High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape. The semi-major axis is one half of the major axis, and thus runs from the centre, through a focus, and to the edge of the ellipse; Essentially it is the measure of the radius of an orbit taken from the points of that same orbit's two most distant points. For the special case of a circle, the semi-major axis is the radius.