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inferior planet

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noun Astronomy.
  1. either of the two planets whose orbits are inside the orbit of the earth: Venus and Mercury.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for inferior planet

inferior planet

noun
  1. either of the planets Mercury and Venus, whose orbits lie inside that of the earth
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

inferior planet in Science

inferior planet

  1. Either of the planets Mercury or Venus, whose orbits lie between Earth and the Sun. Because these planets lie in the general direction of the Sun, they can only be seen a few hours before sunrise or after sunset and are always positioned relatively near the horizon, never overhead. Inferior planets go through a complete cycle of phases as viewed from Earth, although their full phase, which occurs on the far side of the Sun, is lost in its glare. Compare superior planet. See also inner planet.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.