Mean Spherical Radius of the Earth

The Earth's Mean Spherical Radius (Si) is 6371.009 km.

Derivation

The mean radius of an ellipse equation (`r_mu = (2 a +b)/3`) computes the mean radius of an ellipsoid             Ellipsoid with semi-axes of lengths  a and b.

Since the Earth is an oblate spheroid, closely approximated by an ellipsoid, the IUGG defines the Earth's mean radius using:

• a =  Equatorial radius (6,378.1370 km)
• b =  Polar radius (6,356.7523 km)

This computes and Earth mean radius of 6,371.009 kilometers (3,958.761 mi).