3 April 2019

One of the things that I’ve seen on QI a couple of times now is “how many satellites does the Earth have”? It’s a trick question. “Satellites” has a couple of different definitions.

But the thing I did find interesting was when they started talking about something called 3753 Cruithne. 

Pronounced Crew-EEN-ya, it’s a chunk of rock about 5 kilometres across, that orbits – well, it’s not really clear what it orbits. If you’re on Earth, it looks like it’s orbiting Earth. But from other perspectives, it looks it’s orbiting the Sun.

But it gets weirder than that. If you’re on the Earth, it looks as though Cruithne goes around the Earth in a horseshoe-shaped orbit. Yes, that’s right, it seems to orbit us, approach, but then slow down and fall behind. But before the Earth catches Cruithne again, it speeds up and pulls away. I mean, what the…?

It’s not that Cruithne follows the Earth around the Sun. If you looked down on the Solar System from way above the sun, you would see that Earth’s orbit is pretty round, and Cruithne’s is an oval. It gets as close to the Sun as Mercury, and as far out as Mars. In addition, its orbit is tilted differently to ours, so there’s no chance of a collision.

This confusion about what Cruithne orbits is because it takes the same amount of time to travel around the Sun. Clearly there’s some gravitational interaction between the Earth and Cruithne.

According to the orbital experts (the descendants of Tycho Brahe and Johannes Kepler), it only looks like Cruithne orbits the Earth because we go around in the same time and we’re watching it from a synchronised perspective. As Einstein would probably say: “it’s all relative”.

I couldn’t find a good photo of Cruithne, but it probably looks a bit like this asteroid. If you've got a 16" telescope (either a Cassegrain or a Dobsonian) and a DSLR, you can see Cruithne - just. I'll might be able to show you how in a future post.