This image shows the Earth and the moon, and the bulges produced on each side from the tides. Click on image for full size version (55K JPG)

The force of gravity caused by an object gets weaker as you move farther away from that object. In this picture, the Earth is pulling on the Moon, and the Moon is pulling on the Earth. The Moon pulls more strongly on the side of the Earth facing the Moon than on the side facing away from the Moon. Because the gravitational force on one side of the planet is different from that on the other side, it is called a tidal force.

Because planets are not perfectly rigid, they deform when subjected to such tidal forces. They deform as if they are being pushed from the top and bottom, and a bulge forms on either side of the planet (Earth also bulges at the equator all the way around because it is spinning). These two bulges are called tides. On Earth, near the ocean, these tides can actually be seen. The ocean water rises high along the beach, twice each day.

If a body is very rigid or is not held together well, instead of getting pushed and pulled out of shape, the tidal forces can actually tear the body in half, as with comet Shoemaker-Levy 9.