Definition:Ellipse/Major Axis/Semi-Major Axis

Definition

 * EllipseParts.png

Consider an ellipse $K$.

A semi-major axis of $K$ is either half of the major axis of $K$ from its midpoint to its endpoint.

In the above diagram, $a$ is a semi-major axis of $K$, the line segment whose endpoints are $O$ and $\left({a, 0}\right)$.

The other semi-major axis of $K$ is the line segment whose endpoints are $O$ and $\left({-a, 0}\right)$.

Also see

 * Definition:Semi-Minor Axis of Ellipse