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Moon and Venus

April 21, 2015

The crescent Moon and the planet Venus stage a beautiful encounter this evening. Venus is the “evening star,” and stands to the right of the Moon. As a bonus, the bright orange star Aldebaran, the eye of the bull, stands close below the Moon.

Venus has no moons of its own. Several observers in the 17th and 18th centuries thought they saw one. But detailed searches haven’t turned up anything.

A study conducted about a decade ago suggested that Venus might have had a moon early on. In this scenario, the young planet was hit by another large body, blasting debris into orbit around Venus. This material quickly coalesced to form a moon. In fact, that’s just how our moon is thought to have formed.

About 10 million years later, though, a second impact caused Venus to start spinning in the opposite direction. That changed the gravitational interaction between Venus and its moon, dragging the moon back down to Venus itself.

That model fits Venus’s odd spin — it rotates backwards compared to Earth and all the other planets in the solar system. And it takes the planet 243 Earth days to complete one full turn on its axis — longer than it takes to orbit the Sun. As a result, a single day on Venus — the interval from one sunrise to the next — lasts 117 Earth days.

So a pair of giant impacts could have created a Venusian moon and then taken it away — while making it a long wait for dinnertime on our closest planetary neighbor.


Script by Damond Benningfield, Copyright 2015

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