Earth is round, and you can check it yourself. Watch a ship disappear bottom-first over the horizon, look at Earth's curved shadow on the Moon during an eclipse, or compare noon shadows in two cities, the way Eratosthenes did over 2,000 years ago.
A round Earth makes many different predictions that all check out, and a flat Earth would have to explain each one separately. The most convincing evidence is the kind you can test yourself.
Flat-earth belief isn't a religious position for most people, and this page doesn't link it to any religion. It's here because it's a clean example of the main skill this site teaches: how to test a claim yourself instead of trusting a source. Use the same method on any claim.
The seven classic tests
This list follows Live Science's "7 ways to prove the Earth is round."1 The last column says whether you can run the test.
| Test | What you see | Can you do it? |
|---|---|---|
| 1. Ships at sea | A ship's hull disappears below the horizon before its mast. Coming back, the mast appears first. Perspective alone can't do this. | Yes, from a harbor with binoculars |
| 2. Stars change with latitude | The Big Dipper vanishes as you go south; the Southern Cross appears. | Partly: compare star charts |
| 3. Lunar eclipses | Earth's shadow on the Moon is always curved. | Yes, at the next eclipse |
| 4. Climb higher | On a flat plane, a higher viewpoint wouldn't show new horizon. In practice you see farther. | Yes, a tree, building, or hill |
| 5. Fly around the world | You return where you started. The horizon's curve shows faintly at ~35,000 ft. | Possible, costly |
| 6. Weather balloon | A camera on a high-altitude balloon shows the curve. | Yes, with permits |
| 7. Compare shadows | Eratosthenes compared noon shadows in two cities and estimated Earth's size. | Yes, with a stick and a friend |
Experiment 1: The sinking ship
Enter how far away the ship is and how high your eyes are above the water. The calculator estimates how much of the ship the curve hides.
Math: the horizon is at about √(2 × 6,371 km × height). Anything farther is hidden by (distance beyond the horizon)² ÷ (2 × 6,371 km). Real air bends light slightly, which makes ships appear a bit higher than this; that effect (refraction) is real, and you should expect small differences from the calculator on a real day. The ship here is 40 m tall.
At 20 km with eyes at 1.7 m, the curve hides about 18 m of the ship: nearly half the hull and deck.
Experiment 2: Eratosthenes with a stick
About 240 BCE, a Greek scholar named Eratosthenes heard that in the city of Syene at noon on the summer solstice, the Sun lit the bottom of a deep well (so it was straight overhead). In Alexandria, 5,000 stadia to the north, a vertical stick cast a shadow at the same moment, at an angle of about 7.2°. That's 1/50 of a full circle. So the Earth's circumference must be 50 × 5,000 = 250,000 stadia.1
787.5 km is 5,000 stadia at 157.5 m each, one commonly used value. Historians aren't sure which length of "stadion" Eratosthenes used, so his exact accuracy is debated. The method is what matters: on a flat Earth, two sticks at noon would cast parallel shadows. They don't.
Find a friend about 500+ km north or south of you. On the same day, at local solar noon, each of you sets up a vertical stick (check with a plumb line) and measures the shadow's length. The angle is arctan(shadow ÷ stick). The difference between your angles, divided by 360°, is the fraction of the Earth between you. A school class in two cities can do this in an afternoon.
Experiment 3: Day and night on a globe
On a globe lit by one distant Sun, half is always in daylight and half in night. Move the Sun and spin the globe to see where it's day.
Flat-earth supporters say a small, nearby "spotlight Sun" over a disc could also make some places light and others dark. That's the strongest form of the claim, and it's testable. A nearby spotlight would make the Sun look larger and dimmer as it moves away, but measured sizes stay constant; and the angles of the sun from different places (Experiment 2) would give a Sun a few thousand kilometers away, while other measurements put it much farther. Each separate test (ships, southern stars, eclipses, flight times in the southern hemisphere) needs its own special explanation on a flat Earth. On a globe, one idea explains them all.
Sources
- Live Science, 7 Ways to Prove the Earth Is Round (Without Launching a Satellite): ships, stars, lunar eclipses, elevation, circumnavigation, weather balloons, Eratosthenes.
- More DIY tests: PBS NewsHour, 7 DIY experiments to prove the Earth is round; Star Walk, Proof the Earth Is Round; Big Think, Ask Ethan: how do we know the Earth isn't flat?.
- Earth radius (6,371 km) and the horizon formula are standard geometry. Refraction effects and the 157.5 m stadion are noted on the page. The "spotlight Sun" reply is a simplified summary; a linked scholarly source is still to be added.