What is Proper motion?
[[File:Barnard2005.gif|thumb|right|220px|Barnard's star has visibly moved from 1985 to 2005 because of proper motion]] '''Proper motion''' is the name for the way the [[star]]s seem to slowly move ''relative to each other'' when seen from [[Earth]].<ref>The '''proper motion''' of a [[star]] is its angular change in position over time as seen from the [[centre of mass]] of the Solar System.</ref><ref name=Koupelis>{{cite book |title=In Quest of the Universe |author=Theo Koupelis, Karl F. Kuhn |page= 369 |url=https://books.google.com/books?id=6rTttN4ZdyoC&pg=PA369 |isbn=978-0763743871 |publisher=Jones & Bartlett Publishers |year=2007}}</ref> The movement is caused by the fact that all stars (including the [[Sun]]) are moving through [[outer space|space]] at hundreds of [[kilometre]]s 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 powerful telescope to see the difference. Because of this, for a long time, most people thought that the stars did not move at all. Even the [[ancient Greece|ancient Greeks]], who knew a lot about the stars and discovered some of the other ways they move, such as [[stellar precession]], did not discover proper motion (although they probably suspected it). Proper motion was not proved to exist until 1718 when [[Edmond Halley]] noticed the stars [[Sirius]], [[Arcturus]] and [[Aldebaran]] had moved from the places they were in star charts that [[Hipparchus]] had drawn around [[130 BC]]. However, even after more than 1,800 years, they had moved less than half a [[Degree (angle)|degree]]. Proper motion is useful to [[astronomer]]s 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 10.3 [[arcseconds]] per year. This is equal to one quarter of a degree, or half the diameter of the [[Moon]]