c. 500 BCE

The Earth Is Round!

Pythagoras (c. 570–495 BCE)

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

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 the early Greek civilization. Regardless, the issue would be proven in another 250 years or so by 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 their glorious photos of our beautiful, spherical, blue marble floating in space.

Above: One piece of evidence that the Earth is round is the curved shadow of the Earth cast into the Moon during a lunar eclipse, like this one observed from Greece in 2008. Main image: Engraving of the sixth-century BCE Greek philosopher and mathematician Pythagoras, among the earliest scientists to advocate that the Earth is a spherical body.