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.
This entry comes from Encyclopædia Britannica Concise.
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