[DoctorBeaver (OP)]

I was watching QI last night & this came up. Apparently Earth has got 2 moons. Why have I never heard this before?

The second was discovered in 1983 and is called "Cruithne". It orbits the Earth every 770 years, and it is about 3 miles wide.

Here's a description of its orbit (if anyone can understand it - I sure can't)
http://www.astro.uwo.ca/~wiegert/3753/3753.html

[neilep]

What a bizarre object !

According to Wiki Three other objects with similar orbits have been discovered since then.

 Can we really call them moons ?........I don't see why not !

[Andrew K Fletcher]

And there was I thinking planets start small and grow gradually. Certainly fits the pattern

[lyner]

You could look at it this way: If the Earth and  Cruithne were way out, on their own, is space, Cruithne would orbit the Earth every 750 years but not in a circle - a very eccentric ellipse. But they are both going round the Sun. The resulting path of the little one around the Earth is a combination of its orbit around us and its orbit around the Sun. From our point of view, what would be a simple elliptical orbit (without the Sun) is modified into a 'horseshoe' shape by being forced to stick near to a circular path around the Sun. Sometimes it is 'outside' our orbit and sometimes it is 'inside' our orbit.

[DoctorBeaver (OP)]

I'm still confused. But I didn't sleep very well last night.

[Karen W.]

I never heard of that either!

[Karen W.]

I'm still confused. But I didn't sleep very well last night.

Sorry your sleep was not good DOC.. take care of your self and I hope you sleep better tonight!

[DoctorBeaver (OP)]

Thank you, Karen.

[lyner]

I'm still confused. But I didn't sleep very well last night.

The little chap doesn't go on a horseshoe orbit around the Sun. It is going  round about once an Earth year in a similar orbit to ours but, sometimes it is ahead of us and  (350 years later) sometimes is behind us. When it is furthest ahead or behind, it is nearly the other side of the Sun from us. Think of cars on a race track. They may go round 20 times but one car may lag by half a lap and then lead by half a lap during the course of the race.  They are both going quite fast all the time - their relative positions are governed by their relative speeds.