The English used in this article or section may not be easy for everybody to understand. (May 2023)
Earth radius (represented as R🜨 or ) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid, the radius ranges from a maximum of nearly 6,378 km (3,963 mi) (equatorial radius, represented a) to a minimum of nearly 6,357 km (3,950 mi) (polar radius, represented b).
|Unit system||astronomy, geophysics|
|Symbol||R🜨 or ,|
|1 R🜨 in ...||... is equal to ...|
|SI base unit||6.3781×106 m|
|Metric system||6,357 to 6,378 km|
|English units||3,950 to 3,963 mi|
A globally-average value is usually believed to be 6,371 kilometres (3,959 mi) with a 0.3% variability (±10 km) for the following reasons. The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the mean radius (R1) of three radii measured at two equator points and a pole; the authalic radius, which is the radius of a sphere with the same surface area (R2); and the volumetric radius, which is the radius of a sphere having the same volume as the ellipsoid (R3). All three values are about 6,371 kilometres (3,959 mi).
Other ways to define and measure the Earth radius involve the radius of curvature. A few definitions yield values outside the range between polar radius and equatorial radius because they include local or geoidal topography or because they depend on abstract geometrical that are carefully thought about.