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Dictionary of Astrological and Astronomical Terms

Compiled by Julene Packer-Louis, Evan Bortnick, and Jackie Goldstein

Copyright 2000-2020 OCA, Inc. All rights reserved. No portion of the dictionary may be reproduced in any form without written permission.



One of five conical sections (circle, line, parabola, and hyperbola being the others). Kepler's First Law dictates that planets move in elliptical orbits. An ellipse is the focus of all points in a plane whose sum of the distances from two fixed points (called foci) is a constant.

Basically, an ellipse is an elongated, or squished, circle with two centers, equidistant from each other, along the major axis (longer axis). An ellipse has an eccentricity between 0 and 1. A perfect circle has an eccentricity of 0; a parabola has an eccentricity of 1. For example, Earth's orbit has an eccentricity of 0.0167. Neptune's eccentricity is 0.0050 (the closest of the planets to a perfect circle); while, Pluto's eccentricity is 0.2583 (by far, the most elongated of all of the planetary orbits in the Solar System).

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