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The Moon's Tail

December 29, 2010

The Moon has a tail. And at its longest, it can stretch across a quarter of a million miles -- long enough to reach Earth.

The "tail" is a thin cloud of sodium atoms ejected into space from the lunar surface. They most likely were blasted into space by energy or charged particles from the Sun, or by the impacts of meteorites, which vaporize material when they hit the surface.

Some of the sodium atoms fall back to the Moon, but some of them get enough "kick" to climb thousands of miles high. They form a thin atmosphere that completely encircles the Moon.

The atoms have an electric charge, so many of them are swept up by the solar wind -- a million-mile-an-hour stream of charged particles from the Sun. This sculpts the sodium into a teardrop-shaped tail that streams behind the Moon. When the Moon is new -- when it passes between Earth and Sun -- the tail end of the tail can even touch Earth.

A Japanese satellite that's orbiting the Moon found that the atmosphere and tail grow thinner as the Moon moves from first quarter to last quarter. It also found that they're thickest during February -- although no one can yet explain why.

The lunar atmosphere and tail aren't visible to the unaided eye. But the Moon itself puts on a fine display before dawn tomorrow. It's a thin crescent, low in the southeast, with the planet Venus -- the brilliant "morning star" -- to its lower left. More about Venus and the Moon tomorrow.


Script by Damond Benningfield, Copyright 2010


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