Moon and Spica

StarDate: December 10, 2009

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A star that's creeping closer to Earth huddles close to the Moon at first light tomorrow. Spica is just a few degrees to the Moon's left.

Spica is the brightest star of Virgo. But it's not quite as bright as astronomers once thought. That's because its estimated distance keeps getting smaller. A popular reference from the 1970s put the distance at 275 light-years. Last year, the estimate was 260 light-years. Now, though, it's just 250 light-years.

The changes show how hard it is to measure a star's distance. The most accurate technique is called parallax. Basically, astronomers plot a star's position compared to other stars as Earth moves from one side of the Sun to the other. It's like looking at your finger with first one eye, then the other -- the finger appears to shift back and forth a bit compared to the background.

But even the closest stars are trillions of miles away. At that distance, the shift against the background of other stars is tiny. And the problem is compounded by Earth's atmosphere, which blurs the view.

A satellite called Hipparcos did a better job than ground-based telescopes. And recently, astronomers were able to compensate for some flaws in its observations. With the improved data, they came up with a distance to Spica of 250 light-years. It's the most accurate measurement to date. Even so, future observations may change it again, as we slowly close in on the exact distance to this bright star.

Script by Damond Benningfield, Copyright 2009

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