The Moon: A Popular Treatise

by Garrett Putman Serviss

“About half way between Tycho and the south pole of the moon, you will see an enormous irregular plain, with lofty broken walls, interrupted by a number of crater rings. Several similar rings also appear in the interior of the plain. If Tycho is the most perfect in form of the lunar crater rings, this great inclosure, which is named Clavius, is the finest example of the walled valleys. It is more than 140 miles across, and covers an area of not less than 16,000 square miles. Two of the rings within it, which seem so small in comparison, are 25 miles across. A smaller walled plain, yet one of really immense size, is seen half way between Tycho and Clavius, and farther from the terminator than either of them. This is Maginus, and it possesses the peculiarity that at full moon it practically disappears!”

“But how can that be possible? I see nothing behind which it can be hidden.”

“It is the sunlight that hides it. You must have noticed already that the rings and mountains are best seen when at no great distance from the terminator, because there the sunlight strikes across them at a low angle, and their shadows are thrown sharply upon the adjoining slopes and levels. Look at the western part of the moon in the photograph before us. Many of the huge rings and walled plains that were so striking in appearance when the sun was rising upon them are now barely visible. Langrenus and Petavius, for instance, have become no more than whitish blotches, and even Theophilus is no longer conspicuous. The reason is because when the sunlight falls vertically upon any part of the moon there are no shadows there, and without shadows there can be no appearance of relief. Then the mightiest mountains are almost lost from sight in the universal glare. The same thing would be apparent if you were suspended above the earth at a great height in a balloon and looking down upon the tops of the snowclad Rockies. Without shadows serving to reveal their true character and to throw their outlines in silhouette upon the adjacent plains, they would resemble only white spots and lines on the generally darker expanse of the continent. But Maginus is an extreme case. Owing to the relatively small elevation of its walls, and their broken-up state, and owing also, probably, to a similarity of color between the mountain ring and the inclosed plain, when the light is vertical upon them, as at the time of Full Moon, they blend together and become barely distinguishable from one another, and from the surrounding surface of the moon.

“Take now photograph No. 9. The age of the moon here is actually less than it was in the photograph that we last examined, yet, in consequence of libration, which has caused the moon, in effect, to roll a little to one side, the sunlight is farther advanced toward the east, and we see many features of the lunar world that before had not yet emerged from night. Clavius you will notice is much more fully illuminated. See how distinctly the shadow of its vast western wall is cast upon the floor of the valley within, while the opposite eastern wall with its immense cliffs and precipices glows in full sunshine, its shadow, thrown toward the east, blending with the darkness of night still covering that side of the moon. Southeast of Tycho, which is beautifully shown here, two other great walled plains have come into view. The uppermost of these is Longomontanus and the other Wilhelm I. For a considerable distance below these (toward the north) the surface continues broken with rings and craters, but at length these give place to a dark, level expanse. This is a part of the Mare Nubium, or ‘Sea of Clouds.’”

“Not quite so romantic a name as some of the others,” remarked my friend, “but still I think I can be sure that Riccioli had nothing to do with the selection. There is certainly something poetic in the idea of a sea of clouds.”

“It is a very beautiful region when examined with a telescope,” I continued, “and its mountainous shores contain many interesting formations. Farther north, you will observe, near the terminator, and apparently lying in the midst of the Mare Nubium, a large ring, as perfect in form as Tycho itself. This is a very famous object, and it bears the name of the great astronomer Copernicus, who overthrew the Ptolemæic system and established in its place the true idea of the solar system, namely, that the sun is its center, while the earth and the other planets revolve as satellites around him.”

“Surely,” said my friend, “Copernicus deserved to have his name placed in the moon, and very conspicuously, too.”

No. 9. August 2, 1903; Moon’s Age 8.97 Days.

“It could not have been made more conspicuous,” I replied, “for the situation of the great ring mountain called Copernicus, in the midst of an immense level expanse, makes it one of the most marked features of the lunar world. Copernicus is the subject of one of the larger photographs that we are going to examine later, and I reserve a description of its peculiarities. North of Copernicus you will observe apparently a continuation of the Mare Nubium. But it is really another ‘sea’ that we are looking upon there, the Mare Imbrium, ‘Sea of Rains.’ The baylike projection that runs out into the bright highlands west of Copernicus bears the name of the Sinus Medii, ‘Central Gulf,’ and the one just below it is the Sinus Æstuum, ‘Gulf of Heats,’ which is certainly suggestive of dog days on the moon. Observe that the Sinus Æstuum merges on the west with a dark, oval area, which is called the Mare Vaporum, ‘Sea of Mists.’ It is one of the darkest districts on the moon. If you will now turn the photograph upside down you will find that the Sinus Medii constitutes the dark eye of the Moon Maiden, while the Sinus Æstuum and the Mare Vaporum form that portion of her hair which droops upon her forehead.”