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→Calculations: More formulas
<math>a = (Q + q)/2</math>
<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.
For this reason the semi-major axis is often cited as a "mean orbital radius."
The semi-major axis, together with the [[orbital eccentricity]] (''e''), can predict the periapsis and apoapsis:
<math>Q = a(1+e)</math>
<math>q = a(1-e)</math>
See [[orbital eccentricity]] for a detailed derviation
[[Category:Astronomy]]
[[Category:Astronomy]]