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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 Third Law of Planetary Motion

"The squares of the planets' orbital periods are proportional to the cubes of the semi-major axes or their orbits." T = 2ða3/2/Sqrt(µ), where T is the period of the orbit, a is the length of the semi-axis major (half of the major axis) of the elliptical orbit, and µ is the Gravitational Parameter (a constant based on the product of the Gravitational Constant and sum of the masses of the two bodies). It applies to any two objects that orbit each other solely under the influence of mutual gravitational attraction. For instance, this law allows you to determine the amount of time a planet takes to orbit the Sun (sidereal period), which explains why Mercury's 'year' is short and Pluto's is long.

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