Semimajor axis

Average orbital distance from the primary. The semimajor axis of an ellipse (e.g. a planetary orbit) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by

r_p = a(1-e) and r_a = a(1+e).