How many moons does the Earth have?
A simple question, with an easy answer… well if you think it is as simple as 1 then you haven’t thought hard enough (seriously the answer is probably 1 but… ). Nothing new here but interesting nonetheless.
The near-Earth asteroid 3753 Cruithne is now known to be a companion, and an unusual one, of the Earth. This asteroid shares the Earth’s orbit, its motion “choreographed” in such a way as to remain stable and avoid colliding with our planet. It orbits around the Sun in 1:1 orbital resonance with that of the Earth. Due to its unusual orbit relative to that of the Earth, it is a periodic inclusion planetoid. From the Earth’s point of view Cruithne actually follows a kidney bean-shaped horseshoe orbit ahead of the Earth, taking slightly less than one year to complete a circuit (to see some diagrams of this take a look at [http://www.astro.uwo.ca/~wiegert]. Other examples of natural bodies known to be in horseshoe orbits include Janus and Epimetheus, natural satellites of Saturn. So maybe there is a case there?
It has been called “Earth’s second moon”. A term I find hard to agree with but I do like it, and in a way it could been deemed correct.
Cruithne is approximately 5 km in diameter, and its closest approach to Earth is approximately 30 times the separation between Earth and the Moon (12 million kilometres). Cruithne was discovered in 1986, so you have probably heard of it before… but if not this is some good trivia! A couple of others have been proposed in recent times including [http://www.astro.uwo.ca/~wiegert/]/
I would like to call 3753 Cruithne a moon, but it is not, it is a co-orbital asteroid, it shares our orbit. But what does this mean about the planet definition of an object clearing its path?