## Eccentricity and the Semi-Major/Semi-Minor Axes

The *major axis* is the long axis of the ellipse.
The *minor axis* is the short axis of the ellipse.

More often, though, we talk about the semi-major axis (designated a) and the semi-minor axis (designated b) which are just half the major and minor axes respectively. If the (semi-)major and (semi-)minor axes are the same, then we have a circle.

How squashed or “off-circle” the ellipse is is described by the
*eccentricity*
(abbreviated *e*).

The semi-major axis, semi-minor axis, and *eccentricity* are related
such that, if you know any two of the three, you can easily
calculate the third.