File:Forces oblate spheroid.gif

Description:
This sequence of three images shows the forces that are at play on a rotating planet such as Earth. Because of its rotation, the Earth is not spherical in shape, it is an oblate spheriod. The force of gravity is directed towards the center of the Earth. The normal force is perpendicular to the local surface. Except on the poles and on the Equator the two forces are not exactly in opposite direction, so there is a resultant force, that acts in poleward direction. The laws of motion imply that this poleward force is at every latitude precisely the amount of centripetal force that is necessary to maintain an even thickness of the atmospheric layer. The solid Earth is ductile. If the shape of the solid Earth would not match its rotation rate, then shear stress would deform the solid Earth over a period of millions of years until the shear stresses are resolved. (In the case of an oblate spheroid, the center of gravitational attraction and the geometric center are very close to each other, but they do not quite coincide. If all of the earth's mass would be concentrated in a single point, where would that point have to be in order to exert the same gravitational pull as the earth does in its current form? For an object located on the equator, the corresponding center of gravitational attraction is located at a distance of about 10 kilometers away from the geometric center of the earth. (Nonetheless, the equator is at a higher gravitational potential than the poles, for the equator is about 21 kilometers further away from the geometric center than the poles.)) Created: 12 January 2006