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one side of her disk and reappear on the other, This is termed an occultation, and is of practical use in determining the difference of longitude between various places on the earth.

Lunar Seasons; Day And Night, Etc.—As the moon's axis is so nearly perpendicular to her orbit, she cannot properly be said to have any change of seasons. During nearly fifteen of our days, the sun pours down its rays unmitigated by any atmosphere to temper them. To this long, torrid day succeeds a night of equal length and polar cold. How strange the lunar appearance would be to us! The disk of the sun seems sharp and distinct. The sky is black and overspread with stars even at midday. There is no twilight, for the sun bursts instantly into day, and after a fortnight's glare, as suddenly gives place to night; no air to conduct sound, no clouds, no winds, no rainbow, no blue sky, no gorgeous tinting of the heavens at sunrise and sunset, no delicate shading, no soft blending of colors, but only sharp outlines of sun and shade.

What a bleak waste! A barren, voiceless desert! The nights, however, of the visible hemisphere must be brilliantly illuminated by the earth, while its phases "serve well as a clock—a dial all but fixed in the same part of the heavens, like an immense lamp, behind which the stars slowly defile along the black sky."

Telescopic Features.—The lunar landscape is yet more wonderful than its other physical features

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Even with the naked eye we see on its surface bright spots—the summits of lofty mountains, gilded by the first rays of the sun—and darker portions, low plains yet lying in comparative shadow. The telescope reveals to us a region torn and shattered by fearful, though now extinct* volcanic action. Everywhere the crust is pierced by craters, whose irregular edges and rents testify to the convulsions our satellite has undergone at some past time.

Mountains.—The heights of more than 1,000 of these lunar mountains have been measured, some of which exceed 20,000 feet. The shadows of the mountains, as the sun's rays strike them obliquely, are as distinctly perceived as that of an upright staff when placed opposite the sun. Some of these are insulated peaks that shoot up solitary and alone from the centre of circular plains; others are mountain ranges extending hundreds of miles. Most of the lunar elevations have received names of men distinguished in science. Thus we find Plato, Aristarchus, Copernicus, Kepler, and Newton, associated however with the Apennines, Carpathians, etc.

Gray plains or seas.—These are analogous to our prairies. They were formerly supposed to be sheets of water, but have more recently been found to ex

* Several distinguished astronomers assert, however, that the crater Linnaeus has undergone of late certain marked changes. Its sides seem to have fallen in, and the interior to have become filled up, as if by a new eruption. It is said to present an appearance similar to that of the Sea of Serenity.

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hibit the uneven appearances of a plain, instead of the regular curve of bodies of water. The former names have been retained, and we find on lunar maps the "Sea of Tranquillity," the "Sea of Nectar," "Sea of Serenity," etc.

Bills, luminous bands.—The latter are long bright streaks, irregular in outline and extent, which radiate in every direction from Tycho, Kepler, and other mountains; the former are similar, but are sunken, and have sloping sides, and were at first thought to be ancient river-beds. Their exact nature is yet a mystery.

Graters.—These constitute by far the most curious feature of the lunar landscape. They are of volcanic origin, and usually consist of a cup-like basin, with a conical elevation in the centre. Some of the craters have a diameter of over 100 miles. They are great walled plains, sunk so far behind huge volcanic ramparts, that the lofty wall which surrounds an observer at the centre would be beyond his horizon. Other craters are deep and narrow,—as Newton, which is said to be about four miles in depth,— so that neither earth nor sun is ever visible from a great part of the bottom. The appearance of these craters is strikingly shown in the accompanying view of the region to the southeast of Tycho. (Fig. 46.)

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