Moon and Spica
Spica, the brightest star in the constellation Virgo, keeps company with the Moon this evening. They're in the southwest as darkness falls, with Spica not far to the right of the gibbous Moon.
Because it's so bright, Spica's been an important sky marker for many cultures. To the Luiseño tribes of California, for example, the star represented one of the "first chiefs," who ascended to the sky after his death.
Spica also played an important role in the development of the modern science of astronomy. More than 20 centuries ago, it helped the Greek astronomer Hipparchos discover an effect called precession.
Today, Earth's polar axis aims at the star Polaris, so it's called the Pole Star or North Star. But the gravity of the Sun and Moon tug at Earth, causing it to wobble like a spinning top. As it wobbles, its axis points at different stars. In a few thousand years, we'll have a different Pole Star. At the same time, the Sun's position against the constellations also changes. This effect is called precession.
Around 150 B.C., Hipparchos plotted the positions of Spica and Regulus, the brightest star in Leo. He then compared his measurements to some made a couple of hundred years earlier. He discovered that the positions of these stars relative to the Sun on specific dates had moved a bit - thanks to precession.
Look for Spica and the Moon in the southwest at nightfall, with the bright planet Jupiter off to their right.
Script by Damond Benningfield, Copyright 2002, 2005
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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