Semi-major axis

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The Semimajor axis of an orbit is defined as one-half of the long axis of an ellipse or an elliptical orbit, a measure of the size of the orbit.

In an elliptical orbit, the major axis passes through both foci, the central body being at or near one foci. There is nothing at the other foci. The distance between the highest point along the orbit to the barycenter is the apoapsis, the low point is the periapsis. The semimajor axis is one half the sum of the apoapsis and the periapsis.

See also

Semimajor and semiminor axes at Wikipedia