There is therefore at the poles but one day and one night in the year--the day being summer and the night winter. In less northern climates, as England, this exists to a much smaller extent; for half the year the sun is longer above than below the horizon, constituting our summer, the other half of the year the sun is longer below the horizon than above it, and this is winter. Hence, in each case, midway between, the sun is twelve hours above and twelve below the horizon, which occurs in spring and autumn, being called the "equinoxes" (equal nights); the vernal or spring equinox takes place on the 21st of March, the autumnal on the 21st of September. The times just between these, when the sun is the longest and shortest time below the horizon, are called the summer and winter solstices, and occur on the 21st of December and the 21st of June. [Illustration: 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 27-1/2 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. [Illustration: FIG. 10.]