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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 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.

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 [[Issac Newton]]'s [[law of gravitation]].  

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 [[Issac Newton]]'s [[law of gravitation]].  

[[Category:Plane Geometry]]

[[Category:Plane Geometry]]