Cruithne is an Aten asteroid in 1:1 orbital resonance with Earth, around the Sun, making it a co-orbital object. Aten asteroids orbit the Sun in an elliptical orbit which crosses Earth's orbit.
Cruithne was discovered on 10 October 1986 by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. The 1983 apparition (1983 UH) is credited to Giovanni de Sanctis and Richard M. West of the European Southern Observatory in Chile.
It was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Toronto, and Seppo Mikkola, working at the University of Turku in Finland. It has been described as Earth's 2nd moon, but as it orbits the Sun, not Earth, this is incorrect.
Cruithne is approximately 5 kilometres (3 mi) in diameter, and its closest approach to Earth is 12 million kilometres (0.080 AU; 7,500,000 mi), approximately thirty times the separation between Earth and Moon. From 1994 through 2015, Cruithne made its annual closest approach to Earth every November. As yet, we have no observations that reveal surface detail on Cruithne.