3753 Cruithne (Earth’s Second Moon)

If you were to question someone on how many moons are orbiting the Earth, they will most likely answer “one”. They may be wrong.

The Earth has a second satellite orbiting it called Cruithne; with a diameter 3.11 miles.  Why hadn’t you heard of this before now? Probably because it wasn’t discovered until October 10th, 1986! This is because of it’s unusual orbit. You see, Cruithne never fully orbits the Earth, but instead goes back and forth in a horseshoe fashion. This, in effect, makes Cruithne very difficult to view.

Duncan Waldron was working at Royal Observatory in Edinburgh. His job was to create high quality reproductions of astronomical photo plates. On October 10, 1986, while serving at the UK Schmidt Telescope at Siding Spring Observatory, Waldron discovered the asteroid. But it was not until 1997 that it’s unusual orbit was discovered.

While the asteroid does not directly orbit the earth, it does orbit the sun like so:

So when we show only Cruithne’s orbit this is what we see:


This could be considered a “co-orbital moon”. While it is not technically a true second-moon, I like to think of it that way.

4 thoughts on “3753 Cruithne (Earth’s Second Moon)

  1. If this satellite was discovered in October 1986, its eccentricity would not have been discovered until 1997 (possibly 1987) but certainly NOT 1977 (as you have it!) This would mean that its eccentricity would be discovered before the satellite was.

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