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.

proper motion (symbol μ) The progressive change in position of a star due to its motion relative to the Sun. Absolute proper motion is the change in position relative to an inertial reference frame such as that defined by extragalactic objects. Relative proper motion is obtained by measurements relative to selected reference stars at different epochs. The average absolute proper motion of these reference stars is known as the reduction from relative to absolute proper motion. Relative proper motions can be used to separate members of a star cluster from field stars in the same line of sight. Proper motions are usually listed in star catalogues as changes in right ascension and declination per year or century. The star with the largest known proper motion is Barnard's Star. See also common proper motion; reduced proper motion.