Earth radius | |
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![]() Equatorial (a), polar (b) and arithmetic mean Earth radii as defined in the 1984 World Geodetic System revision (not to scale) | |
Other names | terrestrial radius |
Common symbols | R🜨, RE, a, b, aE, bE, ReE, RpE |
SI unit | meters |
In SI base units | m |
Behaviour under coord transformation | scalar |
Dimension | |
Value | Equatorial radius: a = (6378.1370 km) Polar radius: b = (6356.7523 km) |
Nominal Earth radius | |
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![]() Cross section of Earth's Interior | |
General information | |
Unit system | astronomy, geophysics |
Unit of | distance |
Symbol | , , |
Conversions | |
1 in ... | ... is equal to ... |
SI base unit | 6.3781×106 m[1] |
Metric system | 6,357 to 6,378 km |
English units | 3,950 to 3,963 mi |
Geodesy |
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Earth radius (denoted as R🜨 or RE) 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 (an oblate ellipsoid), the radius ranges from a maximum (equatorial radius, denoted a) of nearly 6,378 km (3,963 mi) to a minimum (polar radius, denoted b) of nearly 6,357 km (3,950 mi).
A globally-average value is usually considered 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).[2] All three values are about 6,371 kilometres (3,959 mi).
Other ways to define and measure the Earth's radius involve either the spheroid's radius of curvature or the actual topography. A few definitions yield values outside the range between the polar radius and equatorial radius because they account for localized effects.
A nominal Earth radius (denoted ) is sometimes used as a unit of measurement in astronomy and geophysics, a conversion factor used when expressing planetary properties as multiples or fractions of a constant terrestrial radius; if the choice between equatorial or polar radii is not explicit, the equatorial radius is to be assumed, as recommended by the International Astronomical Union (IAU).[1]
Moritz
was invoked but never defined (see the help page).