Proper Motion
stars seem to be in fixed positions with respect to each other, meaning they always form the same figures, and (for example) Ursa Major looks the same now as forty years ago. More careful observation shows that the constellations change shape very slowly, and that each star has an independent motion.
This motion, caused by the true movement of the stars and the Sun and solar system through space, is called proper motion. It is distinct from the improper motions of the stars, which affect their measured coordinates but are not real motions of the stars themselves. Improper motions are only the apparent motions caused by the motions of Earth (such as precession of the equinoxes and nutation), and by aberration of light.
It can also be described as the apparent change of the star on the celestial sphere, and is measured in seconds of arc per year. Barnard's star has the largest proper motion of all stars, moving at 10.3 seconds of arc per year. Large proper motion is usually a strong indication that a star is relatively close to the Sun.
Proper motion was discovered in 1718 by Edmund Halley, who noticed that Sirius, Arcturus and Aldebaran were over half a degree away from the positions charted by the ancient Greek astronomer Hipparchus roughly 1850 years earlier.
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