Ellipse

An ellipse is a figure that looks like a squashed circle. The more squashed it is, the greater its eccentricity.

To draw an ellipse, place tacks at two points on a piece of paper with a string between them. Trace out the curve of the ellipse with a pencil which is always pushing against the string. The longer the string, the less eccentric the ellipse will be (and more circular).

The terms eccentric and circular are antonyms.

A pair of line segments drawn from one focal point of an ellipse to the curve and from there to its other focal point form an angle whose bifurcating line is always perpendicular to the tangent of the curve. This feature of an ellipse has been exploited in rooms having elliptical walls or (more dramatically) elliptical ceilings. A whisper at one focal point is easily heard at the the other focal point, because the sound waves bounce off the walls and combine again at the other focal point.

Ellipses became important in astronomy in the early 1600s, when Kepler proved that planets orbiting the sun always follow elliptical paths. This helped overturn the Ptolemaic theory, and led to Issac Newton's law of gravitation.

The orbits of planets observable in Kepler's times were very circular. Comets, on the other hand, have highly eccentric orbits with only one focal point inside the sun.