Explanation: Not only did the ancient Greek philosopher Eratosthenes (born and raised in what-is-now Libya) know the earth was spherical, he used that knowledge to measure the circumference of the earth - with two sticks and some carefully applied mathematical thought - to an astounding accuracy!

According to Cleomedes' On the Circular Motions of the Celestial Bodies, around 240 BC, Eratosthenes calculated the circumference of the Earth in Ptolemaic Egypt.[8] Using a vertical rod known as a gnomon and under the previous assumptions, he knew that at local noon on the summer solstice in Syene (modern Aswan, Egypt), the Sun was directly overhead, as the gnomon cast no shadow. Additionally, the shadow of someone looking down a deep well at that time in Syene blocked the reflection of the Sun on the water. Eratosthenes then measured the Sun's angle of elevation at noon in Alexandria by measuring the length of another gnomon's shadow on the ground.[9] Using the length of the rod and the length of the shadow as the legs of a triangle, he calculated the angle of the sun's rays.[10] This angle was about 7°, or 1/50th the circumference of a circle; assuming the Earth to be perfectly spherical, he concluded that its circumference was 50 times the known distance from Alexandria to Syene (5,000 stadia, a figure that was checked yearly), i.e. 250,000 stadia.[11] Depending on whether he used the "Olympic stade" (176.4 m) or the Italian stade (184.8 m), this would imply a circumference of 44,100 km (an error of 10%) or 46,100 km, an error of 15%.[11] A value for the stadion of 157.7 metres has even been posited by L.V. Firsov, which would give an even better precision, but is plagued by calculation errors and false assumptions.[12] In 2012, Anthony Abreu Mora repeated Eratosthenes's calculation with more accurate data; the result was 40,074 km, which is 66 km different (0.16%) from the currently accepted polar circumference.[10]