The stars are not fixed in the sky but are instead moving all the time. The movement is too small to be noticed by eye and as such the constellations have retained their shape since human history began. The angular velocity of a star (with respect to the Sun) is termed the star's proper motion. Only the nearest stars show any discernable proper motion over a human lifetime - Barnard's Star (the second closest star to the Sun) shows a proper motion of only 10.25 arcseconds per year.

The image to the right shows the motion of the stars that make up the familiar constellation of Ursa Major. Moving the mouse pointer over the image will show how the stars looked 50,000 years ago and how they will appear 50,000 years into the future. As can be seen, the common patterns that make up the constellations are transient when viewed on a large time scale.