How we know that Earth isn't flat

Astronomers have known that Earth is a globe for thousands of years

It's a common line to hear: "Five hundred years ago, they thought that Earth was flat!" But it's also completely untrue. Astronomers have been able to prove our planet is a globe for well over 2,000 years, using methods you can recreate at home.

The first known reference to a round Earth is in ancient Indian religious texts. The Rigveda, first composed between 1500 and 1000 BCE, includes the verse "In the midst of the Universe, the spherical Earth stays". Unfortunately, though there is much evidence of ancient India's extremely advanced knowledge of astronomy, there is little surviving record of their methods.

For that, we must turn to the Greek 'natural philosophers. The first written evidence that Earth was a globe comes from Empedocles and Anaxagoras in around 430 BCE, both of whom noticed Earth's shadow appeared rounded during a lunar eclipse. You can look out for its curve yourself on 28 October, when the next lunar eclipse that's visible in the UK is due.

In 350 BCE, Aristotle added even more evidence. He noted that when a ship sailed beyond the horizon, the masts would remain visible after the hull had disappeared, as you would expect if it had sailed around a curved edge. If you're near a port you can do the same with modern ships, though you'll need to look for their upper decks, rather than sails.