Proper motion is the name for the way thestars seem to slowly moverelative to each other when seen fromEarth.[1][2]
The movement is caused by the fact that all stars (including theSun) are moving throughspace at hundreds ofkilometres per second. However, because they are so far away, it takes a long time for us to see that they have moved, and even then it takes a powerfultelescope to see the difference. Because of this, for a long time, most people thought that the stars did not move at all. Even theancient Greeks, who knew a lot about the stars and discovered some of the other ways they move, such asstellar precession, did not discover proper motion (although they probably suspected it).
Proper motion was not proved to exist until 1718 whenEdmond Halley noticed the starsSirius,Arcturus andAldebaran had moved from the places they were in star charts thatHipparchus had drawn around130 BC. However, even after more than 1,800 years, they had moved less than half adegree.
Proper motion is useful toastronomers in figuring out how far away a star is from Earth because stars that are close usually have a larger proper motion (meaning they move faster across the sky) than stars that are farther away. For example,Barnard's star has the largest proper motion of any star, moving 10.3arcseconds per year. This is equal to one quarter of a degree, or half thediameter of theMoon in the sky, in only 87 years. It is the second-closest star to the Earth at 5.98light years away. Also, eight of the ten stars with the biggest proper motions are less than 15 light years away.
