Earth’s circumference was first accurately measured more than 2,000 years ago by the Greek astronomer Eratosthenes.
Eratosthenes heard that midday sunlight shines straight down to the bottom of deep wells, on the same day each year in the nearby town of Syene. This indicated that the Sun was directly overhead in Syene on that day. However, on the same day, sunlight fell only on the sides of the wells in Alexandria.
Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curved surface of Earth. By measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe.
On the day the Sun shone at the bottom of the wells in Syene, Eratosthenes measured the sun’s position in the sky over Alexandria.
It was seven degrees away from the zenith, which meant that Syene was seven degrees away from Alexandria. He then made several calculations considering this angle and the distance between Alexandria and Syene, which is about 800 kilometres.
There is only a difference of five per cent between the answer he got (42,000 km) back then and the value accepted today (40,075 km).
Picture Credit : Google