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. First, Earth is deformed out of a true spherical shape because of the centripetal force of the spin. Thus it has a greater diameter at the equator than at the poles. This means that people standing at the equator are further from the center of Earth than if they were standing at one or other of the poles. Thus the gravitational force is somewhat less at the equator.
The second effect is that all objects at the equator are tending to fly off the spinning earth. The force throwing them off Earth is, fortunately, much smaller than the attractive force of gravity. However, this ejecting force does reduce the over all effect of gravity to some degree. So bodies at the equator have both of these effects apparently reducing the over all attractive force of the earth. The difference between the apparent force attracting bodies to the surface at the equator and the force at the poles is approximately 0.5%. This means a 200 lb (91 kg) man at the poles appears to weigh only 199 lb (90 kg) at the equator.
Earth has an irregular gravitational field for other reasons other than the wider diameter at the equator. Clearly Earth is not a smooth sphere. Mountain ranges and deep valleys will also cause variations in the gravitational attraction. In addition, Earth is not composed of uniform material, or of materials spread uniformly throughout its volume. In other words, Earth has materials of different densities scattered throughout its volume. Earth, variations in ground water storage, ice and snow cover, oceanic mass distribution, and even the variations in the mass of the atmosphere play a part in deforming the gravitational field. Even the oceanic tides cause an observable fluctuation in Earth's gravity.
These variations are comparatively small but they will affect the orbits of satellites around Earth. Small undulations will occur as a result of this variable gravitational field. In fact, tracking the perturbations of orbiting satellites is one way of mapping Earth's gravitational field. Other methods include measurements taken at Earth's surface and measurements taken from orbiting satellites.
Developing a map of the total gravitational field of Earth has been a major scientific enterprise. The ability to observe changes in this field is important to those studying tidal changes, earthquakes, tectonic shifts, and other terrestrial phenomena.
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