Equatorial bulge

An equatorial bulge is a difference between the equatorial and polar diameters of a planet, due to the centrifugal force exerted by the rotation about the body's axis. A rotating body tends to form an oblate spheroid rather than a sphere. An equatorial bulge is a difference between the equatorial and polar diameters of a planet, due to the centrifugal force exerted by the rotation about the body's axis. A rotating body tends to form an oblate spheroid rather than a sphere. The Earth has a rather slight equatorial bulge: it is about 43 km (27 mi) wider at the equator than pole-to-pole, a difference which is close to 1/300 of the diameter. If the Earth were scaled down to a globe with diameter of 1 meter at the equator, that difference would be only 3 millimeters. While too small to notice visually, that difference is still more than twice the largest deviations of the actual surface from the ellipsoid, including the tallest mountains and deepest oceanic trenches. The rotation of the earth also affects the sea level, the imaginary surface that is used to measure altitudes from. This surface coincides with the mean water surface level in oceans, and is extrapolated over land by taking into account the local gravitational potential and the centrifugal force. The difference of the radii is thus about 21 km. An observer standing at sea level on either pole, therefore, is 21 km closer to Earth's center than if standing at sea level on the Equator. As a result, the highest point on Earth, measured from the center and outwards, is the peak of Mount Chimborazo in Ecuador rather than Mount Everest. But since the ocean also bulges, like Earth and its atmosphere, Chimborazo is not as high above sea level as Everest is. More precisely, the surface of the Earth is usually approximated by an indeal oblate ellipsoid, for the purposes of defining precisely the latitude and longitude grid for cartography, as well as the 'center of the Earth'. In the WGS-84 standard Earth ellipsoid, widely used for map-making and the GPS system, the radius of the Earth is assumed to be 6,378.137 km (3,963.191 mi) at the equator and 6,356.7523142 km (3,949.9027642 mi) center-to-pole; meaning a difference of 21.3846858 km (13.2878277 mi) in the radii and 42.7693716 km (26.5756554 mi) in the diameters, and a relative flattening of 1/298.257223563. The sea level surface is much closer to this standard ellipsoid than the surface of the solid Earth is. Gravity tends to contract a celestial body into a sphere, the shape for which all the mass is as close to the center of gravity as possible. Rotation causes a distortion from this spherical shape; a common measure of the distortion is the flattening (sometimes called ellipticity or oblateness), which can depend on a variety of factors including the size, angular velocity, density, and elasticity. To get a feel for the type of equilibrium that is involved, imagine someone seated in a spinning swivel chair, with weights in their hands. If the person in the chair pulls the weights towards them, they are doing work and their rotational kinetic energy increases. The increase of rotation rate is so strong that at the faster rotation rate the required centripetal force is larger than with the starting rotation rate.