Earth Tides

Definition

The earth tide is the deformation of the solid earth caused by the gravitational attraction of the Sun and moon. The most striking gravitational effects of the Sun and moon are the orbital motion of the earth’s center of mass. The moon causes the earth to orbit the earth–moon center of mass, and the Sun causes that center of mass to orbit the Sun.

The Tidal Force

But the Sun and moon also cause points within the earth to be displaced relative to each other: they cause the earth to deform. For example, the side of the earth nearest the moon is attracted toward the moon more than is the center of the earth. And the side farthest from the moon is attracted less than the center of the earth. Consequently, both the far and near sides of the earth are pulled radially outward away from the center. The regions of the earth that are at right angles to the earth–moon vector are pulled radially inward.

This deformation pattern is illustrated in Figure 1. The total gravitational...