BOSTON, Mass., June 10 -- Boston University astronomers report the discovery of a new feature of the Earth's moon -- a sodium-gas tail over half a million miles long. Scientists stumbled across this discovery in November when a Boston University team pointed a sensitive camera in the opposite direction from the moon in an attempt to photograph the Leonid meteor storm. Instead, they found a patch of sodium emission in an otherwise moonless sky. It grew to be larger and brighter on November 19, and then faded slightly on November 20, said Steven Smith, a research associate in the university's center for space physics. Astronomers compared the findings with data recorded earlier that year, when similar observations were made following the new moon of August 21, 1998. They found the same types of emissions appearing in the same shapes over the same three nights spanning the new moon, though the August tail was much fainter. The scientists credit November's strong Leonid storm with the enhanced tail that allowed for relatively easy observation.