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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.


Kepler's Second Law of Planetary Motion

Also known as the Law of Equal Areas. "A straight line joining a planet and the Sun sweeps out equal areas in equal amounts of time." This law accounts for the change in orbital velocity as a planet moves through one orbit. As the planet moves from aphelion to perihelion (or from apogee to perigee in the Moon's case), the planet's velocity increases. The opposite is true when the planet moves from perihelion to aphelion. Basically, the closer the planet is to the source of gravity, the faster it must travel to stay in orbit.

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