The eccentricity of an ellipse is the ratio of the distance from the center of the ellipse to one of its foci and the length of the half of its major axis. It is usually represented algebraically by $e.$

The eccentricity of an ellipse is a measure of how much the ellipse deviates from being a perfect circle, lying between $0$ and $1.$ For an ellipse that is nearly circular, the foci are close to the center, resulting in a $\frac{c}{a}$ ratio close to $0.$ For an elongated ellipse, the foci are near the vertices, resulting in a $\frac{c}{a}$ ratio close to $1.$

$e=0$	The ellipse is a circle.
$e\text{ close to }0$	The ellipse is almost circular.
$e\text{ close to }1$	The ellipse is stretched out and elongated.
$e=1$	The ellipse becomes a line segment.

This concept is fundamental in astronomy, where it describes the orbits of planets and comets around the sun. For example, it was found that the orbit of the Moon has an eccentricity of $0.0549,$ which makes it almost circular.