More Moon and Company

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• 4 of 1996
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The crescent Moon will just slip past the star Antares around dawn tomorrow. They'll be in the southeast beginning a couple of hours before sunrise. Orange Antares is just a little above the Moon, with the brilliant planet Venus to their upper left.

Antares is the "heart" of Scorpius, the scorpion. Astronomers classify the star as a red supergiant, which means it's much larger and more massive than our own star, the Sun. If Antares took the Sun's place, its surface would extend beyond the orbit of Mars, the next planet outward from Earth. Yet Antares is so puffy that its outer layers aren't much denser than a vacuum.

Over the centuries, Antares has played an important role in the mythology of many cultures. That's both because it's bright, and because it lies along the Sun's path across the sky, known as the ecliptic.

The Sun passes the star once a year, with the Moon passing by once a month. When the geometry is just right, the Moon can actually cover the star for a few minutes, blocking it from view. Astronomers have used these events, called occultations, to estimate the size of the star. Since they have a good idea of its distance -- about 600 light-years -- measuring how long it takes Antares to disappear behind the Moon reveals its diameter.

Antares won't disappear behind the Moon this time, but it will come awfully close. Look for it huddling just above the Moon as the color of twilight begins to paint the dawn sky.

Script by Damond Benningfield, Copyright 2007

For more skywatching tips, astronomy news, and much more, read StarDate magazine.