What Makes a 'Moon'? (The Answer Is More Complicated Than You'd Think)

Not all moons look like the spherical glowing orb that hovers above Earth. In fact, to be a moon, a space rock technically only has to be the natural satellite of a star’s satellite.

That said, these rocks don’t all look, or act, alike. They come in a variety of shapes, sizes, and types, and they all have unique behaviors. For example, Jupiter has 53 known moons—including the solar system’s largest moon, Ganymede—and many of them have elliptical, backwards orbits. Meanwhile, Mars has two moons, and they're irregularly-shaped, dark satellites that orbit the planet’s equator in circles.

Since there are hundreds of moons—and even more conditional ones—in our solar system, this raises a question: Should we deem each and every one of these secondary satellites a “moon”? And if not, should the distinguishing criteria include factors like orbit, size, shape, or visibility from a planet’s surface?

MinuteEarth’s Kate Yoshida explores these questions in the video below.