Today is the summer solstice so we should raise a glass to Eratosthenes (c.282-194 BC) who used the day to calculate the circumference of the earth.
Aristotle had proved Pythagoras' assumption that the earth was spherical but establishing longitude without accurate clocks was tricky. Eratosthenes, Head of the Library at Alexandria, drew a meridien from Alexandria to Syrene (modern day Aswan) in Upper Egypt. He calculated that Syrene was on the Tropic of Cancer (in fact it's 70km north of the Tropic). He discovered that at Syrene, a vertical stick cast no shadow at midday on the summer solstice whereas another stick at the same time in Alexandria cast a small shadow. The angle of the Alexandrian stick's shadow to the sun's rays was 1/50th of a circle ie 7.2 degrees. Eratosthenes therefore concluded that the distance from Syrene to Alexandria must be 1/50th of the circumference of the earth. He calculated the distance between the cities to be 5000 stades and therefore the total circumference to be 250,000 stades which is only 1% out from the statistic we recognise today: 40,075 km.