Changes

[[File:Barnard2005.gif|thumb|right|220px|Barnard's star has visibly moved from 1985 to 2005 because of 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=http://books.google.com/?id=6rTttN4ZdyoC&pg=PA369 |isbn=0763743879 |publisher=Jones & Bartlett Publishers |year=2007}}</ref>  

'''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/?id=6rTttN4ZdyoC&pg=PA369 |isbn=0763743879 |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).  

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).