How to debate a flat-Earther

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

A number of ancient cultures believed that the Earth was flat because, simply, they didn’t know any better. But incredibly, there are people today who still believe that the Earth is flat, despite centuries of evidence proving the contrary.

So, why do people believe this, and is it even worth getting into a debate over?

Surprisingly, while there are mountains of proof, the discussion around the idea of a "flat Earth" has nothing to do with evidence at all.

Related: How big is Earth?

You can see Earth is round

To put it bluntly, we know more about the curvature of Earth than almost any other topic in the realm of physical science. There are so many experiments, observations and demonstrations that have, time and time again, revealed the curve of the Earth.

And it all starts with the horizon.

As objects recede from you, they begin to look smaller and slowly disappear in a very unique way: first their bottoms become hidden, and then their tops. If you've ever watched a ship on the horizon, you've seen this for yourself. Similarly, from a great distance, the tops of tall objects like mountains are visible well before their bases.

And Earth's curvature is clearly apparent from high altitudes, as Capt. Albert Stevens of the U.S. Army Air Corps showed in the 1930s. In December 1930, for example, Stevens snapped a photo looking westward while flying at an altitude of 21,000 feet (6,400 meters) above Villa Mercedes, Argentina.

"The Andes Mountains, 287 miles [462 kilometers] away, and although taller than the plane's altitude, lay below the sensible horizon, marked by the white horizontal line in the photograph," NASA officials wrote in a description of the flight. "The Earth's curvature explains this phenomenon, as described in the diagram accompanying the photograph. The Earth's curvature is also visible laterally in the photograph, although the effect is subtle as the image encompasses only 1/360 of the Earth's circumference."

And in November 1935, Stevens and Capt. Orvil Anderson took a photo from a balloon that lifted off from Rapid City, South Dakota and soared to a maximum altitude of 72,395 feet (22,066 m).

"The photograph showed the troposphere-stratosphere boundary and the actual curvature of the Earth and demonstrated the potential for long-range reconnaissance from high-altitude balloons," NASA officials wrote.

Earth's atmosphere is capable of playing funny tricks on our eyes, with different layers of air bending light into interesting directions. This phenomenon, a side effect of Earth's curvature, isn't a sure-fire guarantee of our planet's curve, but it’s a start.

But even if you can't look to the horizon for evidence, you can look up.

Different stars are visible from different parts of Earth, in two very peculiar ways. First, there is the division between the northern and southern hemispheres. So, you can see Polaris, the star nearly directly above Earth's north geographic pole, quite easily in northern latitudes.

But as you travel south, approaching the equator, Polaris sinks lower and lower toward the horizon. Once you’ve crossed that boundary, you can't see it at all — it's blocked by the curve of Earth in that direction.

Similarly, as you travel south, new constellations await your delighted gaze — ones that would be completely obscured by Earth's curve if you stayed up north.

There's another trick you can play, too. If you live in an especially flat area, you'll be able to see stars down to the horizon but no farther (because Earth is in your way). But if you travel up — say, to the top of a mountain — you get a better vantage point and can see stars farther down than you could before.

In fact, the ninth-century Abbasid Caliph al-Ma-mun sent an expedition to do exactly that and used those observations to measure Earth's circumference.

Related: Earth's atmosphere: Composition, climate & weather

Circles everywhere

You might not be able to mount such a scientific investigation to your nearest mountain peak. But there is something you can do to witness the curvature of Earth in the comfort of your own backyard. You just have to be lucky.

During a lunar eclipse, Earth passes between the sun and the moon, allowing Earth to cast its shadow on the moon. That shadow is always, always, always a circle, no matter where you are on the planet, no matter the timing of the eclipse. Always.

The only way to always cast a circular shadow is if the thing casting the shadow — in this case, Earth — is a globe. It's a matter of geometry.

And that's not to mention the countless photos of Earth taken by orbiting satellites and eyewitness testimonies from astronauts hailing from dozens of different countries, space programs and private organizations.

Our curved Earth also aligns perfectly with all of physics. Additionally, all of the other planets ever discovered also appear round, because that's how gravity likes things.

If you use gravity to, say, trust your GPS to give you accurate positions and calculate trajectories, then that same force will form material the size of Earth into a ball.

Related: What is the temperature on Earth?

Reasons behind the belief

I don't think this discussion is really about the actual evidence or the scientific process, however.

People who believe that Earth is flat aren't coming to that conclusion from the same types of observations. Instead, they believe that we are being misled and lied to, that scientists (including me) want you to believe that Earth is round, despite its flatness.

So the question isn't "Why do people believe in a flat Earth?" but rather "Why do people believe in a conspiracy?" And the answer is the same reason it always is: a lack of trust.

Many people don’t trust the society around them, most notably the representatives of that society. That trust often falls even further when it comes to elite representatives of that society, which include government officials, members of academia and scientists like me.

By claiming that Earth is flat, people are really expressing a deep distrust of scientists and science itself.

So if you find yourself talking to a flat-Earther, skip the evidence and arguments and ask yourself how you can build trust.

Additional resources

You can learn more about the first photographs that clearly showed Earth's curvature in this NASA story. This explainer from Arizona State University lays out clearly and concisely the plentiful evidence for our planet's spherical shape. (Nearly spherical, rather: Earth's rotation causes it to be slightly squashed at the poles and swollen at the equator, a shape known as an oblate spheroid.) And this piece by fivethirtyeight.com explains why people believe in conspiracy theories such as the flat-Earth idea.

Bibliography

• Bergreen, L. "Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe." William Morrow, 2003. https://www.amazon.com/Over-Edge-World-Terrifying-Circumnavigation/dp/006093638X
• Nataraj, N. "Earth and Space: Photographs from the Archives of NASA." Chronicle Books, 2015. https://www.amazon.com/Earth-Space-Photographs-Archives-NASA/dp/1452134359
• Novella, S. "The Skeptics' Guide to the Universe: How to Know What's Really Real in a World Increasingly Full of Fake." Grand Central Publishing, 2018. https://www.amazon.com/gp/product/1538760533/