Almost everyone knows and accepts that Earth is not a flat disk but instead a slightly squished-at-the-poles sphere – after all, the ancient Greeks proved this over 2,300 years ago. But in recent years flat-Earthers have been empowered by both the number of "celebrities" lending their status to the cause, and the Internet allowing keyboard warriors to spread their views across a global stage.
Some flat-Earthers are going to quite the expense to prove their theories correct; one guy took an airplane flight, costing around $200, so he could use a level to prove himself correct. Another is building an actual rocket, costing in the realms of $20,000, to launch to space and prove it.
There are many ways to challenge their beliefs. We can discuss how the Sun would not set on a flat world. We can ask them how volcanoes and earthquakes would happen on this wafer-thin planet. Or maybe discuss if an asteroid impact could even flip it like a pancake.
But, if you are looking for a way to disprove it on a budget and without any maths, this is the list for you.
The Fault Is Not In Our Stars But In Ourselves
The simplest, cheapest way to show that the Earth is not flat is to look at the night sky. At any given time, certain stars and constellations will have a certain height on the horizon. Depending on your relative position on Earth you won’t be able to see some of them when others can. The classic example is that you can’t see Polaris, the North Star, from the Southern hemisphere.
As you move anywhere on Earth in a North-South direction you’d be able to see constellations moving up and or down in the sky, new stars appearing and old stars disappearing. There is no way for this effect to happen on a flat disk.
Stick It To The Man
The next step up in the scale of complexity (or if you don’t like to be out late at night) is to use sticks and shadows. To do this, you need two people at different distances from the equator and two sticks with the same length. You place the sticks in the ground and measure the length of the shadows at the same time. If the Earth is flat, the shadow will be the same. But since it’s not, the shadow length will be different.
This is not only a very straightforward way to prove the roundness of our planet. It can also allow you to measure just how big the planet is. This method, done with water wells rather than sticks, was used by Eratosthenes to calculate the circumference of the Earth back in the 3rd century BCE.
Weighing In On The Controversy
The material needed to do this is a piece of string and a little rock. Voilà! We've got ourselves a very precise instrument that can measure something pretty important: gravity. Now, the gravitational pull from a sphere is pretty easy to work out with our instrument. The rock always points directly down.
If we were on a flat Earth, the little rock would be pointing at the center of our disk-shaped planet, and the further we moved away from it (located at the North Pole) the wider the angle between the vertical and the string will be. The latest approach flat Earthers have used to disprove this method is claiming that there is no such thing as gravity. Because they really, really don't know how to deal with facts.
In Full Swing
If you make your little gravity detector a smidgen more sophisticated you can create a pendulum. And by using such an instrument you can demonstrate that the Earth is not only a sphere, but it's rotating too. This method was first performed by French physicists Léon Foucault in 1851. As a pendulum swings, the plane of its oscillations rotate and the time it takes to do a full circle depends on its latitude. This is aptly called Foucault’s pendulum. How could this happen if we were on a static disk? Well, it wouldn't.
The Insane Clown Posse Method
The Detroit musicians became meme-worthy in 2010 for asking “F#cking magnets, how do they work?” and magnets, or the magnetic field of the Earth, is by far my number one way to deal with flat-Earthers.
So our planet has a magnetic field generated by the motion of liquid iron alloy at the center of our planet. We can observe the field generated as it surrounds us and extends all the way into space, and you can follow the field lines (like you might have seen in school experiments with magnets) using a compass. Now the compass gives you a very specific direction, North-South, ending in two distinct points, which we call the magnetic poles. There is no way to create such a magnetic field on a flat disk.
They might claim that Earth is a ring magnet but that doesn't work. A ring magnet has a very specific magnetic field, where the direction of the magnetic field flips when you're inside the ring compared to when you are on the disk. Have you ever seen your compass suddenly point south while working north? And how would the magnetic field of such a ring be produced?
So, there you are. Feel free to use any or all of these if you want to challenge flat Earthers but definitely keep the last one in mind if you want to see them doing some mental acrobatics worthy of the Cirque du Soleil.