How many moons?

This post has been transferred from an older blog, with minor edits and updates.
Original post date: 16-Jul-2009.

What you get from watching QI video clips on YouTube well into the wee hours of the night. The question asked by Stephen Fry was “How many moons does the Earth have?”, and the answer was not the usual “one” but in fact “two”. Now, I half expected that, with “one” being way too obvious, but the nature of the second moon came as something of a surprise.

It’s called Cruithne (pronounced “croo-een-ya”), is about 5km wide and wobbles around in a quite strange horseshoe- or bean-shaped orbit which doesn’t really go around the Earth at all. It takes 770 years to complete one round, and never gets closer to us than some 12 million kilometers, which is a good 30 times as far away as the Moon we all know and love. In fact it spends most of its time on the other side of the sun, as seen from our planet, orbiting not exactly the Earth itself but apparently rather the Earth’s Lagrange points L4, L3 and L5, and then back again before making another round. So it’s not exactly as if you’ll be looking at it in the sky.

More about the wobbling rock on, and at Wikipedia which even includes some nifty animations of the orbit.


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