Except for occasional comets and meteors, the Moon is the celestial body nearest the Earth. Its mean distance from the Earth is 237,640 miles, but as it moves in an elliptical orbit, it has at one point a remoteness of 253,263 miles and opposite to this one of 221,436 miles. The diameter of the Moon is about one-fourth that of the Earth, or 2,160 miles, and its volume is 1/49 that of the Earth. The mass of the Moon (volume multiplied by density) is 1/81 and the density ⅗ that of the Earth. The period of the Moon's revolution about the Earth is 27 days, 7 hours, 43 minutes and 11½ seconds. As its period of rotation on its axis is the same, only one side of the Moon is ever seen from the Earth. Since, however, the Moon's axis is inclined about 83° to the plane of its orbit, we sometimes see a little distance beyond each of its poles, and, since the rate of motion of the Moon in its orbit varies slightly, we sometimes see a little beyond the eastern and western edges of the hemisphere. The total result of these _librations_, as they are called, is to make four-sevenths of the Moon's surface visible to us. Of the remaining three-sevenths, nothing is known. So far as is known, the Moon is not flattened at the poles. Owing to its slow rotation on its axis, the Moon's day has a length of 29½ of our days. Each portion of its surface is therefore exposed to or shielded from the light of the Sun for a fortnight continuously. The Moon has no atmosphere. Hence, it can have no diffused light, and nothing can be seen on it except where the Sun's rays shine directly. "If a man stepped into the shadow of a lunar crag," says Todd, "he would instantly become invisible. For a similar reason, no sound, however loud, can be heard on the Moon. The rolling of a rock down the wall of a lunar crater, will be known only by the tremor it produces." Moreover, changes of temperature on the Moon are rapid and violent. Where the Sun's rays strike, a temperature about that of boiling water is believed to be reached, while in unilluminated portions it is thought to go as low as 100° below zero. The force of gravity upon the surface of the Moon is only ⅙ of that on the Earth. Therefore, a man weighing 150 pounds on the Earth, would weigh only 25 pounds on the Moon, and the same muscular energy by which he could jump 6 feet on the Earth would carry him a distance of 36 feet on the Moon. On the Earth a body falls 16 feet in one second; on the Moon only 2.6 feet in the same time. The surface of the Moon is made up of mountains, valleys and plains, resembling in general appearance those of the Earth. As a whole, however, the surface of the Moon is much more uneven than that of the Earth. Some of the mountains of the Moon have a height of over 20,000 feet. As there is no sea-level to measure from, this figure expresses height above the surrounding surface, it being determined by the length of the shadows cast by the mountains.