Over 2,200 years ago, a Greek librarian named Eratosthenes calculated the size of the entire planet using just a stick and its shadow.
Living from around 276 BC to 194 BC, Eratosthenes was the chief librarian at the great Library of Alexandria in Egypt, giving him access to a vast collection of knowledge.
He noticed a report that on the summer solstice, at high noon, the sun shone directly down to the bottom of a well in the city of Syene, casting no shadows. 
But in Alexandria, about 500 miles north, a vertical stick did cast a shadow at the exact same time.
Eratosthenes measured this shadow and found it created an angle of about 7.2 degrees. He realized this difference was due to the curvature of the Earth.
He reasoned that since 7.2 degrees is 1/50th of a full 360-degree circle, the distance between the two cities must also be 1/50th of the total distance around the Earth. 
By multiplying the distance between Syene and Alexandria by 50, he arrived at a figure for the Earth’s circumference.
His calculation was remarkably close to what we know today using modern satellites, with some estimates putting his accuracy within 2% of the correct figure.
Of course, his calculation relied on a few assumptions, like the exact distance between the cities and that they were on the same line of longitude, which weren’t perfectly accurate.
Still, his method demonstrated a brilliant understanding of our world and remains a landmark achievement in the history of science.