Proper motion is 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. 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, in 1718, was the first to detect proper motions--those of Aldebaran, Arcturus, and Sirius.

Proper motion causes the shape of constellations to distort over time. Below is the changes in the Big Dipper after 10,000 years.

Excerpt from the Encyclopedia Britannica without permission.