Earth’s mysterious ‘second moon’ and its odd orbit


It sounds like one of those crazy conspiracy theories: There are aliens at Area 51. Abraham Lincoln was a lizard. The Earth has a second, secret moon. However, the last of these is actually pretty widely repeated in scientific circles, though only with a very colloquial definition for the word “moon.” Though it’s technically a near-Earth asteroid, the astronomical body called 3753 Cruithne spends much of its time following the Earth like our much more famous satellite, Luna.

First, a quick explanation of why Cruithne is not actually a moon, then an explanation of why many refuse to accept that fact. It’s not a moon because, well, it’s an asteroid. Cruithne orbits the Sun, not the Earth, and its seemingly wonky orbital pattern is definitely not tied to the Earth’s in any satellite-like fashion. By no means is Cruithne actually a secondary body orbiting the Earth — so why is it so often referred to that way?

The answer is that Cruithne has a very similar orbital period to the Earth’s, just a day or so off at 364 days per cycle. This means that when the astroid happens to be close to the Earth, it will also happen to stay close to the Earth for quite a while. The tendency of this small celestial body to move in tandem with the Earth, often for long portions of the year, has led some to incorrectly dub it our second moon.

However, though it may take a similar amount of time to loop around our star, Cruithne’s orbit is anything but regular. As seen from an objective viewpoint (above right), it’s easy to see how the two orbits are independent of one another. Seen from a more realistic perspective, one which keeps the Earth stationary and simulations our view from its surface, shows a very different picture. View the video below, which offers a great three-dimensional look at Cruithne’s so-called “horseshoe orbit.” Seen from this perspective, it’s a lot easier to understand why Cruithne has been so widely mischaracterized as a moon of the Earth.

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