Can you measure the Earth from a mountaintop?

That time, that place

Born in Central Asia, al-Biruni lived at the Ghaznavid court as something close to a hostage. Following the ruler's campaigns into India, he learned Sanskrit and recorded Indian learning and custom — and the effort to set down what differed from him without condemning it still shows.

Why this question

Measuring the Earth the way Eratosthenes did means observing from two distant cities at the same hour — sending people, pacing out the distance.

Al-Biruni looked for a way to do it alone, from one place. Climb a high mountain and look out to the horizon: because the Earth curves, the horizon sits a little below eye level. Measure that angle.

What was found

First he measured the mountain's height, calculating from the angle to its summit taken at two points on the plain. Then, from the summit, he measured how far the horizon dipped below level. Those two numbers give the Earth's radius.

His result came very close to today's. No caravan, no second city, no team — one man, in a day.

He noticed other things too. Seeing rounded, water-worn pebbles across the plain of the Ganges, he wrote that this had once been sea, and had become land as a river laid down what it carried.