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Moon and Spica

February 28, 2013

Since Galileo first turned his small telescope toward the heavens four centuries ago, astronomers have been building ever-bigger telescopes to gather ever more light. Yet sometimes, they learn as much from what they can’t see as what they can.

What brings this to mind is a close conjunction between the Moon and the star Spica. They’re separated by just a fraction of a degree as they rise late this evening.

Spica lies near the ecliptic — the Sun’s path across the sky. The Moon stays close to the ecliptic as well, so it passes close to Spica every month — as well as any other star within a few degrees of the ecliptic.

When the geometry is just right, the Moon can cover up these stars — an event known as an occultation. And astronomers have used occultations to study these stars.

They precisely measure a star’s light as it “winks out” behind the Moon. The time it takes the star to disappear, combined with its distance, reveals the star’s diameter. And if there’s a sudden drop in the light level, it means the star has a companion. This technique has revealed companion stars that were so close to the main stars that they were impossible to see on their own. And watching occultations in different wavelengths reveals details about the environment around the star.

Thanks to better technology, occultations aren’t as important today as they were in the past. But they show us that there’s often a lot more to a star than meets the eye.


Script by Damond Benningfield, Copyright 2012

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