Earth's circumference was first accurately measured more than 2,000 years ago by the Greek astronomer Eratosthenes, who at the time lived in the Egyptian city of Alexandria. He had heard that in the nearby town of Syene midday sunlight shines straight down to the bottom of deep wells on the same day each year, indicating that the Sun was directly overhead in Syene. In Alexandria, however, sunlight on that date never reached the bottoms of wells, but instead fell upon the sides. >>> src -> https://stardate.org/astro-guide/faqs/how-was-size-earth-first-measured

+ mindblowing

Terrence Tao has an old excellent presentation on the “cosmic ladder” which shows how from this measurement you can build up to measure the largest distances in the visible universe: https://terrytao.files.wordpress.com/2010/10/cosmic-distance-ladder.pdf