Ancient Greek scholars measured the Earth with remarkable accuracy using only shadows

Garfield credits the scholar Eratosthenes, working at the Great Library of Alexandria, with calculating the Earth's circumference using a comparatively simple method: measuring the difference in shadow angles cast by the sun at the same moment in two cities a known distance apart, then using that angular difference to extrapolate the planet's full curvature. The result came remarkably close to the modern accepted figure, achieved without any instrument more advanced than careful geometry and observation.

This achievement mattered enormously for how later mapmakers thought about the world, since it replaced purely speculative guesses about the Earth's size and shape with a genuinely empirical, testable measurement, embedding scientific method into geography centuries before most other fields adopted comparable rigor.

Garfield uses this episode to make a broader point about the book's approach: mapmaking has always been entangled with the scientific methods available in a given era, and sometimes those methods produced startlingly good results using tools that look primitive by later standards. Takeaway: rigorous method, not sophisticated instruments, is what let ancient scholars measure the planet with genuine accuracy.