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Moon and the Earth

Updated on March 9, 2016

Moon is the only satellite of the Earth. The diameter of the moon is about one-fourth that of the earth and its mass is about 1\81 that of the earth. The mean distance of the moon from the earth is about 3,84,000 km. The moon rotates on its axis and revolves round the earth in a fixed orbit. The distance between the moon and the earth also keeps changing. At its minimum distance from the earth the moon is said to be in perigee and when the distance between the two is at its maximum, the moon is said to be in apogee. The moon's period of rotation with reference to the sun is about 29.53 days and this period is called a synodic month. In terms of sidereal time the moon takes about 27 1/2 days in completing one rotation on its axis. This period is called a sidereal month or a lunar month. This also the time taken by the moon to complete one revolution around the earth. Due to the fact that the period of rotation and revolution of the moon is equal we always see the same face of the moon from the earth. Incidentally, we can see only 59 per cent of the total surface of the moon from the earth and the remaining about 41 per cent is never visible from the earth.


An eclipse is a complete or partial obscuration of a heavenly body. On the earth we can see the solar and the lunar eclipses. A solar eclipse occurs when the moon in its orbit around the earth is so positioned that the whole of part of the sun is not visible from a certain area of the earth. It can happen on a new moon day when the sun and the moon are in conjunction, i.e. on the same side of the earth and they are in a straight line. A lunar eclipse occurs when the sun, the earth and the moon are in the same line but the sun and the moon are in opposition, i.e. on the opposite sides of the earth so that the earth obstructs sun's light and casts a shadow on the moon. Such a situation may occur on the full moon day. Generally more lunar eclipses are likely to be seen from a place over a given period of time.

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