Moon

Moon, The, the satellite of the earth, is the nearest to us of all the heavenly bodies, being at a mean distance of 240,000 miles. Its diameter is 2,153 miles and, its density being little more than half that of the earth, the force of gravity at its surface is very much less than that at the surface of the earth. A body which weighs a pound here would only weigh about 2J ounces if taken to the moon. Her path is approximately an ellipse with the earth in one focus. [For the causes affecting its motion see Lunar Theory.] Its apparent motion in the sky is from west to east, but she moves much faster than the sun, taking about 27 days 8 hours to travel all round the earth. The time between two successive new moons (synodic period or lunation) is 29$ days. The reason of the difference is that the sun slowly moves in his annual course through the stars in the same direction as the moon, which therefore in its revolution round the earth has to overtake him when it returns. The moon rotates on its axis in the same time as it performs a revolution in its orbit; hence the same half is always turned towards us. [Libration.] Except at opposition - i.e. when the earth is between the moon and sun - the whole of the moon's disc does not appear bright to us, and the amount of the bright surface seen by us is found to depend on the relative positions of moon and sun. Half of the moon is always illuminated by the sun; but when it is in conjunction between the earth and sun the whole of the bright surface is on the side away from us; so that the moon is invisible. As it moves farther from the line joining earth and sun, a small portion of the bright side comes into view as a narrow crescent. This increases till half the disc is illuminated, when the lines joining earth and moon and earth and sun are at right angles. From this time the moon loses its crescent shape and becomes convex on both sides, or gibbous - the maximum brightness, or full moon, occurring when sun and moon are on opposite sides of the earth. After this the moon becomes gibbous, then crescent, and vanishes before the time of new moon. These are known as the phases of the moon. If the moon's orbit coincided with the ecliptic there would be a solar eclipse at every new moon and a lunar eclipse at every full moon; but the inclination of the lunar orbit allows the moon to be sometimes as far as 5° on either side of the sun. It is only when the moon is near one of its nodes (q.v.), at conjunction or opposition, that an eclipse can occur. Every year there must be two solar eclipses, and may be five, while there may be three lunar eclipses or none. From the fact that when a star passes behind the moon the rays from the star are not refracted at the moon's edge, it has been deduced that no atmosphere surrounds the moon. Its surface is seen to be very irregular, and the dark and light parts were thought in earlier times to be seas and continents; but there is now evidence which renders the absence of water certain. Mountains there are, as is proved by the shadows of them cast by the sun, and measurements of these shadows have shown that some of the mountains exceed an altitude of two miles. Many of them appear to have huge craters, often ten miles across, and others form circular rings round low-lying plains. 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.