A scale measured from the ellipse’s center
The major axis passes through both foci and connects the ellipse’s most widely separated endpoints. Its half-length runs from the center to one end.
In an ideal heliocentric Kepler ellipse, the Sun occupies a focus rather than the center. A planet’s solar distance therefore generally differs from a; in the circular limit, the focus and center coincide and a is also the radius.
Relating the two apsidal distances
In one ellipse, focal distances at periapsis and apoapsis add to 2a, giving a=(rₚ+rₐ)/2. The apsidal distances thereby connect directly with orbit size.
For endpoint distances of one and three length units, a is two. This endpoint average should not be confused with a time-weighted orbital average, since motion is slower near the farther end.
Period and the remaining elements
For ideal two-body ellipses under the same central gravitational conditions, Kepler’s third law relates period squared to a cubed. Comparing periods must still account for differing central masses.
Semi-major axis and eccentricity specify size and shape; inclination, other orientation angles and orbital phase supply additional information. One value of a cannot locate a spacecraft or establish an intersection.