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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.
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.
Sills, 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.
Craters.—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.)
Eclipse Of The Sun.—If the moon should pass through either node at or near the time of conjunction or new moon, she would necessarily come between the earth and the sun, for the three bodies are then in the same straight line. This would cause
an eclipse of the sun. If the moon's orbit were in the same plane as the ecliptic, an eclipse of the sun would occur at every new moon; but as the orbit is inclined, it can occur only at or near a node.
The eclipse may be partial, total, or annular.—In Fig. 48, we see where the dark shadow (umbra) of
the moon falls on the earth and obscures the entire body of the sun. To the persons within that region )
there is a total eclipse; the breadth of this space is not large, averaging only 140 miles. Beyond this umbra there is a lighter shadow, penumbra (pene, almost—umbra, a shadow), where only a portion of the sun's disk is obscured. Within this region there is a partial eclipse. To those persons living north of the equator and of the umbra, the eclipse passes over the lower limb of the sun; to those south of the umbra, it passes over the upper limb.* When the eclipse occurs exactly at the node, it is said to be central. If the eclipse takes place when the moon is at apogee, or furthest from the earth, her apparent diameter is less than that of the sun; as a consequence, her disk does not cover the disk of the sun, and the visible portions of that luminary appear in the form of a ring (annulus); hence there is an annular eclipse in all those places comprised within the limits of the cone of shadow prolonged to the earth.
General facts concerning a solar eclipse.—The following data may perhaps guide in better understanding the phenomena of solar eclipses.
(1.) The moon must be new.
(2.) She must be at or near a node.
(3.) When her distance from the earth is less than the length of her shadow, the eclipse will be total or partial.
(4.) When her distance is greater than the length of her shadow, the eclipse will be annular or partial.
(5.) There can be no eclipse at those places where the sun himself is invisible during the time.
* South of the equator the reverse of these phenomena woulJ happen.