Today’s factismal: 3753 Cruithne does not orbit the Earth.
Of late, there have been a lot of blog posts (even, sadly, on supposedly “science oriented” websites) claiming that Earth has a “second moon” named 3753 Cruithne (pronounced “CREW-eee-nuh”; it is the name of a Pictish king). As is often the case with things found on the internet, the truth is both less and more interesting. First, the less interesting part: 3753 Cruithne is not a moon of the Earth or any other planet; instead, it orbits the Sun all by itself. This may sound like nitpicking, but it is an essential part of the definition of the word “Moon”. Until 1655, everything that we saw in the sky was either a star, or a comet, or a planet with the sole exception of the Moon. Galileo’s discovery of four new things orbiting Jupiter was taken in stride; those things were planets according to the astronomers (even though Galileo called them stars). But in 1655, they started seeing planets orbiting Saturn as well. Before long, Saturn had five planets and a ring orbiting it while Jupiter’s planet count grew to ten. So the astronomers decided that they would redefine the word planet. If it was big enough to see and orbited the Sun, it was a planet. If it was big enough to see and orbited another planet, it was a moon. And so, because the asteroid 3753 Cruithne orbits the Sun and not the Earth, it isn’t a moon. (It isn’t a planet because it isn’t big enough; at just three miles across, it is too small to be round.)
So where did all of this nonsense about 3753 Cruithne being a second moon of the Earth get started? With the astronomers, of course. You see, astronomers love to think about what things look like, especially orbits. And they started looking at the orbits of Near Earth objects (i.e., things that had an orbit similar to Earth’s) and found several that had amusing (to the astronomers) orbits. If you flew abovethe Sun and watched 3753 Cruithne orbit, you would see it moving out toward Mars and back in toward Venus, crossing Earth’s orbit twice on each trip. And, thanks to the odd shape of 3753 Cruithne’s orbit, it actually takes about a year to complete each go-round. It would look something like this:
But if you stand on Earth and watch 3753 Cruithne orbit, it looks much different. Because Earth passes 3753 Cruithne in its orbit, it appears that the asteroid is making a “horseshoe” in space. So the astronomers giggled for a while about some asteroids being close enough for horseshoes and left it there. Which is where the internet found it. Unfortunately, most of the people on the internet aren’t astronomers. (You are shocked, I know.) As a result, they don’t know that the horseshoe “orbit” of 3753 Cruithne only happens when you look at the asteroid from the moving Earth; that it is a geocentric view. Since we know that the heliocentric view is much closer to reality, using a geocentric one to claim that an asteroid is the Earth’s second moon makes about as much sense as claiming that the Sun orbits the Earth. And 3753 Cruithne is hardly the only asteroid to look like it is orbiting Earth when it isn’t; just last year, 2014 OL339 was shown to also have a horseshoe orbit.
But that isn’t to say that the Earth doesn’t have a second moon every once in a while. (This is where life gets even more interesting than the internet thinks it is.) Due to the odd orbital interactions of all of the various bits of junk out there, every so often a small asteroid will get trapped in orbit around the Earth for a few days or a few weeks or a few years. When this happens, Earth truly does have a “second moon”; because these asteroids aren’t trapped by Earth’s gravity and are just “passing through”, they are referred to as coorbiting asteroids. In 1999, asteroid 2003 YN107 began a coorbit of Earth that lasted for seven years. And some experts estimate that we have a small, temporary “second moon” almost all the time!
So why aren’t we sure about how often the Earth has a “second moon” (even if it never is 3753 Cruithne)? Simply because asteroids are small and space is vast. As anyone who has ever tried to find a remote control in a room has discovered, it can take a long time to locate something if it is very small compared to the room that you are looking in. But having more people looking can help. And that’s where you can join in on the fun! The Asteroid Survey is looking for folks who are looking to be looking for asteroids! (Here’s looking at you, KD!) You’ll sort through photos, identifying objects as stars, asteroids, or “junk”. And you’ll be helping to identify the millions of bits of junk that fly through our Solar System and give us our second moons. To join in on the fun, orbit over to: