In November 1998, when the annual Leonid meteor shower coincided with the New Moon, astronomers working in Texas noticed an unexpectedly high concentration of sodium atoms localized above the Earth. This 'Na spot' was identified as a stream of atoms escaping from the Moon. Now, a comprehensive study by Majd Matta and colleagues of the brightness of this spot over 31 consecutive lunar months has provided clues as to how sodium atoms escape the Moon's surface (Icarus http://dx.doi.org/10.1016/j.icarus.2009.06.017; 2009).
All-sky observations made between 18–20 November 1998 at the McDonald Observatory in Fort Davies, Texas, revealed an intense spot of 589 nm light — the same wavelength as sodium D lines — in a 3° × 3° area of the night sky. This Na spot was seen near the antisolar point and only on the three nights of the New Moon, when the Earth is roughly aligned with the Sun and the Moon. The explanation was that the Moon's sodium tail was being focused by the Earth's gravitational field. It was also postulated that the spot was made particularly bright by an increase in the rate of sodium escape from the Moon's surface during the Leonid meteor shower, which had been at its most intense a couple of days earlier (it takes the sodium atoms roughly two days to travel the distance between the Moon and the Earth).
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