Eccentricity

Eccentricity: how much a conic section (a circle, ellipse, parabola or hyperbola)
varies from being circular.

A circle has an eccentricity of zero, so the eccentricity shows you how "un-circular" the curve is. Bigger eccentricities are less curved.

Eccentricity

Different values of eccentricity make different curves:

Eccentricity is often shown as the letter e (don't confuse this with Euler's number "e", they are totally different)

Animation

Try the slider to see what happens:

Calculating The Value

circle For a circle, eccentricity is 0
ellipse on xy graph

For an ellipse, eccentricity is:

ellipse eccentricity

parabola For a parabola, eccentricity is 1
hyperbola on xy graph

For a hyperbola, eccentricity is:

hyperbola eccentricity