proper motion

proper motion

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.

Apparent motion of a star across the celestial sphere at right angles to the observer's line of sight, generally measured in seconds of arc per year. Any radial motion (toward or away from the observer) is not included. Edmond Halley was the first to detect proper motions; the largest known is that of Barnard's star, about 10 seconds yearly.

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