Moon and Antares

StarDate: June 6, 2009

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As seen from much of the United States, the celestial scorpion will be missing his bright orange heart early this evening. It'll still be there, it'll just be hidden behind the Moon.

The scorpion's heart is the star Antares. It's one of the highlights of the summer sky. Right now, in fact, it rises around sunset, scoots low across the south during the night, and sets around sunrise.

Antares lies along the ecliptic -- the Sun's path across the sky. The Moon stays close to the ecliptic, too. It passes by Antares once every four weeks. Most months, it just misses the star because its path is tilted a little compared to the ecliptic. But some months the geometry is just right, and the Moon passes directly in front of Antares, blocking the star from view.

Such an event is called an occultation. Astronomers have used occultations to help measure how big Antares is. They know about how far away the star is, so measuring how long it takes Antares to disappear from view reveals its size -- hundreds of times bigger than the Sun.

As seen from about the eastern half of the country, this occultation will be in progress as the Moon rises. Antares will emerge from behind the Moon an hour or two later.

For those in the western part of the country, the occultation will be over by the time the Moon and Antares climb into view. But they'll form a beautiful pairing, separated by just a whisker. They'll remain quite close throughout the night.

Script by Damond Benningfield, Copyright 2009

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