Eratosthenes and the Size of the Earth (Mouse-over image to see
detail at Alexandria.)
Eratosthenes noticed that the Sun shown straight down a vertical well in
the city of Syrene (near present-day Aswan) at local noon on the summer
solstice. Back in Alexandria, he measured the length of the shadow
of a vertical stick at local noon on the summer solstice. Using the
length of the shadow and the height of the stick he was able to work out
the angle (or fraction of a circle) that the Sun's rays made with the stick.
Knowing the Earth was a sphere and using simple geometry, he was able to
work out that the fraction of a circle that the Sun's rays made with the
stick was equal to the fraction of a circle represented by the distance
from Syrene to Alexandria (see diagram). Using the known distance
between the cities he could then work out the size of the circumference
of the Earth (and of course the radius as well). The exact distance
units Eratosthenes used has been lost to history, but it is thought that
he was able to get the correct answer to within 20% of the modern measurement.
More than a 1000 years before Columbus, this Greek astronomer not only
knew the world was round but just how big it was as well!