Previous: Poor, Misunderstood Poison Ivy
Next: We're Running Out of These Elements — Here's How



View count:250,595
Last sync:2022-11-20 18:30
Thanks to Oribel for sponsoring this episode. Go to and use code “SCISHOW15” for 15% off on your wall toy or extension sets. Or get it on Amazon - Make Science fun with VertiPlay STEM Marble Run!

We owe a lot to our moon. Beautiful moonlit walks, higher tides, and regular seasons - all are made possible by our little rocky friend. But what would happen if we picked up a second moon?

Thanks to Dr. Neil Comins at the University of Maine for his orbital mechanics expertise and help with this episode!

Hosted by: Hank Green

SciShow has a spinoff podcast! It's called SciShow Tangents. Check it out at
Support SciShow by becoming a patron on Patreon:
Huge thanks go to the following Patreon supporters for helping us keep SciShow free for everyone forever:

Eric Jensen, Matt Curls, Sam Buck, Christopher R Boucher, Avi Yashchin, Adam Brainard, Greg, Alex Hackman, Sam Lutfi, D.A. Noe, Piya Shedden, Scott Satovsky Jr, Charles Southerland, Patrick D. Ashmore, charles george, Kevin Bealer, Chris Peters
Looking for SciShow elsewhere on the internet?
Correspondence with Dr. Neil Comins

Image Sources:,_GA_2018.webm
Thanks to VertiPlay STEM Marble Run for supporting this episode of SciShow.

Click the link in the description to see the more than 100 ways to play and learn some critical STEM skills! [♪ INTRO]. We owe a lot to the Moon.

Without it, our tides wouldn't be as high, our seasons wouldn't be as regular, and our days would be much shorter. But you know what? What we call “the Moon” isn't the only moon the Earth has ever had.

Once in a while, our planet's gravity captures a tiny, rocky body, forcing it to make a few orbits around us before it continues its jaunt around the Sun. Because they're so small, these objects usually go undetected. But what if Earth managed to capture something a lot bigger?

Something much closer in mass to our current Moon? It might sound like a lot of fun, but once you look at the consequences, you might start to hope this never happens. Now, not all of the consequences would be catastrophic.

For instance, if this second moon orbited close to Earth, we would generally have more moonlight. And while that would affect nocturnal animals and make it easier for some predators to hunt, it likely wouldn't destroy the food web. The light probably isn't the thing we'd be most concerned about, though.

We'd likely be thinking more about stuff like tides. If we picked up another moon, its gravity would cause Earth's tides to reach both higher highs and lower lows over the course of a month. That means a lot of people would probably have to move away from coastal areas, shifting the population of our planet.

If the moons were orbiting at different distances from Earth, they'd also be traveling at different speeds. And that would further mess up our tidal system, where we currently have a nice, regular spacing between high and low tides. The time between those tides would speed up, causing more shoreline erosion.

And that would threaten not just aquatic life, but human infrastructure, which would get pummeled by waves all the more often. This extra force wouldn't just pull on Earth's water, though:. It would pull on everything on and underneath the Earth's surface, too.

This would increase Earth's volcanic activity, and the frequency of other natural disasters, like earthquakes and tsunamis. Which are rarely good for life - or for humans. On an even longer scale, this second moon would even affect how long our day is.

Right now, our Moon is gradually slowing. Earth's rotation because it's orbiting around us more slowly than we're spinning. Its gravity is tugging on Earth and creating a sort of drag.

But a second satellite — especially one close to the Moon's size — would interfere with that process. Exactly how, though, would, of course, depend on its orbit, but for the sake of simplicity, let's say it orbited in the same direction as our current Moon, and was located a bit closer to Earth. That satellite would help slow down our rotation, and our days would get longer.

Which might sound kind of nice, because I could use a couple extra minutes a day, until you realize that satellite would also gradually creep outward because of some specific orbital mechanics. Eventually, it would smash into the Moon, which would almost definitely bring about some kind of Armageddon as debris rained down upon the Earth. Like I said… the moonlight probably isn't the thing we should be most worried about.

At the end of the day, of course the odds of the Earth acquiring another Moon-like satellite are very slim, but there is something to learn here. These kinds of scenarios remind you of how important the specific conditions around our planet are. Like, if our Moon had a different orbit or mass, our world could be much different.

So even though we usually take it for granted, maybe we should appreciate our Moon a little bit more… look up there and say like - hey buddy, thanks, you're really great. You're always helping us out. Thanks for being a friend.

But don't try and pick up a third friend. Just us. Just the two of us.

Building castles in the sky. Understanding how a second moon would affect the Earth depends on an understanding of gravity and other physics. And if you're looking to build up your STEM skills and learn how to solve problems like a scientist,.

Oribel's VertiPlay Marble Run is one place to start. The Marble Run is a “toy system” that offers a way to explore concepts like gravity and propulsion by building something yourself and learning through trial and error. It comes in a modular set, so you can always start small and add to your creation later.

The Marble Run is perfect for kids five years or older —or even adults who are up for a fun challenge. No matter who you are, it will help you master basic. STEM skills and expand your creativity.

If you want to give it a try, you can get 15%t off any VertiPlay Marble Run set or extension. Just click the link in the description and use promo code SCISHOW15. [♪ OUTRO].