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.