Al-Biruni

Khwarazmian polymath of the Islamic Golden Age - astronomer, physicist, geographer, linguist (he read seven languages) - called the 'father of modern geodesy'.

Around 1020 he invented a new way to measure the Earth's radius: climb a mountain, measure its height, then measure how far the horizon dips below the horizontal from the top, and do the trigonometry. Carried out at Nandana (present-day Pakistan), his value - 12,803,337 cubits - works out to a radius within about 2% of the modern mean, nine centuries before modern surveying. He never corrected for atmospheric refraction (the dip he measured, 34 arc-minutes, is inflated by it), which is exactly the subtlety our $10 clinometer experiment still has to respect.

Built on Eratosthenes' spherical Earth and the trigonometry of the Islamic Golden Age. Enabled the horizon-dip method that navigators and surveyors still use today - the dip tables in nautical almanacs are his method, industrialised.