c. 500 BCE

Earth Is Round!

Pythagoras (c. 570–495 BCE)

We take it for granted: the Earth is a beautiful, blue, spherical marble set against the blackness of space. But without the relatively recent benefit of being able to go out into space and look back, someone had to advance the idea that the Earth might be round rather than flat, as it appears to anyone on the ground. By many accounts, that someone was Pythagoras of Samos, a sixth-century-BCE philosopher, mathematician, and part-time astronomer from Greece, also famous for his Pythagorean theorem in geometry.

One piece of evidence that the Earth is round is the curved shadow of the Earth cast onto the Moon during a lunar eclipse, such as this one, observed in 2008 from Greece.

The argument made by Pythagoras and his followers for a spherical Earth was an indirect one based on a variety of observations. For example, sailors traveling south from Greece reported seeing southern constellations higher in the sky the farther south they went. Expeditions that departed for destinations along the African coast south of the equator, for example, reported that the Sun shone from the north rather than from the south (as it does in Greece). Another important piece of evidence came from observing lunar eclipses: when the full Moon passes directly behind the Earth relative to the Sun, the Earth’s curved shadow is clearly visible as it eclipses the Moon.

It is a matter of some debate whether Pythagoras himself actually “discovered” that the Earth is spherical or whether he was simply the most outspoken (and famous) advocate of what was becoming relatively common wisdom among educated people of early Greek civilization. Regardless, the issue would be proven in another 250 years or so in the experiments of Eratosthenes, and nearly 2,500 years later the first astronauts to leave Earth orbit, aboard the Apollo 8 mission, would share with the world the first glorious photos of our beautiful, spherical, blue marble in space.

Our precious blue planet is a sphere of rock and metal, covered by a thin layer of air and (in many places) liquid water. To our distant ancestors, it wasn’t obvious that the world was round.