British astronomer Edmund Halley is best known for predicting the return of the bright comet that now bears his name. But in 1718, Halley made a key discovery that would help later astronomers unlock some of the secrets of the stars. He found that year after year, decade after decade, century after century, stars move a tiny bit across the sky.
This steady movement is known as "proper motion." It occurs because every star follows its own path.
The size of a star's proper motion depends on just two things. The first is how fast the star moves across our line of sight: Faster stars have greater proper motion. Most stars that are near the Sun speed across our line of sight at tens of thousands of miles per hour. The second is the star's distance: Closer stars have greater proper motion.
Even so, proper motion is hard to measure. It can take centuries for a star's changing position to become obvious to the unaided eye. So you need good telescopes and maps of the sky -- and a space-based telescope is even better.
From the northern hemisphere, the bright star with the greatest proper motion is yellow-orange Arcturus, which is visible in the west tonight. In fact, Arcturus is one of the stars that Edmund Halley used to make his discovery. He saw that the star had a slightly different position in his time from those recorded in ancient times -- a sign that the star was moving as it followed its own path through the galaxy.
Script by Ken Croswell, Copyright 2008
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