EARTH has only one true moon, but astronomers have found that we also have a “quasi-moon” – a travelling companion through space that is circling the Earth but is actually orbiting the sun. The object, the only quasi-moon discovered so far, is an asteroid called 2003 YN107 which circles the sun in an orbit almost identical to Earth’s, but follows a corkscrew path that from time to time means that it appears to orbit Earth.
Relative to Earth, the asteroid usually follows a horseshoe path that sometimes lags behind us and is sometimes ahead. But since 1996 its path has taken…
