How Eratosthenes measured the diameter of Earth. (Taken
from Cambridge Latin Course pg. 157)
"At
noon, when Eratosthenes had calculated that the sun was directly overhead
in Syene, he measured the length of the shadow of an object in
Alexandria. From this he
could calculate the angle A between the sun's rays and the object.
Since the sun's rays are parallel, by simple geometry angle B is
the same size as angle A. Knowing angle B and the distance between Syene
and Alexandria (which was north of Syene), he was able to calculate the
circumference of the earth.
"

Eratosthenes of Cyrene Born: 276 BC in Cyrene,
North Africa, now Liberia. He was a highly regarded mathematician
who worked in the Museum in Alexandria. He was the director of the
library of Alexandria for couple of decades. He was the first person
to accurately calculate the diameter of Earth using only simple
geometry. He also created "Sieve of Eratosthenes" a method
of identifying prime number. Eratosthenes also measured the tilt of
the Earths' axis by 23.5, which gave us the seasons. He was bind at an
old age and died of voluntary starvation in 195 B.C.

