The rotation of the Earth causes it to bulge slightly in the middle. Centrifugal force makes the Earth’s material move away from the centre — the faster the spin, the greater the force. As places at the Equator are moving faster than places at the poles, the centre of the Earth pushes out slightly more than the rest.
The simplest model for the shape of the entire Earth is a sphere. The Earth’s radius is the distance from Earth’s center to its surface, about 6,371 kilometers (3,959 mi). While “radius” normally is a characteristic of perfect spheres, the Earth deviates from spherical by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term “the radius of the Earth”.
The concept of a spherical Earth dates back to around the 6th century BC, but remained a matter of philosophical speculation until the 3rd century BC. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes’s measurement ranging from 2% to 15%.
The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947 – 3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 kilometers (3,959 mi). Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 – 3,963 mi). The difference 21 kilometers (13 mi) correspond to the polar radius being approximately 0.3% shorter than the equator radius.
Picture Credit : Google