Why Is the Earth Round? Why Can’t it be Flat?

(Last Updated On: May 11, 2020)
Earthrise, taken on December 24, 1968, by Apollo 8 astronaut William Anders.

Why Is the Earth Round?

Alright, this question assumes you know the Earth is a sphere. We could go into the evidence, like how we’ve been to space, how you can see the curvature of the Earth on the horizon, and how you can loop around the Earth without falling off the edge. Those are intuitive arguments, but we also know that the Earth has to be round. But what makes that work? Why exactly is the Earth round?

How Planets Are Formed

Part of the answer has a lot to do with how planets are formed and the way gravity works. Well… Most of the answer lies here, in honesty. 

Luckily, the way planets are formed is actually quite simple. There’s a bunch of space dust and stuff floating around. Normally this follows the formation of a starit’s just the leftover stuff that was left unused. By chance, the space dust collides, and eventually makes bigger chunks of space dust. Those bigger chunks of space dust continue colliding with other chunks, eventually making like… Space pebbles or something. 

This process continues until you get some really big chunks floating around. Alternatively, gases can freeze and form gas giants. 

But as it turns out, everything has its own gravitational pull (even you!). Of course, most objects on our scale are not anywhere near massive enough to create any meaningful pull. But that’s a different story in space, wherein these large chunks can pull other chunks towards their centers. 

What Makes Things Round?

Eventually, there’s a point where our space chunks’ gravity starts having a noticeable impact on themselves. Most of our celestial objects’ mass is concentrated at their centers–it’s why the cores of stars and planets are so dense. It’s also the reason the cores of planets and stars are hot–assuming they haven’t cooled down yet. There’s more to it, but suffice to say that the centers of planets and stars are generally where most of the mass is concentrated.

So what happens when a potential planet is so massive it starts affecting itself? Well it’s a simple product of gravity. Gravity acts on everything equally–it doesn’t discriminate. If it acts on everything a planet is made of, then all parts of the planet will have to be equidistant from the planet’s center. It’s why no matter where you are on Earth, you weigh about the same amount! 

So what’s the only shape where all points on the outside are equidistant from its center? If you guessed “sphere,” you have spatial awareness!

Yes, it’s a sphere. 

In fairness, you can get kind of non-spherical planets if they’re spinning really really fast. They’re still relatively spherical, they become ellipsoids. Which are like 3-D ovals.

What Happens if The Earth Is Flat?

Well you’re seeing the first big issue with having a flat Earth if you remember that gravity acts on everything equally, and that the center of the Earth is super dense as a result (so dense that it compresses solid metals into liquids!). 

So what does that mean for a hypothetical flat Earth?

Well, assuming gravity didn’t push the Earth back into a sphere (it would) but still keeps you on the Earth, nobody would be able to inhabit the edges of the disk. No, it’s not because that’s where the ice poles are.

Because if gravity is pulling you towards the center of a flat plane, you would start to feel gravity pull in directions that were… Not down the closer to the disk’s edge you were. That’s because well, you’re not in the middle. Which means the farther away from the middle of a flat Earth’s center you go, the more you’ll actually feel like you’re running uphill. It would be exactly like tying a bungee cord to  your wall and then running away from it. The bungee cord is gravity.

Basically, on a flat Earth, you would feel heavier depending on where you stood.

Also if the Earth was flat your cat would have already pushed everything off the edge. 

Earth and space are cool, so go look at some pictures here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.