Changes

A whisper at one focal point is easily heard at the other focal point, because the sound waves bounce off the walls and combine again at the other focal point.

A whisper at one focal point is easily heard at 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 revolving the sun always follow elliptical [[orbit]]s, with the sun at one of the foci. This helped overturn the [[Ptolemaic theory]], and led to [[Newton|Issac Newton]]'s [[Gravitation|law of gravitation]].  

Ellipses became important in [[astronomy]] in the early 1600s, when [[Kepler]] proved that [[planet]]s revolving the sun always follow elliptical [[orbit]]s, with the sun at one of the foci. This helped overturn the [[Ptolemaic theory]], and led to [[Newton|Issac Newton]]'s [[Gravitation|law of gravitation]].  

==Area and Circumference==

==Area and Circumference==

Unlike a [[circle]], there is no closed form for the [[circumference]] of an ellipse. The [[area]] of an ellipse is given by:

Unlike a [[circle]], there is no closed form (simple equation) for the [[circumference]] of an ellipse. The [[area]] of an ellipse is given by:

<math>A = \pi ab</math>

<math>A = \pi ab</math>

[[Category:Plane Geometry]]

[[Category:Plane Geometry]]