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Semi-Major Axis

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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