In the area of astronomy, proper motion is the term used to describe a star's apparent movement across the celestial sphere, usually measured in arc seconds per year. The symbol for proper motion is the Greek letter μ - mu.

The English astronomer Edmond Halley was the first person to detect proper motions. In 1710 he discovered discrepancies with his own observations and Ptolemy's catalog, and deducted that the stars might move around on their own. Later he managed to measure the movement, the proper motion, of Aldebaran, Arcturus and Sirius.

It's important to note that proper motion alone does not fully describe a star's motion in space, as it does not take into account the star's distance, nor the movement along the line of sight from the observer; the radial velocity.

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