ellipse: the sum of the distances from any point on an ellipse to each focus is constant and equal to the major axis $2a$.

$c= \sqrt{a^2 - b^2}$ $a$: distance from the center to a vertex

$b$: distance from the center to a co-vertex

$c$: distance from the center to a focus

$e= \frac{c}{a}$ $e$: eccentricity; the larger the value of $e$, the more "squished" the ellipse

$c= \sqrt{a^2 - b^2}$ $a$: distance from the center to a vertex

$b$: distance from the center to a co-vertex

$c$: distance from the center to a focus

$e= \frac{c}{a}$ $e$: eccentricity; the larger the value of $e$, the more "squished" the ellipse