Semi-major axis
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The semi-major axis of any ellipse is the axis of symmetry that passes through the foci of that ellipse.
In astrophysics, the semi-major axis of an orbit is the distance between the geometric center of the orbit and either apsis. It is a commonly reported orbital element that can determine the size and shape of an orbit.
Calculations
The semi-major axis, usually labeled a, is the arithmetic mean of the two apsides:
The semi-major axis, together with the orbital eccentricity, can predict the periapsis and apoapsis. See orbital eccentricity for details.