proper motion

American  

noun

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

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proper motion British  

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

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proper motion Scientific  

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.

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Etymology

Origin of proper motion

First recorded in 1595–1605

Example Sentences

"The Court specifically reserves the right to retroactively amend or modify child support without the need for the filing of a proper motion by either party to modify support," the judge wrote.

From Fox News • Feb. 19, 2020

The Orange Whip Wedge uses a super-flexible shaft and heavy head to ingrain the proper motion, and it even lets you hit balls as you train.

From Golf Digest • Apr. 30, 2018

He used the motion of the star to retrace the star's path — the first time, to his knowledge, anyone has used the proper motion of a star this way.

From Washington Post • Aug. 30, 2017

He practiced walking properly, landing right and squatting with proper motion.

From Washington Times • Nov. 22, 2016

The proper motion of a star near to us will ordinarily be large, and, in the case of solar stars, the larger their proper motion the greater their number.

From Astronomy: The Science of the Heavenly Bodies by Todd, David Peck