semimajor axis
The semimajor axis is half the longest distance (major axis) across an ellipse. The semimajor axis is one of the orbital elements – a standard parameter used to describe an elliptical orbit. The semimajor axis is also the average distance of an orbiting object from its primary. The periapsis and apoapsis distances, r_{p} and r_{a}, can be calculated from the semimajor axis, a, and the eccentricity, e, by the formula
r_{p} = a(1  e) and r_{a} = a(1 + e)
semimajor axis
