Solar and Lunar Tides

Recall that the force of gravity depends inversely on the square of the distance, so that nearer objects are more strongly attracted to each other than are more distant objects. The Sun as well as the Moon exerts tides on the Earth, sometimes adding to create very large tides (the "Spring" tides, at new and full moon), and sometimes working 90° apart to create weaker tides (the "neap" tides, at 1st and 3rd quarter), but because the tide-raising force depends very strongly on the distance (the force varies as the inverse cube of the distance), the Sun's tidal effects are only half those of the Moon. In the Earth-Moon system, the side of the Earth facing the Moon experiences a stronger attraction than does its center, which in turn has a stronger attraction than has its far side. The result of this differential gravitational attraction is the creation of tides—one on the side nearer the Moon, and the other on the far side.25 Early studies of the motions of the Moon may have been in part motivated by a need to predict the tides. See §6.2.2 for further discussion of the effects of the tides in the context of early cultures.

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