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Moon and Spica

July 8, 2011

Spica -- a star whose light has been traveling for two and a half centuries by the time it reaches your eye -- stands just to the upper right of the Moon tonight.

Spica is a pair of stars locked in orbit around each other. They're about 250 light-years away. A light-year is the distance that light travels in one year. If an object is 250 light-years away, that means its light has taken 250 years to reach Earth.

Professional astronomers seldom use light-years, though. Instead, they prefer a measurement known as a parsec -- a word that means "parallax-second."

Parallax is a technique that's used to measure the distances to nearby stars. As Earth moves in its orbit around the Sun, the direction to those stars shifts, so the positions of the stars appear to jump a tiny bit when compared to the background of more-distant stars. It's like holding your finger in front of you and looking at it with just one eye at a time; the finger appears to jump back and forth against the background of more-distant objects.

Parallax is measured in seconds of arc. A full circle around the sky consists of one million, 296 thousand arc-seconds. The distance from Earth at which an object would show a parallax of one second is a parallax-second -- a parsec -- about three and a quarter light-years.

Spica is so distant that its parallax is only a tiny fraction of an arc-second -- an angle that tells us that it's about 77 parsecs away.


Script by Damond Benningfield, Copyright 2011


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