# Relation between semi-major axis and radius of an orbit?

From what I understand, the semi-major axis of a circular orbit should be equal to its radius.

However, checking Wikipedia's info on Hubble, which is in a nearly circular orbit, I find :

sMA $= 6,919$ km

apogee $\approx$ perigee $\approx 540$ km

I'm having trouble understanding that difference.

Assuming I approximate an orbit as being circular, how can I get its radius from the semi-major axis?

Screenshot from https://en.wikipedia.org/wiki/Hubble_Space_Telescope

• You are right, there is a problem with the table. – uhoh May 3 '17 at 10:11

• For these types of calculations, you use (by convention -- don't ask me why) earth's equatorial radius of 6,378 km. You'll then see that the semimajor axis is just equal to this number added to the arithmetic mean of the apogee and perigee altitudes, i.e., $6378 + \frac{1}{2}(539 + 543) = 6919$ – Tristan May 3 '17 at 14:45