What is the radius of the earth? The answer of this question can vary from person to person. Mostly the answer you will get is 6371 Km or approximately 3959 miles. This is earth's average radius as the earth is not a perfect sphere. Due to the earth's revolution, at poles earth is flattened and at equator the earth is round that forms ellipsoid. Radius is the distance between point on the surface of sphere and the centre of sphere. In perfect sphere radius is same at every point on the surface of the sphere but this is not case for the earth. The distance of center of the earth from poles and equator is different. The center of earth lies 6356.8 Km away from poles and 6378.1 Km from equator.  

The difference between the radius of earth from poles and equator is 21.3 Km. Mean radius is calculated from the equatorial radius and polar radius. Only spherical solids can have radius but earth not being a sphere also uses radius for many applications. Earth is ellipsoid and fixed distance from specified center and fixed point is called radius of the earth at that particular point. Earth is treated as a perfect sphere because it is deviated only 3 percent from being a perfect sphere.  Earth has two fixed radii which remain constant and its magnitude does not depend on different points. These radii are equatorial and polar radius. The equatorial radius is used whenever earth is compared with other numerous planets.You can get every information realted to it. 

Radius of the earth also depends on different locations. The maximum radius of the earth is 6384.4 Km from the tip of Mr. Chimborazo and minimum radius is approximately 6352.8 Km from the Arctic Ocean deepest point. The radius of earth is used to calculate many other geophysical formulas for correct output. To calculate the volume, density, area of the earth is determined with the help of earth's radius. Earth is so vast and to study different aspects of earth, many experiments have been carried out to find the effective radius of earth that is used in all the calculating formulas. Thus, telling the radius of the earth is a not a straight forward answer for any person.