Moon and Spica

StarDate: February 13, 2009

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If you could come back to Earth 3,000 years from tonight, the constellations would look the same. Yet the overall look of the night sky would be quite different. Each star would rise about three hours later than it does tonight, and at a different point on the horizon -- as if someone had taken the entire starry dome of the night sky and shifted it around a bit.

That shift is caused by a "wobble" in our own planet -- an effect known as precession.

A star that played a role in the discovery of precession is in good view tonight. Spica, the brightest star of Virgo, is just a little to the left or upper left of the Moon as they rise in late evening.

The Greek astronomer Hipparchus is credited with discovering precession almost 2200 years ago. He did so by comparing Spica's position during his own time with its position a century and a half earlier. He found that Spica had moved a little with respect to the Sun.

Precession is caused by a "wobble" in Earth's rotation that's set up by the gravitational tug of the Sun and Moon. As Earth wobbles, the Sun shifts position against the background of stars. Because of that, the stars rise at different times, and at different points on the horizon.

So if you come back in 3,000 years, Spica will rise about three hours later than it does tonight, and a little farther south. But Spica and the stars around it will have maintained the same pattern, so Virgo will look just the same.

Script by Damond Benningfield, Copyright 2008

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