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Moon and Venus

May 21, 2015

Venus is slowing down.

Our closest planetary neighbor takes about 243 Earth days to make a single turn on its axis — far longer than any other planet in the solar system. But observations by Venus-orbiting spacecraft showed that from 1986 to 2012, that time increased by about six-and-a-half minutes.

We can’t see the surface of Venus from Earth because the planet is blanketed by clouds. In the 1980s, though, the Magellan spacecraft used radar to peer through the clouds. It mapped thousands of surface features. Scientists used the motions of those features to measure Venus’s rotation rate to within a few seconds.

A few years ago, another craft, Venus Express, found that those features weren’t where they were supposed to be. The features hadn’t actually moved, though. Instead, the time of a single rotation had gotten longer — by six-and-a-half minutes.

Scientists are still trying to understand the reason for the change. Some of it is probably caused by the planet’s atmosphere. It’s about 90 times denser than Earth’s air, so as Venus turns, the atmosphere exerts a drag on the planet itself. And strong weather systems may magnify the effect.

But the atmosphere can’t explain the entire difference, so scientists are still pondering why Venus is slowing down.

And Venus teams up with the Moon to put on a grand showing this evening. Venus is the brilliant “evening star” to the right of the Moon as night falls. More tomorrow.


Script by Damond Benningfield, Copyright 2015

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