1. The effect arising from the differential gravitational effect of one body on another.

This usually manifests itself in the distortion of the shape of the body; especially, the surface layers.

2. Periodic changes in the shape of a planet, moon, or star caused by the gravitational attraction of a body near it.

The moon tugs on earth's oceans, causing high and low tides; while Jupiter's gravitational attraction on its moon Io causes ground tides; and when two stars are very close together, they pull each other's atmospheres into distorted shapes.

A monthly tide of decreased range that occurs when the moon is at apogee or the farthest from the earth.

A tide with one high water and one low water in a tidal day of approximately 24 hours.

1. A tide of decreased range that occurs twice a month, when the moon is in quadrature; that is, during its first and last quarters, when the sun and the moon are at right angles to each other relative to the earth.
2. The tide raised on the earth when the sun and moon are in positions forming a right angle at the earth's center.

The two pulls (sun and moon) largely cancel each other out, resulting in an extra low high tide and an extra high low tide.

A monthly tide of increased range that occurs when the moon is at perigee or closest to the earth.

The tide on the earth's surface when the effect of the moon and sun is greatest; that is, when the two bodies are in line.

This occurs at new and full moon.