Surface of the Moon. The moon is an opaque, cold globe, covered with mountains, extinct volcanoes, and plains. She has neither water nor atmosphere, and always presents the same surface to the earth in consequence of rotating on her axis in the same time as she revolves round the earth. Moonlight is only reflected sunlight, the illuminated hemisphere being always turned towards the sun.
The face of the moon has been studied and mapped on a large scale. Its chief features are three in number: (1) the numerous volcanic craters, such as Tycho and Copernicus, which are mostly named after distinguished men of science; (2) the wide, dark plains which are known as seas, because they were formerly thought to consist of water; (3) the curious systems of bright streaks, which radiate from many of these craters, of which the most remarkable extend in all directions from the great crater Tycho, near the moon’s south pole, and are conspicuous even to the naked eye at the time of full moon.
The Moon and the Tides. The moon has long been known to have an effect upon the tides, and may perhaps influence the winds. It is of enormous importance to navigators for the determination of longitude, and hence its movements have been investigated with the greatest care and precision.
HOW THE MOON FORMS “TIDES” IN THE CRUST OF THE EARTH
By reason of its power of attraction, it is well recognized that the Moon exercises a greater influence on the side of the earth which is nearest to it. In consequence the earth is subject to a stress or pull that tends to lengthen it out toward the moon, and then to recede as the earth turns away on its axis.
The Planet Mars. Nearest to the earth, with the single exception of Venus, resembles the earth more closely than any other of the planets, and is most favorably situated for our observation of all the heavenly bodies, except the moon. It is a globe rather more than half the size of the earth. When Mars comes nearest to the earth its distance from us is about 35,000,000 miles. At these favorable moments its brightness is about equal to Jupiter, and only surpassed by that of Venus. Mars has a very pronounced red color, which is supposed to be due to the prevalence of a rock like our red sandstone on its surface, or possibly to the color of its vegetation.
Its density is much less—about three-quarters that of the earth; so a pound weight placed on its surface would not weigh much more than six ounces, and a ponderous elephant would, if there, be able to jump about with the agility of a fawn.
The heat and light which Mars receives from the sun, therefore, vary enormously, and so cause a difference in the lengths of winter and summer in his north and south hemispheres, the seasons in the north hemisphere being far more temperate than those in the south. Viewed with the telescope, large dark green spots are seen, the rest of the surface being of a ruddy tint, except at the two poles, where two white spots are observed and considered to be due to large masses of snow and ice. It has been supposed that the greenish spots are oceans, and the ruddy parts land. The spectroscope has shown that watery vapor is present in Mars’ atmosphere, and appearances like huge rain-clouds sometimes obscure a part of the planet for a considerable period. Physical processes seem to go on there much the same as on our planet; hence many believe that Mars is inhabited and forms, in fact, a miniature picture of the earth.