Tip to Wes.
3753 Cruithne ( /ˈkrɪnjə/, from Old Irish [ˈkrɪθnɛ]; Modern [ˈkrɪhnʲə] or [ˈkrɪnʲə]) is an asteroid in orbit around the Sun in approximate 1:1 orbital resonance with the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit. It has been incorrectly called “Earth’s second moon”, but it is only a quasi-satellite.
hmmm…. I’m not even sure I would call it a “moon” but, still cool!