Moon Earth Sun

The moon is the earth’s only natural satellite.

Its revolution period around the earth is the same length and direction as its rotation period, which results in the moon always keeping one side turned toward the earth and the other side turned away from the earth.

The moon has a noticeable effect on the earth in the form of tides, but it also affects the motion and orbit of the earth.

The sun acts on the earth and its moon as one entity with its center at the barycenter.

The Sun

Because of the elliptical nature of the earth’s orbit and constant changes in the earth’s rate of spin because of the previously mentioned phenomena, the sun, as seen from earth, is moving at a non-uniform rate. This makes it difficult to use the real position of the sun as a reference for time keeping. For these purposes, a point which moves at a constant rate around the earth is used instead of the real position of the sun. This point is called the mean sun and is the basis for mean solar time.

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