Semi-major and semi-minor axes
term in geometry; longest and shortest semidiameters of an ellipse
(Redirected from Semi-major axis)
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.
![](http://up.wiki.x.io/wikipedia/commons/thumb/a/a1/Ellipse_semi-major_and_minor_axes.svg/260px-Ellipse_semi-major_and_minor_axes.svg.png)