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Moon and Venus

November 5, 2013

The beautiful crescent Moon slips past another beautiful crescent the next couple of evenings — Venus, the “evening star.” The brilliant planet is to the left of the Moon early this evening, and closer to its lower left tomorrow.

The Moon appears as a crescent because it lines up roughly between Earth and the Sun. At that angle, sunlight illuminates only a small fraction of the lunar hemisphere that faces our way. Yet we can still see the entire lunar disk because the nightside is lit up by earthshine — bright sunlight that’s reflected by our own planet.

Venus also lines up between Earth and the Sun right now. So when seen through a telescope, the planet forms a crescent that’s about as thick as the Moon is. And over the centuries, many Venus watchers have reported a glow over the planet’s nightside that’s like a washed out version of earthshine.

The effect is known as the “ashen light.” It's visible mainly when Venus is a thin crescent in the evening sky, as it is now. But it doesn’t show up all the time — sightings are somewhat rare.

One idea says the glow is similar to the “northern lights” here on Earth. Another says it’s caused by several quick lightning flashes that appear to light up the whole disk. And yet another says that it’s an optical illusion — the light from the bright crescent makes you “see” a glow that isn’t really there.

Whatever its cause, the ashen light is a mystery that’s persisted for centuries — and continues even today.


Script by Damond Benningfield, Copyright 2013

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