and occupies most of the eastern half of the disk. Her head is under Tycho, toward the northeast, I suppose you would say. The bright double ray that you pointed out in one of the preceding pictures lies across the top of her head and over her ear. Her face seems to be formed by a part of the Mare Nubium—you observe how well I have learned your selenographical terms—and her hooked nose is composed of a kind of bay, projecting into the bright part below Tycho. Her front hair is banged, and the Mare Humorum constitutes her chignon. She has a short neck, and a humped back, consisting of the Oceanus Procellarum. Copernicus resembles a starry badge that she wears on her breast, and Aristarchus glitters on the inner side of the elbow of her long arm. The Mare Imbrium seems to be a sort of round, bulky object that she carries on her knee, and she appears to be gazing with intentness in the direction of the Mare Tranquillitatis.”

“Ah, yes,” I said, laughing, “I see her plainly enough now. I really cannot say that your discovery is likely to be recorded in astronomical annals, but nevertheless I congratulate you upon having made it, if only for the reason that henceforth you can never forget the names and locations of the lunar ‘seas’ and other objects that you have been compelled to remember in pointing out your ‘dark woman.’ In truth, her features are almost as well marked as those of the Moon Maiden, but you will hardly be able to find her again, except in a photograph, or with the aid of a telescope, because you must recollect that this picture shows the moon reversed top for bottom as compared with her appearance to the naked eye, or with an opera glass. But please look again at the objects along the western edge, for we are about to turn our attention to photograph No. 15 in which this will be no longer visible. You must say ‘good-by,’ or rather ‘good night,’ to the Mare Crisium and the Mare Fœcunditatis; for you will see them no more, until another lunar day has dawned.”

We next picked up photograph No. 15.

NO. 15. AUGUST 28, 1904; MOON’S AGE 17.41 DAYS. ]

“Here the age of the moon has increased to nearly seventeen and a half days. The sunset line has advanced to the borders of the Mare Nectaris and the Mare Tranquillitatis. Toward the south a vast region which was very brilliant in the morning and midday light with the reflections from mountain slopes and the rays of Tycho, has passed under the curtain of night. The great crater rings on the eastern border of the Mare Nectaris, and thence upward to the South Pole, are beginning to reappear, but with the shadows of their walls thrown in a direction opposite to that which they assumed before. By a little close inspection you will recognize Theophilus and its neighbors which were so conspicuous for many days while the sunrise was advancing, but which have been almost concealed in the universal glare of the perpendicular sunshine since the Full Moon phase was approached. On the Mare Tranquillitatis and the Mare Serenitatis it is late afternoon, and your favorite ‘Marsh of a Dream’ has become a true dreamland.”

“This oncoming of night,” said my friend, “seems to me more imposing, and more suggestive of mystery than was the advance of day.”

“Surely it is. Do we not experience similar sensations when night silently creeps over the earth? But it imparts a feeling of loneliness and desolation when we watch it swallowing up the barren mountains and plains of the lunar world that we do not experience in terrestrial life. There are no cheerful interiors on the moon to which one can retreat when darkness hides the landscapes. There is another thing about the lunar night to which I have made but scant reference thus far. I mean it’s more than Arctic chill. Imagine yourself standing there in the midst of the broad plain of the Mare Tranquillitatis. Toward the east you would see the sun close to the horizon, yet blazing bright and hot, without clouds or mists to temper its rays. The rocks or soil beneath your feet would perhaps be cold to the touch, because the surface of the moon radiates away the heat very quickly, but your face and hands would be almost scorched by the intense solar beams. Looking toward the west you would see the shining tips of mountains suddenly extinguished, one after another, and when the sharply defined edge of the advancing night passed over you it would be as if you had plunged into a cold bath. In a little while, if you remained motionless, you would be frozen. No clothing would suffice to keep you warm. Nothing that polar explorers have ever experienced can be likened to the cold of the lunar night. Only the apparatus of the laboratories for producing temperatures, capable, when combined with pressure, of liquifying and solidifying the air itself, can bring about upon the earth a lowering of temperature comparable with that which occurs during the lunar night.”

“But I do not exactly see why night should be so much colder on the moon than on the earth. She is not farther from the sun.”

“No, her average distance from the sun is the same as that of the earth. The reason why her nights are so cold is to be found in the absence of an atmosphere like ours. The air is the earth’s blanket, which serves a double purpose, tempering the heat by day with its vapors and winds, and keeping the earth warm at night by preventing the rapid radiation into space of the heat accumulated during the daylight hours. If there is any atmosphere at all upon the moon—and I shall tell you by and by what has been learned on that subject—it is so rare as compared with ours that it can exercise very little effect upon the temperature of the lunar surface.

