Which of the following is often referred to as Earth's "Second Moon"?

The asteroid 3753 Cruithne is often referred to the "Second Moon" of the Earth because its orbit is highly synchronized with the Earth's. In fact, it's orbit takes it inside the orbit of Mercury and beyond the orbit of Mars and comes within 0.08 a.u. (12 million km) of the Earth, making it's closes approach around the end of November each year. While this seems really close, there is no danger of Cruithne hitting the Earth any time soon.

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