[SciShow Intro Plays]

Caitlin: Hey, Hollywood! If you’re watching this, I have an idea for you! You’re all about making disaster movies, right? Especially ones where astronomical events cause The End of the World As We Know It? Like, when the ocean currents got messed up and New York City froze? Or when the planets lined up, and Earth ripped open? Well, here’s my pitch: What if the Earth just stopped spinning?

It may sound implausible -- even to you, Hollywood -- but physicists /have/ actually considered what we could expect if our planet’s rotation were to stop. And I gotta say: It’s not pretty. And it may actually happen! But just... over the course of a couple billion years. First, let’s imagine that the Earth’s rotation just stopped suddenly -- like, because of an impact with some giant, planet-sized object. If that were to happen, and you somehow survived it, the first effects would be pretty obvious. Because, everything would be flying away at supersonic speeds.

That’s because Earth rotates at about 1700 km/h at the equator. And if the planet suddenly stopped, its atmosphere would keep going, creating a windstorm way stronger than anything we’ve ever experienced. Anything that wasn’t anchored to bedrock would be swept away... cities, forests, rock, and topsoil -- all just zooming sideways and crashing into each other. Meanwhile, the oceans would lurch into huge tsunamis. Just think how cool the special effects would be!

But the truth is, if Earth really were to stop spinning, it wouldn’t happen all at once. It would do it very slowly. And in fact, that process is already underway -- mostly because of the moon’s gravitation pulling on us, a phenomenon known as tidal braking. As a result, the length of a day on Earth is getting longer, by about 14 thousandths of a second, every 100 years. So theoretically, after several billion years, our rotational spin would decay enough that the Earth would just... stop. And OK, yes: By this time, the sun might have already swelled up to a red giant and devoured everything.

But, for the sake of argument: What would happen as the Earth’s rotation slowed way down? Well, most of the effects would come from a big shift in our planet’s equatorial bulge. See, the spin of the Earth creates a bulge around the equator -- an accumulation of mass that makes a cute little spare tire around the planet’s waist. If the spin slowed down, the first thing that would stop bulging would be Earth’s most fluid component: the oceans. Computer models predict that the oceans’ mass would retreat from the equator and move toward the poles -- where Earth’s gravity would be the strongest. This redistribution of water would completely submerge northern Europe, Russia, and parts of South America, while my home here in Montana would now be oceanfront property.

And, if that weren’t enough, there would also be changes that would happen on a more day-to-day basis. By which I mean, the whole idea of “a day” would be totally different. After all, Earth’s rotation controls how much daylight different places get around the world. If our rotation slowed down, parts of Earth would get much more daylight, and much longer nights. By the time our planet came to a dead stop, a “day” would be entirely controlled by our orbit around the sun: Daytime would be six months long, followed by a six month night.

But there’s one more major thing that would change -- something less tangible than where the oceans are, or the length of our day. But in terms of our planet’s habitability, it’s probably the most important thing. Without its rotation, the Earth would no longer have a magnetic field. We’re one of the few rocky planets in the solar system that’s lucky enough to have a nice, robust magnetic field, or magnetosphere, around us. This magnetosphere protects us from cosmic radiation, and a lot of the most dangerous radiation from the sun. And we owe its existence, in part, to our planet’s outer core of metallic rock.

Molten metal inside the Earth rises and cools and then sinks again, creating convection currents, while also spinning like a dynamo, to create electrical and magnetic energy. If the Earth stopped spinning, this magnetic field would gradually fade, exposing us to more and more cosmic radiation until life became -- scarce, if not impossible.

As a cautionary example -- you know who used to have a magnetic field, and then lost it? Mars. Astronomers think that, if Mars was ever habitable, it was four billion years ago, when its rocks show that it had a strong magnetosphere. And then, when that magnetic field disappeared, so did its atmosphere and, as far as we can tell, its ability to sustain life. So, there’s a disaster scenario for you, Hollywood -- one that’s totally scientifically plausible but also, in its own way, butt-clenchingly terrifying. You’re welcome!

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