Think twice before you say that Earth has two moons


Duncan Forgan, a research fellow at the University of St. Andrews, recently wrote in The Conversation about Earth’s “second moon”.

While this is a sexy title that others have used in the past, it’s not an accurate description for this asteroid, called 3753 Cruithne, circled in orange in the five-series image above. 

Cruithne is more of a companion than a moon. 

It flies near Earth about once every 385 years, the last time being around 100 years ago. Don’t worry about any catastrophic impact, however. The closest that Cruithne ever reaches Earth is about 40 times farther than the moon, according to Wiegert and his colleagues in a paper on the object’s odd orbital behaviour.

And you can check it out for yourself in this interactive timeline of Cruithne’s orbit that allows you to see where it will be relative to Earth for the next 185 years.

This orbital behaviour is what makes Cruithne so intriguing. The asteroid traces a horseshoe shaped path through space, shown in the GIF below where Earth is in blue, Cruithne’s path around the sun is in red and how this path looks from Earth is in white:


Cruithne completes a full orbit around Earth once every 800 years, but during that same amount of time, it completes roughly 800 orbits around the sun.

That’s because Cruithne is more gravitationally bound to the sun than Earth, and for that reason it does not qualify as our moon.

To compare, the moon, which is more gravitationally bound to Earth, orbits Earth once every 27 days and orbits the sun once every 375 days — the same as Earth.

So, think twice before you start saying that Earth has two moons.

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