The Moon slips past the planet Venus this evening -- the brilliant "evening star." They're well up in the west as night falls, with Venus just to the right or lower right of the Moon. They set around 9 or 9:30.
The Moon is a thin crescent right now; sunlight illuminates only about 10 percent of the lunar hemisphere that faces our way. But the rest of the Moon is visible because it's lit up by earthshine -- sunlight reflected off of Earth.
Venus is also a thin crescent now -- about the same fraction as the Moon. And at times, the "dark" portion of a crescent Venus may show a glow that's similar to earthshine. It's not as bright as earthshine, and it doesn't have the same cause -- there's no nearby body to reflect sunlight toward Venus.
The effect is known as the "ashen light." It was first seen more than three and a half centuries ago. Since then, hundreds of people have reported seeing it through their telescopes. It's visible mostly when Venus is a thin crescent in the evening sky. But it's not seen very often.
One idea says the glow is caused by a process that's similar to the one that produces the "northern lights" here on Earth. Another says it's caused by lightning.
And yet another says that it's an optical illusion -- that the light from the bright crescent makes you perceive a glow that isn't really there.
Even without the ashen light, though, Venus is always a beautiful sight -- tonight, near the crescent Moon.
Script by Damond Benningfield, Copyright 2008
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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