horizon; Md = moves each day. the moon sinks below the horizon the distance ab, and when mov Fig. 44. Mb = 13°, the distance the moon ter only cd; and the moon will rise correspondingly later in the one and earlier in the other. NODES.-The orbit of the moon is inclined to the ecliptic about 5°, the points where her path crosses it being termed nodes. The ascending node (8) is the place where the moon crosses in coming above the ecliptic or toward the north star; the descending node (8) is where it passes below the ecliptic. The imaginary line connecting these two points is called the "line of the nodes." OCCULTATION.-The moon, in the course of her monthly journey round the earth, frequently passes in front of the stars or planets, which disappear on 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. 4 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. 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 irregu lar 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 Linnæus 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 ap pearance similar to that of the Sea of Serenity. |