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As a SciShow viewer, you can keep building your STEM skills with a 30 day free trial and 20% off an annual premium subscription at Brilliant.org/SciShow. [intro jingle] From Aristotle to Coldplay, humans throughout history have connected astronomy with music. Especially when they thought of how everything moves through space.

And there are two moons in orbit of Saturn, named Janus and Epimetheus, that do a special dance to that music that seems to be unique in our Solar System. Our story begins on December 15, 1966, when a French astronomer named Andouin Dollfus discovered a new moon of Saturn. He named it Janus, after the Roman god of gates and doorways.

And three days later , the American Richard Walker observed a moon that was in the same orbit. “Yes, definitely a moon there,” astronomers must have thought, patting themselves on the back and moving on. But twelve years later, two researchers revisited the data. And they concluded that there  wasn’t one moon in Janus’s orbit.

They were very likely two. When the Voyager 1 spacecraft  visited Saturn in 1980, it confirmed that hypothesis. Now, in our Solar System, it’s not so unusual for  multiple objects to orbit another r at the same distance.

It’s known as a 1:1 resonance. But usually when that happens, one is much much larger than the other. Like one is Jupiter, and another is a dinky little asteroid.

Janus and Epimetheus are quite similar in size. Janus is 196 kilometers long on its longest side, and Epimetheus is 135 kilometers. Their orbits, meanwhile, differ from each other by an even smaller distance.

Just 50 kilometers. But what makes these moons truly  special is the dance they do. Every four years, they switch places.

To explain the steps of this dance, let’s start with Janus on the outside orbit, and a little bit ahead of Epimetheus. Because Epimetheus is just that little bit  closer to Saturn than Janus, that means it completes its orbit just a  tiny bit faster and starts to catch up. And as Epimetheus catches up,  its gravity pulls on Janus, slowing it down and making it  fall a little closer to Saturn.

Janus’s orbit shrinks. But gravitational pulls work both ways. So at the same time, Epimetheus is pulled forward by Janus.

It speeds up, and its orbit gets a little bigger. They actually switch tracks! Four years later, the moons are once again close enough that they can do their dance again.

Janus will go back to the outside orbit, and Epimetheus the inside. And lucky for all of us, in 2006, Cassini was able to capture  these steps in exquisite detail. There’s even video of the two moons crossing, but that’s just because Cassini  was also moving at the time.

The moons never get that close  to one another physically. Thanks to Cassini, astronomers know that both of these moons are probably piles of rocky rubble, and because they’re both covered in craters, they’ve clearly been around for quite some time. The impacts that caused all those craters might also be responsible for a faint ring that Cassini found, too, which also shares the same orbit.

Janus and Epimetheus may be two  small moons of a distant planet, but they’re sharing a very  important message from the cosmos: Even space rocks know how to get down and boogie. Unlike me I don’t This SciShow video is supported by Brilliant: an online learning platform with thousands of interactive lessons in science, computer science, and math. Like their course, Geometry I!

That course will have you wondering if these moons are dancing or if it’s coordinated geometry … or maybe coordinate geometry. Yeah, the course definitely  covers coordinate geometry. And when you solve the geometry puzzles in this course, you might even consider that accomplishment as impressive as moons swapping  places with each other.

Anyway, you don’t need me  to keep making bad segues from the video to the sponsor because you can just try it yourself  right now for free for 30 days by visiting Brilliant.org/SciShow or clicking the link in  the description down below. That link also gives you 20% off an  annual premium Brilliant subscription. Thanks for watching! [ OUTRO ]