(Redirected from

Earth Radii)

*Note: ***Earth radius** is sometimes used as a unit of distance, especially in astronomy and geology. It is usually denoted by *R*_{E}.

The radius of the Earth is the distance from the Earth's centre to its surface at mean sea level. The Earth is not a perfect sphere, but instead is somewhat flattened at the North and South Poles, and it bulges at the equator. This shape is known as an oblate spheroid. The Earth's non-spherical shape means that its radius differs depending on where you measure it.

## Polar radius

The Earth's polar radius is the distance from its center to the North or South Pole, and is approximately 6,356.9 kilometres (3,950 statute miles) .

## Equatorial radius

The Earth's equatorial radius is the distance from its centre to the equator, and is approximately 6,378.5 kilometres (3,963 miles).

## Mean radius

The Earth's mean radius is approximately 6,371.3 kilometres (3,959 miles). This number is derived by averaging the centre-to-surface distances on all points on the globe. Equivalently, the mean radius is

where *A* is the surface area of Earth. This would be the radius of a hypothetical perfect sphere which had the same surface area as the Earth.

*See also: Effective Earth radius*

## Quadratic mean radius

The *quadratic mean radius* (*Q*_{r}) of an ellipsoid is a more accurate method of expressing the Earth's radius.

where *a* is the equatorial radius and *b* the polar radius.

For the Earth, *a* = 6,378.135 km, *b* = 6,356.75 km, and *Q*_{r} = 6,372.795477598 km.

Last updated: 05-14-2005 21:33:45