Glossary term: Ellipse

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Description: An ellipse is a two-dimensional shape that resembles a squashed or elongated circle. The widest distance across an ellipse is called the major axis and the shortest distance is called the minor axis. An ellipse has two foci (focusses) which lie along the major axis with both having the same distance from the center. At any point on the ellipse, the sum of the distances to the two foci is constant. The eccentricity of an ellipse defines how squashed it is. A circle has zero eccentricity. An eccentricity of one or more indicates the shape is not an ellipse but an unclosed shape such as a parabola or hyperbola. Orbits follow ellipses. An orbit will be centered on one focus of the ellipse and is usually defined by the semimajor axis (half the size of the major axis) and eccentricity.

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The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".