He measured the whole planet with a stick and a shadow
Around 240 BCE, Eratosthenes — the chief librarian of Alexandria — noticed that at noon on the summer solstice the sun shone straight down a well at Syene but cast a shadow at Alexandria. From the angle of that shadow and the known distance between the two cities, he calculated the circumference of the entire Earth, landing within a small margin of the true figure — more than seventeen centuries before anyone actually sailed around the globe.
How we know
Eratosthenes of Cyrene, c.276–194 BCE, chief librarian of Alexandria; his solstice-shadow measurement c.240 BCE yielded a circumference within roughly 1–16% of the true value depending on the stadion used. First circumnavigation: Magellan–Elcano 1519–1522 CE, about 1,760 years later.
One of many fact-checked wonders across eight traditions at The Spring of Wisdom.