“Now, look at the great range of the lunar Apennines. You will see that the eastern faces of these mountains are in the sunlight, and they cast no shadows, as they did in the lunar morning, over the Mare Imbrium. The same is true of the lunar Caucasus, and the lunar Alps. All of these mountains are very steep on the side facing the plains, and that is the side presented sunward in the lunar afternoon. By turning to photograph No. 16, we shall see this phenomenon more clearly displayed. This photograph, measured by the age of the moon when it was taken, is more than a day older than the other, but once again the effect of libration has, in part, counteracted for us the advance of the line of sunset. Still it has distinctly advanced. You will observe that it has now passed completely across the Mare Nectaris, and more than half across the Mare Tranquillitatis, while only the mountain tops along the western edge of the Mare Serenitatis remain to indicate its outlines in that direction. Theophilus, Cyrillus, and Catharina, on the eastern border of the Mare Nectaris, have again become very conspicuous, but this time in evening instead of morning light. See how sharply the western wall of Theophilus stands out against the darkness of night behind it, and how its central peak glows in the setting sun while all the vast hollow beneath it is black. The floors of Cyrillus and Catharina, being less profoundly sunken, are still illuminated. Below the Mare Serenitatis, the twin rings, Aristoteles and Eudoxus, are very conspicuous, and they show the same change of illumination as Theophilus, their western sides being strongly illuminated on their inner faces, while the eastern walls cast shadows into the interior. The mountainous character of the surface in the neighborhood of the North Pole of the moon seems to be more clearly brought out in evening than in morning light. In this picture the North Polar Region seems to be almost as much broken up with gigantic rings as is that surrounding the South Pole. In both cases, you observe, many of the rings are poised just on the edge of the lunar disk, and their libration alternately swings them in or out of view.”

NO. 16. AUGUST 29, 1904; MOON’S AGE 18.62 DAYS. ]

“Then the other side of the moon may not be very different from the side that is turned toward us.”

“In its general features I doubt if it is at all different. There was once a theory, which had considerable vogue, that the side of the moon turned away from the earth presented a great contrast with its earthward side. A German mathematician, Hansen, drew conclusions, which are no longer accepted, as to the form of the moon. He thought that the moon was elongated in the direction of the earth, somewhat like an egg, her center of figure being about thirty miles nearer to us than her center of gravity. This, if true, would make the part of the lunar surface that we see lie at a great elevation as compared with the other part, and the center of gravity being toward the other side would cause the atmosphere and water to gravitate in that direction.”

“What a pity that so interesting a theory should have been abandoned!”

“If interest were the only test of the value of a scientific theory knowledge would not advance very fast. Notice how this very photograph before us vindicates the true scientific attitude toward nature. It records all the facts within its range, and leaves the theories to us. The features of your ‘dark woman’ are, in their way, as clearly marked in the photograph as is the range of the lunar Apennines. It is for us to recognize the essential difference between the interpretations which we choose to put upon these two phenomena. Giving play to fancy, we see the figure of an old woman in the one case, and employing our reason we find a chain of unmistakable mountains in the other.”

“But surely you do not mean to aver that science has no other business than that of recording facts.”

“By no means. It is also the business of science to find hypotheses and to build up theories that will explain its facts and connect them together systematically, according to some underlying law. But as I have just intimated it is the mark of true science that it never retains a theory merely because it is interesting. The truth is the only touchstone. Still, even the most conscientious scientific investigator may be misled by his imagination. His greatest virtue is that he never lets his fancies deceive him after he has recognized their false character. Point out your ‘dark woman’ to the child, or the savage, and it will be in vain afterward to explain that her profile is made up of plains and mountains. The child and the savage are not scientific but imaginative, and only after a long education will they abandon the apparent for the real.

“I will ask you now to take up photograph No. 17. The age of the moon here is twenty days. Comparing it with the last photograph we see that Theophilus has disappeared, although Cyrillus and Catharina, being a little farther east, are yet visible. Half of the Mare Serenitatis is buried in night, and only a little of the eastern edge of the Mare Tranquillitatis remains visible. Aristoteles and Eudoxus are now very close to the terminator, and the shadows of their eastern walls are spreading farther over their floors. Aristarchus is very brilliant, as it is still early afternoon on that part of the moon, and the sunshine is intense. Observe that Kepler, the crater ring directly east of Copernicus, has become more conspicuous than we have seen it in any preceding photograph. This is especially true of the system of bright rays surrounding it, and it is due to the change of illumination. In the southern part of the moon, west of Tycho, you will now recognize many gigantic formations which we first saw when the sun was rising over them. Some of them are even more prominent in the sunset light. Among these is our old acquaintance Maurolycus, whose western wall is so brilliant that it resembles a tiny crescent moon. The double row of broad, dish-shaped walled plains along the central meridian has also become visible once more. In fact the amount of delicate detail and the sharpness of the definition in these photographs are very remarkable. Observe the curious mottling of the ‘seas.’ It is in some of the differences of tint, which correspond in telescopic views of the moon more or less closely with the varying shades in the photographs, that some selenographers have thought they could detect evidences of the presence of vegetation on the moon. We shall talk about that more in detail another time. It is sufficient just now to notice that the beds of the mares are by no means uniform either in tint or in level. All of them are more or less ‘rolling,’ like many of our prairies, and often winding chains of hills and huge cracklike ravines are visible in them. In this photograph the amount of detail shown in the Mare Imbrium is particularly striking. Notice how some of the crinkled rays from Copernicus extend almost to the center of the ‘sea,’ and how in front of the precipitous base of the Apennine range the lighter-colored ground, with three prominent ring plains in it, presents the appearance of shallows. Lying off the shore south of Plato and the Alps a number of isolated mountain peaks are seen, mere white specks on the gray background. The undulating character of the ‘bottom’ of the ‘Bay of Rainbows’ is also distinctly indicated. By the way, I should perhaps mention the names of the three rings lying off the front of the Apennines, for although they are among the most interesting on the moon they have hitherto escaped our special attention. The largest of the three is Archimedes, the second in size is Aristillus, and the smallest is Autolycus. You will hear of them again when we come to the large photograph of the Mare Imbrium and the Mare Serenitatis.