FIG. 9.
The earth has a smaller sphere or "satellite," circulating round it, this is the Moon; it is placed at a distance of about 238,000 miles from the earth, is about one-fiftieth of its bulk or volume, and revolves round it every 27½ days, keeping always the same side towards it, so that the other side has never been seen. This is effected by rotation on its axis, which takes place once for every circuit round the earth; if it did not rotate, or kept the same side always to the same point of the heavens, then, when it had half-way revolved, the other side would be turned towards the earth. There are good reasons for believing that the moon has no atmosphere or air around it, for the concave edge (when only a small portion of the half illuminated by the sun is seen) instead of being gradually shaded off into darkness, as would be the case if there were an atmosphere, is well defined and uneven, showing the tops of some of the mountains and their shadows (fig. 10). Nor does there appear to be any clouds, and as a consequence no water; neither has anything been observed by the aid of the best telescopes which could be considered sea, on the contrary, the whole surface seems torn up and rent into chasms and immense jagged mountains, enclosing circular portions like gigantic walls. The darkened parts which are always seen on the surface of the moon were formerly supposed to be seas, but of late, by the use of improved instruments, they have been seen to be rough like the other parts, and cannot therefore be water (fig. 11). As there appears to be no water, it may be confidently inferred that there are no inhabitants.
FIG. 10.
The moon having always one half illuminated by the sun, while the other half is dark, presents different "phases" to us on the earth, according to the position in which we view it; thus, in fig. 12, e represents the central position of the earth, and S the direction from which the light of the sun comes, the outer circle of moons represents the various real positions of that luminary with respect to the earth and sun, and the inner circle shows its appearance when in these various positions. When the moon is between the earth and sun, as at a, it is said to be in "conjunction," and as the darkened side is towards the earth, of course it is not visible, this is "new moon" (a corruption of "no moon"); when at b b, but a small part of the illuminated half is seen, and it is then said to be "crescentic" (first and last quarter); at c c, half of the bright side is seen, the moon is said to be at "quadrature," and the appearance is that of a "half-moon;" at d d, the greater part of the bright side is seen, and it is called "gibbous" and appears as a "three-quarter-moon," and at e the whole of the illuminated side is seen; it is then "full-moon," and is said to be in "opposition."
FIG. 12.
Beyond the earth the planet Mars (fig. 13) moves in its orbit round the sun; it is the smallest, but one, of the larger planets, having a diameter of but 4085 miles, and being only about one-seventh the magnitude of the earth; it makes its revolution in 687 days, at a distance of 144,780,000 miles from the sun.
