Ancient Greeks used geometry to measure the Sun's size with 0.2% precision
A stick, a well, and a ship revealed the Sun's true scale 2,500 years ago.
Elliott Thornley's essay on LessWrong walks through the ancient Greeks' step-by-step reasoning to estimate the Sun's size. Starting with evidence of a round Earth—lunar eclipse shadows and ships disappearing hull-first—they calculated Earth's radius using Pythagoras and a mast height. Then Eratosthenes used a vertical stick in Alexandria and a well in Syene at summer solstice noon to measure the Earth's circumference within 0.2% of its true value (40,000 km).
By cross-checking Earth's size from two independent methods, they confirmed the Sun is extremely distant. From its apparent angular size, they deduced it must be much larger than Earth. This demonstrates how primitive tools and clever reasoning can solve grand cosmic questions—a lesson still relevant in AI-driven science today.
- Eratosthenes' shadow trick estimated Earth's circumference at 40,000 km, accurate to 0.2%
- Disappearing-ship method gave a rough Earth radius via Pythagorean geometry
- Similar estimates from both methods proved the Sun is far away and thus physically larger than Earth
Why It Matters
Shows that fundamental reasoning, not advanced tech, can yield profound scientific insights.