proper motion

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

[1595-1605]

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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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▪ astronomy

      in astronomy, the apparent motion of a star across the celestial sphere at right angles to the observer's line of sight; any radial motion (toward or away from the Sun) is not included. It is observed with respect to a framework of very distant background stars or galaxies. Proper motion is generally measured in seconds of arc per year; the largest known is that of Barnard's star in the constellation Ophiuchus, about 10″ yearly. The English astronomer Edmond Halley (Halley, Edmond), in 1718, was the first to detect proper motions—those of Aldebaran, Arcturus, and Sirius. The symbol for proper motion is the Greek letter μ (mu).

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