How Do We Know the Earth Isn't Flat?

The Flat Earth Theory: some people still believe it. But how do we know the Earth is round? Science steps in to help.

Sabrina Stierwalt, PhD
5-minute read
Episode #251

The Earth is a sphere. We have known this since the ancient Greeks, but those who claim to disagree have persisted for just as long. One group, known as the Flat Earth Society, suggests the Earth is instead a flat disc surrounded by the Antarctic, a wall of ice around the edge, with the Arctic circle in the center. In this picture, the Sun and Moon move in circles above the Earth’s plane with stars orbiting in a plane just above that one.  Although a small minority, the idea of a flat Earth occasionally gains a little publicity, most recently thanks to rapper b.o.b. who famously picked a fight (and lost) with Astrophysicist Neil de Grasse Tyson.

Although there’s no actual scientific evidence to back up the flat version of the Earth, it does provide an interesting thought experiment. How, exactly, do we know the Earth isn’t flat? How do we know that we are all wandering the surface of a (slightly oblate) sphere as it hurtles through space around the Sun? It turns out that even though the portion of the Earth’s surface that we interact with everyday may appear flat, there are quite a few ways we can tell that the Earth is, in fact, round.

For starters, if you watch something or someone approach from a distance, they typically appear to fade into view. But most of the time, we are looking over distances that are a small of a fraction of the Earth’s circumference since inevitably something, like a mountain or a building, will block our view. If you could instead view that approach over a distance long enough so that the curvature of the Earth comes into play, what would it look like?

One of the longest range, unobscured views we have to test this scenario is the view of ships on the ocean’s horizon. On a flat Earth, we would expect the the ships to appear to fade into view just as objects over shorter distances do. Instead, however, the ships appearing on the horizon seem to rise out of the water, the result you would expect from a round planet. Next time you’re at the ocean, check it out.

Another clue that our Earth is round comes from the stars. Most of us know that we see different stars in the sky at different times. The summer constellations are different from those in the winter sky. On a flat Earth with stars orbiting in another flat plane above us, this might still be true, but we would expect each set of stars to pass over our spot on the plane eventually. Instead, we know that certain portions of the planet see entirely different stars.

For example, if you live in or have traveled to the southern hemisphere, you may have seen the constellations Carina (the Keel), Centaurus (the Centaur), the Southern Cross, or even our nearest extragalactic neighbors, the smaller dwarf galaxies known as the Large and Small Magellanic Clouds. These are all difficult if not impossible to spot from more northern latitudes. Certain groupings of stars or constellations near the equator, like the Big Dipper also appear “upside down” in the north or the south depending on which hemisphere you are from.

We also get a direct look at the shadow of the Earth cast onto the Moon during a lunar eclipse, when the Moon passes behind the Earth. That shadow, noticed by astronomers as far back as Aristotle, appears curved. While you can come up with orientations of a flat Earth that would also produce a curved shadow, they are highly specific. The curvature of the Earth during lunar eclipses viewed throughout the year and across the planet is always consistent, clear evidence that the Earth is spherical.

And speaking of shadows, philosophers and astronomers in ancient Greece used the shadows cast by objects on Earth not only to show that our planet is round, but also to measure its circumference. On a flat Earth, two identical objects will cast identical shadows as the Sun hits them at the same angle. On a curved Earth, the incoming light from the Sun will approach from different angles thus altering the resulting shadow. Around 240 BC, Eratosthenes, a resident of Alexandria, had heard from travelers that in a city in Egypt, the Sun cast no shadow in a well at precisely noon on the summer solstice suggesting that the Sun was directly overhead. He then measured the angle of the shadows in Alexandria at that precise date and time and found them to be angled at 7.2 degrees. Knowing the distance between Alexandria and the well in Egypt allowed for the calculation of the Earth’s curvature.

We also know the Earth must be spherical because millions of calculations that we rely on everyday are based on this idea. Our GPS satellites are launched into orbit and continue to accurately (at least when we have good cell service) determine our locations. Planes, especially those on transcontinental flights, are able to take off and land in accurately predicted locations. We base these predictions in part on the gravitational pull of the Earth which would be largely different for a flat plane (i.e. stronger towards the middle of the plane) versus a sphere (i.e. consistent across the surface).

Another huge clue for the roundness of the Earth is that we are able to quite literally go around it. The globe has been circumnavigated by ships as early as the 1500s. Planes can also travel around the world without stopping.

We also know the Earth is round because we have pictures of the Earth itself! Just as it would be hard to take a photo of your house from inside your bedroom, we have to leave the surface of the Earth to gain a firsthand perspective of its shape. By now, we have viewed the curved Earth from multiple angles, including the famous Earthrise photo from Apollo 8, revealing that it is not only round, but spherical.  The curvature can be seen from the International Space Station and some report seeing it from the now defunct Concorde jet.

Finally, we have circumstantial evidence to suggest that our planet is an orb and not a disk. Thanks to our outside perspective, we know the other planets, as well as our Moon, are all round. Our understanding of how planets form, an active area of astronomical research, is further deeply rooted in those planets being round. Pythagorus may have been the first to hypothesize that the Earth was round, based on the fact that the Moon was round, in 500 B.C..  

The universe also has a way of favoring easy symmetries – like spheres - that don’t emphasize the importance of one location over any others. History is full of examples of humanity trying to place ourselves in a more special spot but none of those have yet to pan out.

Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Image of Earth from the International Space Station courtesy of nasa.gov

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.