# Semi-Major Axis - Ellipse

## Description

In geometry, the major axis of an ellipse is the longest diameter: a line (line segment) that runs through the center and both foci, with ends 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 radius of an orbit at the orbit’s two most distant points. For the special case of a circle, the semi-major axis is the radius. One can think of the semi-major axis as an ellipse’s long radius.

Related formulas## Variables

a | semi-major axis (m) |

l | semi-latus rectum (m) |

ϵ | eccentricity (dimensionless) |