In
geometry, the
major axis of an
ellipse is its longest diameter: a
line segment that runs through the center and both
foci, with ends at the widest points of the
perimeter. The
semi-major axis is one half of the major axis, and thus runs from the centre, through a
focus, and to the perimeter. Essentially, it is the radius of an orbit at the orbit's two most distant points. For the special case of a circle, the semi-major axis is the radius. One can think of the semi-major axis as an ellipse's
long radius.