This is an alternate expression of the size of the orbit, or altitude of
the satellite, and can be derived from the Mean Motion and the Eccentricity.
If the orbit is a circle, the semi-major axis is the radius of the circle. If
it is an ellipse, the major axis is the longest diameter, while the
minor axis is the shortest. The semi-major axis is half the major axis,
in km or mi.
Note that for elliptical orbits, the earth is at one focus. These
points (the foci) are not centered in the ellipse, so the semi-major
axis does not directly describe the altitude at perigee.
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