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How many moons does the Earth have?

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458822.  Tue Dec 16, 2008 4:19 pm Reply with quote

Under this heading, on page 61 of TBOGI, it says that Cruithne has 'an odd horseshoe-shaped orbit.' I am at a loss to imagine what this means. A horseshoe isn't even a complete closed loop. So what does it mean? This is a serious question, by the way, though I expect I may get some humorous replies.

458877.  Tue Dec 16, 2008 5:26 pm Reply with quote

The simple and serious answer is that Cruithne is not a moon of the Earth; it's an asteroid. So far as is known, the Earth has just one moon, the one we call The Moon.

If you search the forums you'll see that Cruithne has come up quite a few times. In particular, see post 113742 in which John Lloyd himself explains why it's "QI's talisman".

Dr. Know
458880.  Tue Dec 16, 2008 5:28 pm Reply with quote

Jeremy Hardy wrote:

"Blue moon I saw you standing alone." Not- 'With a small friend.'

458894.  Tue Dec 16, 2008 5:38 pm Reply with quote

Stephen Fry then said, not wrote:

Please don't correct me, I know it's wrong, but I'm just quoting.

459091.  Tue Dec 16, 2008 9:11 pm Reply with quote

Just to clear this up once and for all. The Earth has one moon. It may or may not have numerous other natural satellites (perhaps with eccentric, perhaps even satirical, orbits probably designed by God to annoy atheists) but it definitely has one moon.

In the same way, there is only one Sun.

459099.  Tue Dec 16, 2008 9:33 pm Reply with quote

My question was not really about the answer to the question : How many moons does the Earth have? It was really trying to find out what is meant by a horseshoe-shaped orbit. TBOGI really contradicts itself as to the answer to the question. First of all it says, without any qualification whatsoever, 'At least seven'. then it says that the Moon is the only body to observe a strict orbit of the Earth, which is what a moon has to do (in the opinion of all normal people, i.e. people who think the same way that I do). These other objects may well go round the Sun in a similar orbit to the Earth's, but if they don't go round the Earth, then they are not moons. By the way, if there are at least seven co-orbitals, and probably more that have not been discovered yet, doesn't that mean that the Earth is not a planet, since it hasn't 'cleared the neighbourhood' around its orbit?

459104.  Tue Dec 16, 2008 9:38 pm Reply with quote

It goes round in a circle but doesn't quite finish the circle, then goes back the other way but still doesn't quite finish the circle, etc. etc. To simplify it, think of it going back and forth across the letter U.

Sadurian Mike
459107.  Tue Dec 16, 2008 9:40 pm Reply with quote

I assume that "horseshoe-shaped" means more like an egg outline than a sphere or oval.

Not that I am any sort of expert here, though. Jan might know more so I'll try to direct her here.

459124.  Tue Dec 16, 2008 10:24 pm Reply with quote

er no. Horse shoe shaped means - horse shoe shaped. you know - up to the top, then back down again, round to the other extreme, then back down again. never quite completing a full oribit. more like a pendulum really.

459128.  Tue Dec 16, 2008 10:26 pm Reply with quote

and if you're going to include objects that are influenced by the Earth but which don't fully orbit it, then you'll have to include a number of stars too.

459130.  Tue Dec 16, 2008 10:26 pm Reply with quote

bobwilson wrote:
er no. Horse shoe shaped means - horse shoe shaped. you know - up to the top, then back down again, round to the other extreme, then back down again. never quite completing a full oribit. more like a pendulum really.

That's what I said: sort of like the letter U. Though probably bigger at the bottom. Like an egg with its tip off.

459132.  Tue Dec 16, 2008 10:31 pm Reply with quote

I bet Mr Fry wishes he'd never mentioned bloody Cruithne

459405.  Wed Dec 17, 2008 10:52 am Reply with quote

Cruithne does not in fact have a horseshoe shaped orbit - being a well-behaved asteroid it goes around the Sun in an elliptical orbit just as it's supposed to.

But as seen from the Earth, it appears to have a horseshoe shaped orbit. A slightly technically explanation of how that works, together with a mention of "tadpole orbits", can be seen at

459736.  Wed Dec 17, 2008 7:04 pm Reply with quote

As suze says, it has a perfectly normal asteroidal orbit relative to the sun. However it's orbital period is close to one Earth year, and so relative to the Earth appears to travel about the sun with us, as if in orbit. From the point of view of Earth this orbit appears to change direction before it has made a complete circuit.

Here are some Java applets which show what is going on...

460082.  Thu Dec 18, 2008 8:09 am Reply with quote

Well, I'll take Suze's word for it. I'd like to see a demonstration, though, if possible. Has Sir Patrick Moore ever done one? I don't think he made them himself - in fact I'm pretty sure he credited the chap who did make them - but they were good, seemingly made of black card with slots and holes in, held together with those tags that you push through a hole and then splay out the two 'limbs' (- what are they called?) and turned by hand.
Just a niggle - why couldn't TBOGI add 'as seen from the Earth' after 'orbit' and before the full stop? I guess that all the planets and asteroids have odd-shaped orbits as seen from the Earth, so it seems hardly worthy of comment.


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