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[[Image:658px-Elipse svg.png|right|thumb|300px|In the figure, ''a'' is the semi-major axis, ''b'' is the semi-minor axis, F1 and F2 are the two focal points. The distance F1-X-F2 is constant.]]

[[Image:658px-Elipse svg.png|right|thumb|300px|In the figure, ''a'' is the semi-major axis, ''b'' is the semi-minor axis, F1 and F2 are the two focal points. The distance F1-X-F2 is constant.]]

An '''ellipse''' is a geometric figure that looks like a squashed [[circle]]. The more squashed it is, the greater its eccentricity. It is defined as the set of all points in a plane where the sum of the distance from two points, the ''foci'', is the same. A special case of the ellipse is the circle, where the two points are coincident. A circle has an eccentricity of 0. The terms ''eccentric'' and ''circular'' are antonyms.

An '''ellipse''' is a geometric figure that looks like a squashed [[circle]]. The more squashed it is, the greater its eccentricity. It is defined as the set of all points in a plane where the sum of the distance from two points, the ''foci'', is the same. A special case of the ellipse is the circle, where the two points are coincident (in the same place). A circle has an eccentricity of 0. The terms ''eccentric'' and ''circular'' are antonyms.

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]].  

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]].