In geometry, the semi-major axis is the distance from the center of an ellipse to the farthest point on the perimeter of the ellipse. The semi-major axis is half of the major axis, which goes all the way across the ellipse at the widest part. The semi-minor axis is a line segment that is at 90 degrees with the semi-major axis. For the circle, the lengths of the semi-axes are both equal to the radius.