Moon and Regulus

StarDate: October 13, 2009

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The crescent Moon flirts with the heart of the lion early tomorrow -- Regulus, the brightest star of the constellation Leo. They're well up in the east at first light, with Regulus to the upper left of the Moon.

If you use binoculars to scan the line separating lunar night from day, you'll see some long shadows stretching across it -- shadows cast by the rims of craters, and by lunar mountains.

From these shadows, you might expect the mountains of the Moon to be sharp and jagged, but they're not. The mountains are gentle and rounded -- and that reveals a lot about their history.

Sharp, jagged mountains here on Earth are young and fresh. Over time, though, they erode. Wind, rain, and living organisms wear the mountains away, chipping off tiny bits of rock that wash down the sides. Over hundreds of millions of years, the mountains turn from rugged crags, like the Rockies, into rounded mounds, like the Appalachians.

So the mountains of the Moon must be old and eroded, too. But there's no air, water, or life on the Moon, so they're eroded by a different source -- meteorites. Small bits of rock scattered throughout the solar system rain down on the lunar surface, chipping away at the mountains. Most of the meteorites are no bigger than grains of sand. But since most of the lunar mountains formed more than three billion years ago, there's been plenty of time to wear them down -- forming gentle hills on the rugged surface of the Moon.

Script by Damond Benningfield, Copyright 2001, 2004, 2009

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