Gravity is a universal attractive force that is directly related to the mass of an object. One mass will always attract another mass, and the size of the attraction is related to the size of the masses involved. Gravity is a comparatively weak force until one or other of the masses gets very large. Earth, being a large mass, exerts an appreciable attraction on any mass close to it. The gravitational force of Earth reaches far out into space. To reach a spot where Earth's gravitational attraction is one millionth of its value at Earth's surface it would be necessary to travel approximately four million miles into space.

If Earth were a stationary sphere composed of a uniform substance, the gravitational force would be identical at any place on its surface. Earth, however, is none of these things. It is rotating, which produces two notable effects...