The Moon

Tides on our planet are caused by the gravitational pull of the Moon and Sun. Earth's oceans "bulge out" because the Moon's gravity pulls a little harder on one side of our planet (the side closer to the Moon) than it does on the other. The Sun's gravity raises tides, too, but lunar tides are twice as big. When the Sun and the Moon are approximately aligned (as they are when the Moon is New or Full) we experience especially high tides called "Spring Tides." When there is a 90-degree angle between the two, we get lower tides called "Neap Tides."

Ocean tides on Earth can be as high as 12 meters (40 ft) depending on local geography and the alignment of the Moon and Sun. Earth has solid ground tides too, but they amount to less than 20 centimeters (about 8 inches).

Although the Moon is small compared to behemoths like the Sun and Jupiter, lunar tidal forces are dominant because the Moon is so nearby. Tidal stretching is proportional to the inverse cube of the distance to the source of gravity. (If you moved the Moon to twice its present distance, its tidal influence would decrease by a factor of 23=8). Thus, nearby objects -- even small ones like the Moon -- raise high tides, while distant giants like Jupiter don't produce much of an effect.

"The force of Newtonian gravity falls off as the square of the distance," explains Lebo, "but tidal forces decline as its cube. Tides are caused by gravity, so why aren't they the same? The reason is simply that tidal stretching comes from a difference in the gravitational pull felt on two sides of a body. In a 1/r2 gravitational field, the difference in gravity between any two points falls off as the cube of the distance, not the square."