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Moon and Venus

June 14, 2010

A beautiful pairing of the Moon and the planet Venus highlights the western sky this evening. Venus is the "evening star" just above the Moon.

Venus has no moons of its own. But for more than a century, many astronomers thought it did. And a recent study says that the planet might have had a moon early in its history.

Beginning in the late 17th century, astronomers reported dozens of sightings of a Venusian moon. But a detailed study in the late 19th century found that almost all of those sightings were actually stars that just happened to line up close to Venus in the sky.

In 2006, though, two astronomers suggested that billions of years ago, Venus did have a short-lived moon.

Under their scenario, the moon formed in the same way that ours did. Another planet-sized body slammed into Venus, spewing material out into space. This material coalesced to form a moon.

But a few million years later, a second body hit Venus at a different angle, changing the planet's gravitational interaction with the moon. As a result, the moon began moving toward the planet. It was pulverized by Venus's gravity, with the debris falling to the planet's surface.

The double impacts would also explain Venus's odd rotation. Venus turns on its axis in the opposite direction from Earth and the other planets, and takes about eight months to complete a single turn. So a double wallop could have given Venus a moon, taken it away, and spun the planet backwards.

Script by Damond Benningfield, Copyright 2010

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