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This story was originally published on Smarter Than That in 2008. We are republishing a lightly edited version on Popular Science in light of recent interest in the subject.

Humanity has known Earth is round for a few millenia, and I’ve been meaning to show more methods that prove the world is not flat. I’ve had a few ideas on how to do that, but recently got an interesting incentive, when Phil Plait, The Bad Astronomer, wrote about a recently published BBC article about “The Flat Earth” society. (Most recently, rapper B.o.B. went on a Twitter rant on the topic.) Phil claims it’s ridiculous to even bother rebutting the Flat Earth Society—and I tend to agree. But the history of our species’ intellectual pursuit is important and interesting. You don’t need to denounce all science and knowledge and believe in a kooky conspiracy theory to enjoy some historical factoids about humanity’s quest for space.

Earth from the ISS

Earth from the ISS

The curvature of the Earth is visible in this 2014 photo, which ESA astronaut Samantha Cristoforetti snapped from the International Space Station.

NASA/Samantha Cristoforetti

On we go, to the top 10 ways to know the Earth is unequivocally, absolutely, positively, 100% not flat!

1. The Moon

Now that humanity knows quite positively that the Moon is not a piece of cheese or a playful god, the phenomena that accompany it (from its monthly cycles to lunar eclipses) are well-explained. It was quite a mystery to the ancient Greeks, though, and in their quest for knowledge, they came up with a few insightful observations that helped humanity figure out the shape of our planet.

Aristotle (who made quite a lot of observations about the spherical nature of the Earth) noticed that during lunar eclipses (when the Earth’s orbit places it directly between the Sun and the Moon, creating a shadow in the process), the shadow on the Moon’s surface is round. This shadow is the planet’s, and it’s a great clue about the spherical shape of the Earth.

lunar eclipse

Since the earth is rotating (see the “Foucault Pendulum” experiment for a definite proof, if you are doubtful), the consistent oval-shadow it produces in each and every lunar eclipse proves that the earth is not only round but spherical—absolutely, utterly, beyond a shadow of a doubt not flat.

2. Ships and the horizon

If you’ve been next to a port lately, or just strolled down a beach and stared off vacantly into the horizon, you might have noticed a very interesting phenomenon: Approaching ships do not just “appear” out of the horizon (like they should have if the world was flat), but rather seem to emerge from beneath the sea.

But—you say—ships do not submerge and rise up again as they approach our view (except in Pirates of the Caribbean, but we are hereby assuming that was a fictitious movie series). The reason ships appear as if they “emerge from the waves” is because the world is not flat: It’s round.

Ant on an orange

What you would see if you watched an ant crawling toward you over a curved surface.

Moriel Schottlender

Imagine an ant walking along the surface of an orange, into your field of view. If you look at the orange “head on”, you will see the ant’s body slowly rising up from the “horizon” because of the curvature of the orange. If you would do that experiment with the ant approaching along a long road rather than a round object, the effect would change: The ant would slowly “materialize” into view (depending on how sharp your vision is).

3. Varying star constellations

This observation was originally made by Aristotle (384-322 BCE), who declared the Earth was round judging from the different constellations one sees while moving away from the equator.

stargazing on a round Earth

On a round planet

Stargazing on a round Earth

Moriel Schottlender

After returning from a trip to Egypt, Aristotle noted, “There are stars seen in Egypt and…Cyprus which are not seen in the northerly regions.” This phenomenon can only be explained if humans were viewing the stars from a round surface, Aristotle continued, claiming that the sphere of the Earth is “of no great size, for otherwise the effect of so slight a change of place would not be quickly apparent.” (De caelo, 298a2-10)

The farther you go from the equator, the farther the “known” constellations go towards the horizon, to be replaced by different stars. This would not have happened if the world was flat:

Stargazing on a flat Earth

On a flat plane

Stargazing on a flat Earth

Moriel Schottlender

4. Shadows and sticks

If you stick a stick in the (sticky) ground, it will produce a shadow. The shadow moves as time passes (which is the principle for ancient Shadow Clocks). If the world had been flat, then two sticks in different locations would produce the same shadow:

stick shadows on a flat Earth

Stick shadows on a flat Earth

Imagine the Sun’s rays (represented by yellow lines) hitting two sticks (white lines) some distance apart. If the Earth were flat, the resulting shadows would be the same length, no matter how far apart you place the sticks.

Moriel Schottlender

But they don’t. This is because the Earth is round, and not flat:

stick shadows on a round Earth

Stick shadows on a round Earth

Because the Earth is round, sticks placed at distant locations will throw shadows of different lengths.

Moriel Schottlender

Eratosthenes (276-194 BCE) used this principle to calculate the circumference of the Earth quite accurately. To see this demonstrated, refer to my experiment video about Eratosthenes and the circumference of the Earth.

5. Seeing farther from higher

Standing on a flat plateau, you look ahead toward the horizon. You strain your eyes, then take out your favorite binoculars and stare through them, as far as your eyes (with the help of the binocular lenses) can see.

Next, climb up the closest tree—the higher the better, just be careful not to drop those binoculars and break their lenses. Then look again, strain your eyes, and stare through the binoculars out to the horizon.

The higher up you climb, the farther you will see. Usually, we tend to relate this to Earthly obstacles—like the fact we have houses or other trees obstructing our vision on the ground, and climbing upwards we have a clear view—but that’s not the true reason. Even if you stood on a completely clear plateau with no obstacles between you and the horizon, you would see much farther from the greater height than you would on the ground.

This phenomenon is caused by the curvature of the Earth as well, and would not happen if the Earth was flat:

point of view on a flat Earth

Point of view on a flat Earth

How far can you see from a height? On a flat Earth, elevation doesn’t make a difference.

Moriel Schottlender

point of view on a round Earth

Point of view on a round Earth

How far can you see from a height? On a round Earth, elevation makes a big difference.

Moriel Schottlender

6. Ride a plane

If you’ve ever taken a trip out of the country, specifically long-distance trips, you could notice two interesting facts about planes and the Earth:

  1. Planes can travel in a relatively straight line for a very long time and not fall off any edges. They can also circle the Earth without stopping.
  2. If you look out the window on a trans-Atlantic flight, you can, most of the times, see the curvature of the Earth on the horizon. The best view of the curvature used to be on the Concorde, but that plane’s long gone. I can’t wait to see the pictures from the new plane by Virgin Galactic—the horizon should look absolutely curved, as it actually is from a distance.
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I do not doubt the existence of God., I doubt His responsibility towards the invention of the human diverse ways of worshiping. I.e Religion. But I am good though. Love to meet you too.

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