Distance to Sirius

StarDate: January 8, 2009

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The brightest star of the night sky shines in the southeast on January evenings. The star is Sirius, and it's one of our closest stellar neighbors. Even so, astronomers took a long time to measure its distance. And in fact, for more than a generation, they got the distance wrong.

Astronomers measure a nearby star's distance by measuring the size of its parallax -- a tiny shift in its apparent position in the sky that's caused by Earth's motion around the Sun. It's like holding your finger in front of you and looking at it with one eye, then the other. As you blink back and forth, the finger appears to shift against the background of more-distant objects. The size of the shift tells you how far away the finger is, and the same thing applies to stars -- closer stars have a larger parallax.

But even the nearest stars are so far that their parallaxes are tiny and hard to measure. The first measurements of the parallax of Sirius, in the mid 1800s, placed the star almost 14 light-years away.

A couple of decades later, though, two astronomers in South Africa -- David Gill and William Elkin -- remeasured the parallax. Their work indicated that the star was just 8.6 light-years away -- the same distance that modern astronomers find for the star.

This distance means that Sirius is the fifth-closest star system to the Sun. The light we see from Sirius tonight left the star 8.6 years ago -- in the spring of the year 2000.

Script by Ken Croswell, Copyright 2008

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