Neptune is thought to have an immense atmosphere, and, like the other outer planets, one of a composition not wholly familiar to us. Consequently we do not know as yet just what this atmosphere does for the planet. It has a fairly good reflecting power, though the planet, on the whole, is darker in color than Jupiter or Saturn. Its color is of that bluish cast which sometimes suggests a leaden appearance. The color, as well as the fact that Neptune is denser than any of the other outer planets, indicates that it may be in a more advanced stage of development than at least Jupiter and Saturn are, and perhaps than Uranus is. That Neptune has made greater progress toward solidity (though it is still very far from that state) than the other outer planets is suggested also by its size; for, as we have seen, the smaller planets develop more rapidly than the larger ones. The diameter of Neptune is a little less than thirty thousand (29,827) miles. The planet is somewhat smaller, therefore, than Uranus, and much smaller than Jupiter or Saturn. But as compared with the earth, the largest of the inner planets, it is a vastly greater body. Its mass is seventeen times more than that of the earth; its surface is as much as sixteen times more extensive than the earth’s; and its volume is more than eighty times greater than the volume of the earth. Of the time of Neptune’s rotation on its axis very little is known. That little, however, indicates a slower rotation than the other planets seem to have, and the alternations of day and night on Neptune are, therefore, probably less swift than on Jupiter and on Saturn. The planet is too far away for us to see its surface markings with any distinctness, but there are indirect processes by which we can get approximate information concerning the facts about rotation. One of these processes is by observation of the motions of the satellites. Of these useful bodies Neptune, fortunately, has one--a very excellent moon about the size of our own. It has some eccentricities, such as revolving about the planet in the opposite direction from that which the more conventional satellites follow, and having an orbit a good deal inclined to the plane of the equator of the parent body. But it is a very interesting moon to astronomers, and will no doubt in time help to make clear some things in the history of Neptune which are now not quite understood. Being so far from the sun, Neptune moves, of course, very slowly in comparison with the nearer planets, though his speed is at the rate of three and a half miles a second, which, after all, does not denote any high degree of sluggishness. His change of position in the sky amounts to a little more than two degrees a year; so that in an ordinary lifetime he does not make any very great progress along the zodiac.