Moon and Venus

StarDate: February 26, 2009

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The Moon is racing toward the planet Venus tonight -- as seen from Earth, that is. Venus -- the dazzling "evening star" -- is millions of miles farther, but it and the Moon follow roughly the same path across the sky. As they both circle through the sky, they pass each other about once a month. Tonight, Venus stands well above the crescent Moon. By tomorrow night, though, they'll stand side by side.

Comparing the Moon's position relative to Venus the next couple of nights will show how quickly the Moon moves across the sky.

The Moon orbits Earth once every 27 and a third days -- an interval known as the "sidereal" period, because it's measured against the background of stars. During that time, though, Earth moves a good distance in its own orbit around the Sun, so it takes the Moon a couple of extra days to complete its month-long cycle of phases.

As it races around Earth, the Moon moves about half a degree against the background of stars every hour. That's equivalent to the width of the Moon itself. It moves about 13 degrees every day -- a little more than the width of a fist held at arm's length.

So if you see the Moon and Venus tonight, lighting up the western sky beginning shortly after nightfall, note how far apart they are. Then look again tomorrow night, and you'll see that they're standing shoulder to shoulder, as the Moon shoulders past the brilliant planet.

We'll have more about Venus and the Moon tomorrow.

Script by Damond Benningfield, Copyright 2008

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