BOSTON — Scientists have known that the Moon has a tail, like a comet, since the late 1990s, but now they know where it comes from — and why it's brighter sometimes than others.

According to research published in the Journal of Geophysical Research: Planets by a team of Boston University physicists, the tail is made of millions of sodium atoms that are blasted into orbit by meteoroids hitting the Moon's surface. These are shaped into a tail by photons arriving from the Sun.

Earth periodically passes through the tail. When this happens, Earth's gravity focuses the tail into a beam that wraps around the planet and shoots out behind it.

The researchers found the Moon's tail glows more brightly during sporadic meteor showers, as opposed to annual meteor showers, which can make the tail glow more brightly but less so than sporadic meteor showers.