The Moon isn’t alone in the sky. Sure, we’ve launched hundreds of man-made objects into the space around Earth, but even before we got there, Earth and the Moon had a little companion. This tiny, rocky world is named 3753 Cruithne, which comes from Old Irish and refers to the early Irish people and their king, Cruidne. You can forgive your parents and your grade-school teachers for not mentioning it though: The object was only discovered in 1986 and its orbit mapped in 1997.
Cruithne is technically not a moon but a quasi-orbital satellite of Earth. Duncan Forgan, a researcher at the University of St Andrews, in Fife, Scotland, explains over at The Conversation:
This simply means that Cruithne doesn’t loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner solar system in what’s called a “horseshoe” orbit.
To help understand why it’s called a horseshoe orbit, let’s imagine we’re looking down at the solar system, rotating at the same rate as the Earth goes round the sun. From our viewpoint, the Earth looks stationary. A body on a simple horseshoe orbit around the Earth moves toward it, then turns round and moves away. Once it’s moved so far away it’s approaching Earth from the other side, it turns around and moves away again.
Here’s a video of the wacky loops that Cruithne’s orbit traces in that imagining:
But if you look down on the solar system from above the sun and watch both the Earth and Cruithne spin through space, it looks like this:
It takes Cruithne 770 years to wobble in its horseshoe-shaped movement around the Earth. The object can be thought of as an asteroid caught up between the Earth’s orbit and its own around the sun or as a minor planet. It does takes Cruithne about 364 days to go around the Sun — so that’s why it appears to follow the Earth and also why it's been called poetically (but technically incorrectly) our "second moon."
Several other objects also have these orbits that resonant with the Earth—so they too could be called (again, incorrectly) our sort-of moons. Mars, Jupiter and Saturn have their own quasi-orbital asteroids.
But Cruithne might not be around forever. Forgan writes:
Cruithne is expected to undergo a rather close encounter with Venus in about 8,000 years, however. There’s a good chance that that will put paid to our erstwhile spare moon, flinging it out of harm’s way, and out of the Terran family.
This is not the model of space that was once taught in grade school. Keep up: Pluto is really a dwarf planet; there are more dwarf planets out there, (including Ceres, which will soon get some spacecraft close-ups); the Moon isn’t the only object hanging out near Earth.
It's not that everything once thought about our solar system was wrong, it’s that researchers are constantly new learning things. They suspect that Cruithne and other objects like it can tell us about the changing nature of the solar system and formation of planets. And at the very least, new information keeps things fun.