Eli5: How did people in olden days (such as Galileo) come to the conclusion that the earth was spherical without very advanced technology?

In: Physics

Astronomy, shadows, math.

Using data collected from available sources like stars, moon, sun, shadows, distance between shadows, then do math.

A smart guy from Egypt figured this out a couple millennia before Galileo.

I would explain, but instead I’ll defer to the king of ELI5, Carl Sagan…

The first estimates were made by Eratosthenes, in about 240 bce.

Astronomical dates like midsummer and the solstice had been known for thousands of years. What Eratosthenes did was confirmed a rumour of a place where the midsummer sun perfectly illuminated the base of a well (the equator), then on the same date measured the length of shadow cast by a vertical pole at a location that was several hundred km away. By knowing the distance apart, and the fact that one place had vertical sun Ray paths, and the other Ray paths at an angle, he was able to use basic trigonometry to estimate the earth’s diameter. His value is within measurement error of the true value.

I imagine the earliest people to think about the earth being round wondered why they couldn’t see boats after a while as they sailed away from the beach. The horizon is only about 5 km/3 mi out if you are about 6 ft/180cm or so tall.

Every civilization that lived near an ocean knew the Earth is round. They can SEE the curvature on the water surface.

What ancient Greek had done is actually calculate its circumference.

Here’s a video explaining how simple experiments can show that the Earth is a sphere, without modern technology.

Shadow length difference from same height object at same time of day, at different points on the globe.