# Earth radius

**Earth radius** (denoted as ** R_{๐จ}** 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**, denoted

*a*) to a minimum of nearly 6,357 km (3,950 mi) (

**polar radius**, denoted

*b*).

Earth radius | |
---|---|

General information | |

Unit system | astronomy, geophysics |

Unit of | distance |

Symbol | โorโ, R_{๐จ} |

Conversions | |

1 R_{๐จ}in ... | ... is equal to ... |

SI base unit | 6.3781ร10^{6} m[1] |

Metric system | 6,357 to 6,378 km |

English units | 3,950 to 3,963 mi |

Geodesy |
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A *nominal Earth radius* is sometimes used as a unit of measurement in astronomy and geophysics, which is recommended by the International Astronomical Union to be the equatorial value.[1]

A globally-average value is usually considered to be 6,371 kilometres (3,959 mi) with a 0.3% variability (ยฑ10 km) for the following reasons.
The International Union of Geodesy and Geophysics (IUGG) provides three reference values: the *mean radius* (R_{1}) 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 (R_{2}); and the *volumetric radius*, which is the radius of a sphere having the same volume as the ellipsoid (R_{3}).[2] All three values are about 6,371 kilometres (3,959 mi).

Other ways to define and measure the Earth's 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 considerations.