Eratosthenes was a Greek polymath. He was a Mathematician, Geographer, Astronomer, Historian, Philosopher, Poet, and Theater Critic. One of his contemporaries called him beta, the second-best in the world in everything. But it seems clear that in almost everything he was alpha. He is best known for being the first person to calculate the size of the earth. Many others before him had done it, but their  methods of measurement was not archived and he was the first whose method of measurement was well preserved. This is how he found the circumference of the earth...


Alexandria and Syene (now Aswan) are two cities of Egypt. Aswan is in the southeast of Alexandria and the distance between them is about 800km. One day while reading he red in a book that, in  Aswan, at noon, on June 21, summer solstice, vertical sticks cast no shadows. This would happen when the sun is directly overhead and making zero degrees with the stick. It was an observation that someone might have easily ignored, but his presence of mind made it into a discovery. He asked himself, whether sticks in Alexandria cast a shadow at the same time. And, he discovered sticks do. He was startled to know that, how at the same moment a stick in Aswan could cast no shadow and a stick in Alexandria could cast a promising shadow.  He supposed that two sticks of equal length one in Alexandria, the other in Aswan at a certain moment, cast no shadow. He thought it is completely fine-provided the earth is flat. If the two sticks cast shadows of equal length, that would also make sense on a flat earth, and the sun ray's would then be inclined at the same angle to the two sticks. 





First person to measure the size of the earth.


But how could it be that at the same instant there was no shadow at Aswan and a pronounced shadow at Alexandria? The only possible answer, he imagined, was the surface of the earth is curved. And the greater the curvature, the greater the difference in the shadow length. He visualized that, sticks placed at different angles to the sun's rays cast shadows of different lengths. For the observed difference in the shadow lengths, the distance between Alexandria and Aswan had to be about 7 degrees along the surface of the earth. Seven degrees is something like 1/50th of 360degrees, the full circumference of a curve. He knew the distance between Aswan and Alexandria was approximately 800km, and 800 multiplied by 50 (800*50 = 40000) equals forty thousand Kilometers, so that must be the size of the earth. 

This is the correct answer. Eratosthenes' only tools were sticks, eyes, and brain. With them, he calculated the circumference of the earth with an error of only a few percent. A remarkable achievement for 2200 years ago. He was the first person to accurately measure the size of the earth.