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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:
<math>a = (Q + q)/2</math>
The semi-major axis, together with the [[orbital eccentricity]], can predict the periapsis and apoapsis. See [[orbital eccentricity]] for details.
[[Category:Astronomy]]
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:
<math>a = (Q + q)/2</math>
The semi-major axis, together with the [[orbital eccentricity]], can predict the periapsis and apoapsis. See [[orbital eccentricity]] for details.
[[Category:Astronomy]]