Eratosthenes measures the Earth
A shadow at noon in two Egyptian cities gives the planet's circumference to within a few percent.
- When
- Known to
- ± 20 years
- Era
- Classical, Holocene
Around 240 BCE Eratosthenes, chief librarian at Alexandria, notes that at noon on the solstice the sun is directly overhead at Syene but casts a measurable shadow at Alexandria. From that angle and the distance between the cities he computes the Earth's circumference — accurate to within a few percent, depending on the length of the stade he used. He also sketches a system of latitude and longitude and calculates the tilt of the Earth's axis.