Hipparcos

StarDate: August 8, 2009

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Three of the brightest stars in all the night sky are in good view this evening. Vega stands high overhead -- about 25 light-years away. Yellow-orange Arcturus is over in the west, at a distance of 37 light-years. And even oranger Antares is low in the south -- shining on us from 600 light-years.

Astronomers determined the distances to these and other stars using a technique called parallax. And the grand champion for measuring parallax was the European satellite Hipparcos, which was launched 20 years ago today.

To see how parallax works, hold a finger in front of your face, then look at it with one eye at a time. As you blink from one eye to the other, the finger appears to move back and forth against the more distant background.

In astronomy, this is accomplished by looking at a star when Earth is on opposite sides of the Sun. The size of the star's back-and-forth shift is known as its parallax. Stars with a big parallax are closer than those with a small parallax.

Despite a booster failure that left it in the wrong orbit, Hipparcos was a spectacular success. It provided by far the best distances to date for more than a hundred thousand stars, and slightly rougher measures for a million more.

Recently, astronomers recalibrated the distances based on new information about how Hipparcos moved in orbit. The new measurements are even better, providing accurate distances to stars out to thousands of light-years.

Script by Damond Benningfield, Copyright 2009

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