proper motion

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proper motion, in astronomy, apparent movement of a star on the celestial sphere, usually measured as seconds of arc per year; it is due both to the actual relative motions of the sun and the star through space. Proper motion reflects only transverse motion, i.e., the component of motion across the line of sight to the star; it does not include the component of motion toward or away from the sun. The most distant stars show the least proper motion. Barnard's Star, one of the closest stars, has the largest measured proper motion, 10.27 sec of arc per year. The average proper motion of the stars that can be seen with the naked eye is 0.1″ per year.

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proper motion The apparent movement of a star relative to the other ‘fixed’ stars, at right angles to the observer's line of sight. For example, Barnard's star moves 10 seconds of arc per year; this is the largest proper motion recorded. There is no absolute frame of reference, and the positions of stars in the familiar constellations change significantly within a few thousand years.