Earth radius
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.
The Earth's polar radius is the distance from its center to the North or South Pole, and is approximately 3950 miles (6356.9 kilometres).
The Earth's equatorial radius is the distance from its centre to the equator, and is approximately 3963 miles (6378.5 kilometres).
The Earth's mean radius is approximately 3959 miles (6371.3 kilometres). This number is derived by averaging the centre-to-surface distances on all points on the globe. Equivalently, the mean radius is
- <math>r = \sqrt{\frac{A}{4\pi}}<math>
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.
Earth radius is sometimes used as a unit of distance, especially in astronomy and geology. It is usually denoted by RE.
See also: Effective Earth radius
