How the Ancients knew the Earth was round (Part 2 of What we can learn from Flat Earthers)

Flat Earthers can pique our curiosity about history. How DID the ancients figure out the Earth was round? It’s not like they used giant catapults to launch themselves into space for a look-see.

Picture It… Alexandria, 240 BCE

The world of the Mediterranean in the age of Antiquity was booming with trade, commerce, and the exchanging of ideas. Here lived a scholar named Eratosthenes of Cyrene (c.276–c.195 BCE). In 255 BCE, after studying in Athens, Eratosthenes settled in the great city of Alexandria—the cultural crossroads of many civilizations [1-3]. There he became the director of the famous Library of Alexandria just a few decades after it was first founded by the Pharoh Ptolemy I Soter [4, 5].

Eratosthenes was a notable scholar of many fields. He wrote about ethics, astronomy, mathematics, and even poetry. Reportedly, a rival nicknamed Eratosthenes “Beta” after the second letter of the Greek alphabet, because it seemed Eratosthenes was the second best at everything. I guess my rivals might call me “Omega” because I am usually dead last in everything, except for being and a mega pain in the ass (just ask my spouse) [1-3].

Eratosthenes’ most notable contributions were (re)confirming the Earth’s shape (long suspected to be round) and calculating the Earth’s size using only shadows, footsteps, logic, and some mathamasurgery [1-3].

One day, Eratosthenes read that no shadows could be seen at noon on the summer solstice in a city named Seyne (modern-day Aswan, south of Alexandria). However, shadows could be seen in Alexandria on the same day and time. Eratosthenes knew the only way this could happen is if the Earth is round [1-3].

Aristotle and Aristarchus

Now, some of y’all may be wondering, “hypothetically, what about a flat Earth with a small, close sun? How did the ancients rule out this possibility?

For one thing, the mast of tall ships would be the last part of the vessel to disappear over the horizon, which is excellent evidence for a curve [2].

But was this simply due to waves?

Not likely. Before Eratosthenes, Aristotle (384–322 BC) reasoned that the Earth was round. In Book II, part 14 of his work On the Heavens, Aristotle spoke of how the curve of Earth’s shadow can be seen moving across the Moon during lunar eclipses, and how different stars can be seen in Cyprus which cannot be seen further north. Aristotle reasoned the Earth had to be a sphere [6].

Eratosthenes also knew the sun must be large and distant, which was mathematically confirmed by a renowned astronomer and mathematician, Aristarchus of Samos (310 BCE–230 BCE). Aristarchus knew (regardless of Earth’s shape) the Moon orbited the Earth, and its phases were the result of the moon at different positions relative to the Sun and Earth.

The moon’s phases are a result of the moon’s ~27-day orbit around the Earth (a lunar orbit is roughly one month). (Note: Unlike Aristotle, Aristarchus believed in a heliocentric universe, where the Earth and other planets orbit the Sun. This contrasts with the geocentric model, where everything orbits the Earth. It took centuries before the heliocentric model was accepted, but the lunar phases can technically be achieved in both models).

Aristarchus knew that during the half-moon phase (the first or second quarter), the moon was ~90° with respect to the Earth and Sun.

During a lunar eclipse, the Earth is blocking the Sun, causing a temporary, moving shadow on the moon. Using this knowledge, he estimated the relative diameters of the Earth, Sun, and Moon.

Aristarchus calculated that the Sun was between 18 and 20 times further than the Moon. This is, of course, incorrect. The Sun is much further away, but this error was not totally his fault. The angles were too narrow for his instruments, but despite this, Aristarchus managed to confirm the sun was indeed enormous and far away [7, 8].

Determining The Earth’s Size

So after reading about the shadows, Eratosthenes knew THE only real answer was the Earth had to (still) be a sphere. But how big was it? What was the circumference of the Earth? That would take some clever ingenuity.

If vertical sticks gave no shadows at noon on the summer solstice in Syene, then that meant the sticks in Syene were pointed directly at the sun (or zero degrees). Eratosthenes measured the angle of the shadow in Alexandria for this same time and day, which was about 7.2 degrees (or about 1/50th that of a circle). If Eratosthenes knew the distance between the two cities, all he had to do was multiply that by 50, and he would know the Earth’s circumference [1-3].

So Eratosthenes hired someone to pace the distance between the two cities, and to their credit, they did a remarkably accurate job, counting about 5,000 stadia (or roughly 800 kilometers). Multiply this by 50, and you get 250,000 stadia, which is approximately 40,000 kilometers. Using nothing more than sticks, shadows, some footwork, sound reasoning, and math, Eratosthenes calculated the circumference of the Earth to within a few percent, around 240 BCE [1-3].

The flat earth idea has been debunked for more than two millennia by people using simple tools—long before anybody physically viewed the Earth as a complete sphere [1-3].

Next to Part 3

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