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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.


Sidereal Period

The amount of time expressed in Earth years that it takes a celestial body to make one complete orbit around the Sun. Kepler's Third Law states, "The square of a planet's sidereal period is proportional to the cube of the length of its semimajor axis."

In simplistic terms; the closer a planet is to the Sun {axis (distance) cubed}, the more rapidly it orbits {period (yr) squared}. This law is why Mercury's 'year' is short and Pluto's is long. This applies to any two objects that orbit each other solely under the influence of mutual gravitational attraction as proved by Newton.

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