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`mu"Radius"_"Earth" = 6371.009 " km"`

6371.009

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

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).