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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.


Semimajor Axis

The semimajor axis is a segment of the major axis of an ellipse that has one endpoint on the center and the other on the ellipse. The major axis of an ellipse is a line segment containing the foci of the ellipse whose endpoints are on the ellipse. The average distance a planet is from the Sun equals the semimajor axis of the planet's elliptical orbit. 1 Astronomical Unit (AU) is the semimajor axis of Earth's orbit.

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