Quick: How many moons does the Earth have?

You might be forgiven for saying “one,” but it turns out the question isn’t all that easy to answer … because it depends on what you mean by “moon.”

Of course, we have one moon, the one we call the Moon. It physically orbits the Earth, so it counts. In this case, what we firmly call a moon would orbit inside what’s called the planet’s Hill sphere, a region around the planet where the planet’s gravity dominates over everything else.

But there are other orbits that make itlook like an object is orbiting the Earth, but it doesn’t really. There are subtly different flavors of objects like these, but one kind orbits the Sun in elliptical orbits, with an orbital size and shape just right that they take almost (but not quite) exactly one year to go around the Sun once. Typically their orbits are elliptical, taking them closer in and farther out from the Sun than the Earth.

This is precisely the case with 2014 OL339, a 150-meter (or so) diameter rockdiscovered serendipitously in July by a team of asteroid hunters. Its orbit is highly elliptical; it crosses the orbit of Venus and gets out as far as Mars. It’s also slightly tilted compared with the Earth’s orbit by about 10°.