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proper motion

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noun
  1. Astronomy. the angular motion of a star relative to a suitably defined frame of reference, expressed in seconds of arc per year.

Origin of proper motion

First recorded in 1595–1605
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for proper motion

proper motion

noun
  1. the very small continuous change in the direction of motion of a star relative to the sun. It is determined from its radial and tangential motion
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

proper motion in Science

proper motion

  1. Movement of a celestial object in the sky that is the result of the object's own motion in space rather than of how it is observed from Earth. All celestial objects are in motion with regard to each other, but because objects outside the solar system are so distant from Earth most of them seem fixed in the sky. Over long periods of time, however, their proper motions result in gradual changes in their relative positions as viewed from Earth. Measurements of these motions by modern instruments can be extrapolated forward or backward in time to produce a celestial sphere on which the stars have somewhat different positions than they have today. In general, objects nearest the Earth have the greatest proper motions and will move the farthest on the celestial sphere in such extrapolations. Extremely distant objects, although they may be moving through space at equal or higher speeds than nearby objects, will appear to move little in the sky even over thousands of years.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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