Moon and Regulus 
The Moon lines up near a bright star late tonight: Regulus, the "heart" of Leo, the lion. It's just 85 light-years away. Or 77. Or how about 79?
Astronomers measure the distances to nearby stars like Regulus by measuring their parallax -- how far they appear to shift back and forth against the background of more-distant stars as Earth moves from one side of the Sun to the other.
The technique is like holding your finger in front of your face and looking at it through just one eye at a time. As you blink from eye to eye, your finger appears to shift position. Measuring the angle of that shift tells you how far away your finger is.
But even the closest stars are so far away that the back-and-forth shift is tiny. If you draw a circle around the sky, and divide it into about 30 million segments, Regulus shifts position by the width of just one segment.
And to make matters worse, Earth's atmosphere blurs the view, so a star looks like a fuzzy blob. A couple of decades back, the best observations from the ground gave a distance to Regulus of about 85 light-years.
But in the 1990s, a European satellite measured the positions of stars from above the atmosphere. Its observations put the distance to Regulus at just 77 light-years. Recently, though, astronomers recalibrated the satellite's observations. That yielded a distance of 79 light-years -- the most accurate measurement of the distance to Regulus -- so far.
Script by Damond Benningfield, Copyright 2010