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More Moon and Venus

March 30, 2011

In 1651, a pair of Italian astronomers published the best map of the Moon to that time, complete with place names that are still in use today. Craters were named for scientists, while the large, dark features were called seas or oceans. The two men named a couple of features for themselves. They showed some restraint, though; the features are of modest size, and they're tucked away on the Moon's western limb instead of right in the middle of the disk.

If you have binoculars, you can pick out these features around first light tomorrow. The crescent Moon is low in the sky then, with Venus, the "morning star," close to its lower right.

The features are oval-shaped dark spots near the thickest part of the crescent. They're known as Grimaldi and Riccioli. Grimaldi is the larger of the two -- it's about 150 miles across.

Both features are volcanic basins. They formed several billion years ago, when molten rock oozed up to the surface, filling wide, shallow impact craters.

Grimaldi pulls orbiting spacecraft toward it a little bit. The attraction comes from the dense volcanic rock that fills the basin, plus a bulge in the denser rocks below the crust.

There are hints that Grimaldi could still be volcanically active. Several observers have seen flashes of light from the region, including an Apollo astronaut who was in lunar orbit. These flashes could be outbursts of gas from volcanic vents -- the last gasps of an almost-dead world.


Script by Damond Benningfield, Copyright 2011


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