Five Simple Ways to Prove the Earth Isn't Flat

Five Arguments To Use Against Flat-Earthers

The quickest answer: you can show the Earth is round without a rocket, a plane ticket or any maths. Watch which stars rise over your horizon as you travel north or south, compare noon shadows in two cities, swing a long pendulum, or read a compass's dip angle. Each test gives a result that only makes sense on a spinning globe, and each has been repeated for centuries by people with no stake in the argument.

Greek thinkers had worked out the shape of the planet by the fourth century BCE, and by the third century BCE Eratosthenes had a decent estimate of its size. Yet flat-Earth belief has enjoyed an odd revival online. Around 2016 and 2017, rapper B.o.B argued with astrophysicist Neil deGrasse Tyson on Twitter about it, and basketball player Kyrie Irving said in public that the Earth was flat before later backing away from the claim. Some believers spend real money trying to prove their case: one reportedly paid for a flight just to watch a spirit level on his tray table, and another spent years building his own rockets.

Below are five budget-friendly tests, roughly in order from easiest to most demanding, along with what each one actually shows and where the flat model breaks down.

1. Look up: the night sky changes as you travel

The cheapest test needs nothing but clear weather. Stars sit at fixed positions on the sky, yet the part of the sky you can see depends on where you stand. Polaris, the North Star, hangs about as high above the northern horizon as your latitude: low over the horizon in southern India, halfway up the sky in Paris, almost overhead in northern Norway. Cross into the Southern Hemisphere and it sinks out of view, while the Southern Cross and the Magellanic Clouds climb into sight.

On a globe this is exactly what you would expect, because the curve of the ground tilts your horizon as you move. On a flat disc, every observer would be looking up at the same dome, so a star visible from one spot would be visible, at least low down, from everywhere at the same moment. Southern observers would all see Polaris. They do not.

2. Compare shadows in two places

This test needs a friend some distance north or south of you and two identical sticks. Plant them upright at the same moment, ideally local noon, and measure the shadows. If the ground were flat and the Sun far away, the shadows would match. On a curved Earth the shadows differ, because each stick points in a slightly different direction relative to the sunlight.

This is a stripped-down version of what Eratosthenes did in the third century BCE. He knew that at noon on the summer solstice the Sun shone straight down a well at Syene (modern Aswan), while a vertical rod in Alexandria still cast a shadow at about 7.2 degrees, one-fiftieth of a full circle. With roughly 5,000 stadia between the two cities, the planet's circumference came out at about 250,000 stadia. Depending on which length of stadion he used, that lands close to the true figure of about 40,000 km. His own book is lost; the method survives mainly through the later writer Cleomedes.

Flat-Earth supporters sometimes argue that a small, nearby Sun could explain different shadows. It could explain two shadows, but add a third or fourth stick in other places and the numbers stop fitting a single nearby Sun, while they fit a distant Sun and a curved surface perfectly.

3. Let a plumb line point the way

Tie a small weight to a string and you have a plumb line, one of the oldest tools in building. Gravity pulls the weight toward the centre of mass of whatever you stand on. On a sphere, that centre sits directly beneath your feet wherever you go, so the string always hangs at right angles to level ground and water lies flat in every harbour.

On a disc, the centre of mass would sit in the middle of the disc. Near the centre a plumb line would hang straight, but the farther out you went, the more it would lean inward. Builders on every continent would find their walls tilting toward the North Pole, and oceans would slosh toward the middle like water in a tilted tray. Surveyors have measured plumb lines for centuries and found nothing of the kind, apart from tiny deflections near mountains, which themselves match the gravity of a slightly flattened sphere. Some flat-Earth writers respond by denying gravity altogether, which leaves everyday things like falling objects and the tides without any explanation.

4. Swing a Foucault pendulum

A heavy weight on a long wire can show that the planet is not only round but turning. In February 1851, physicist Léon Foucault demonstrated this at the Paris Observatory, then a few weeks later at the Panthéon, where a 28 kg bob swung on a 67-metre wire. As the hours passed, the plane of the swing slowly rotated relative to the floor.

The key detail is that the speed of that rotation depends on latitude, following the sine of the latitude. At the poles the swing turns through a full circle in about a sidereal day, roughly 24 hours. In Paris it takes around 32 hours. On the equator it does not turn at all. That precise pattern falls straight out of a rotating sphere. A flat, motionless disc offers no reason for the swing to turn, let alone at a rate that changes smoothly with distance from the equator. Many science museums still run a Foucault pendulum, so you can watch this for the price of a ticket.

5. Read what your compass is telling you

The last test comes with a pop-culture nod. In 2009 the Detroit duo Insane Clown Posse released "Miracles", whose line asking how magnets work became a long-running internet joke. Magnetism is in fact a useful clue, though it is the most technical of the five.

Earth's magnetic field is produced by currents in the liquid iron and nickel of the outer core, a process called the geodynamo. Close to the surface it behaves much like a bar magnet at the planet's centre, tilted about 11 degrees from the spin axis. A compass needle that is free to tilt shows this clearly: at the magnetic equator it lies flat, while toward the magnetic poles it dips more and more steeply until it points straight down in the north and straight up in the south. That smooth change in dip angle matches a dipole inside a sphere.

Some flat-Earth models treat the planet as a ring-shaped magnet with the Antarctic coast as an outer rim. A ring magnet produces a very different field, with directions that reverse depending on whether you are inside or outside the ring, and it gives no natural reason for a south magnetic pole near the middle of Antarctica, where expeditions have measured one. To be fair, magnetism alone is not a knock-out proof, because the field is messy and the poles wander, but taken with the other tests it fits only one shape.

Common flat-Earth claims, briefly answered

"The horizon always looks flat." Earth is so large that at eye level the curve is hard to see. What you can see is ships disappearing hull first over the horizon, and distant coastlines hidden from the bottom up, which is what curvature predicts.

"Photos from space are faked." Images come from many agencies and countries, including rivals such as the United States, Russia, China, India and Japan, and amateur balloon launches routinely capture the curve too. During a lunar eclipse, anyone can see the Earth's shadow on the Moon, and it is always round, as Aristotle pointed out more than 2,300 years ago.

"Water always finds its level." It does: it settles at right angles to local gravity, which on a sphere means a gently curved surface. That is the plumb-line test again.

None of these will necessarily change a committed believer's mind, since such beliefs often rest on distrust rather than evidence. Doing the experiments yourself, though, is a fine way to see how much ordinary observation can tell you about the world.

The rocket builder who wanted to see for himself

The man building his own rocket was the daredevil "Mad Mike" Hughes. In March 2018 he launched himself in a home-built steam-powered rocket to about 1,875 feet (572 metres) over the Mojave Desert and landed hard. He said he hoped one day to photograph the Earth as a flat disc from space, although after his death his publicist said the flat-Earth angle had been a publicity stunt, and people who worked with him disagreed about how sincere it was. On 22 February 2020, aged 64, he was killed near Barstow, California, when the parachute of another of his rockets deployed early and tore away during launch.

Sources: Based on an idea from IFLScience; Wikipedia: Eratosthenes; Roger Pearse: Cleomedes on how big the Earth is; Wikipedia: Foucault pendulum; Wikipedia: Earth's magnetic field; Physics Forums: field inside a ring magnet; Wikipedia: Mad Mike Hughes; FOX 35 Orlando, 24 February 2020.

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