# Definition:Ellipse/Minor Axis

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## Definition

Consider an ellipse $K$ whose foci are $F_1$ and $F_2$.

The **minor axis** of $K$ is the line segment through the center of $K$ perpendicular to the major axis of $K$ such that its endpoints are the points of intersection with $K$.

In the above diagram, $C_1 C_2$ is the **minor axis** of $K$.

### Semi-Minor Axis

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

## Also see

## Linguistic Note

The plural of **axis** is **axes**, which is pronounced **ax-eez** not **ax-iz**.

Compare basis.

## Sources

- 1933: D.M.Y. Sommerville:
*Analytical Conics*(3rd ed.) ... (previous) ... (next): Chapter $\text {IV}$. The Ellipse: $2$. To find the equation of the ellipse in its simplest form