One of the common sources of information for the elves is the latest scientific news. We scan all the science magazines and blogs that we can, hoping that a boffin or egghead somewhere has crushed a common belief or found something groundbreakingly interesting. However, such is the nature of science, that it is always moving forward, and so it can sometimes make you look a little silly.
Cruithne was first discovered in the late 80s, before QI was even a twinkle in John Lloyd's eye, but it was after he read about the "moon" in a 1999 New York Times article, where it was clearly described as "Earth's Second Moon," that it found its way into the QI database.
When the episode of QI was first broadcast, in 2003, QI was one of the very first to bring Cruithne to the public conciousness. In subsequent series, we told of new discoveries - in series two, 2 more "moons" and in series three a further 4 - but by this time we were careful to specify that they were not "moons" as such, but co-orbital bodies that orbited the Sun rather than the Earth. In fact - on one occasion, we were able to unveil one of these co-orbital bodies before its discoverer (with whom we were corresponding) had published his findings.
Cruithne, named after a Celtic deity and pronounced Cru-een-ya, and its friends are most certainly not moons in the astronomical sense, and under the new IAU definition should probably be describe as Small Solar System Bodies. But they track the Earth's orbit around the Sun and could, one-day, become moons in their own right.
So if they're not really moons, then why do we say, in the Book of General Ignorance that the Earth has "at least seven moons"?
Well, since Cruithne found its way into the first few series of QI, it has become something of an unofficial mascot. As an iconic piece of General Ignorance, we felt that the book would be empty without it.
The idea that Earth has two moons is certainly a lot older than ours (or any) TV show. In 1846, Frederic Petit, director of the observatory of Toulouse, stated that a second moon had been discovered. He became somewhat obsessed with the second moon, and used it to explain hitherto unexplained eccentricities in The Moon's orbit, though his peers generally dismissed Petit's claims as nonsense. When Jules Verne heard about this story, he wrote the novel "From the Earth to the Moon." The idea still did not die, and at least two other astronomers claimed to have discovered this second moon, which was given the name "Lilith"
The earth only has one moon. As Alan Davies said, there can only be one moon - that's why it's called "The Moon".
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