Flattening

${\displaystyle f=\frac{a-b}{a}.}$

The compression factor is ${\displaystyle b/a}$ in each case; for the ellipse, this is also its aspect ratio.

Definitions

There are three variants: the flattening ${\displaystyle f,}$ sometimes called the first flattening, as well as two other "flattenings" ${\displaystyle f^{\prime }}$ and ${\displaystyle n,}$ each sometimes called the second flattening, sometimes only given a symbol, or sometimes called the second flattening and third flattening, respectively.

In the following, ${\displaystyle a}$ is the larger dimension (e.g. semimajor axis), whereas ${\displaystyle b}$ is the smaller (semiminor axis). All flattenings are zero for a circle (a = b).