<P> Eratosthenes calculated the circumference of the Earth without leaving Egypt . He knew that at local noon on the summer solstice in Syene ( modern Aswan , Egypt ) , the Sun was directly overhead . ( Syene is at latitude 24 ° 05 ′ North , near to the Tropic of Cancer , which was 23 ° 42 ′ North in 100 BC ) He knew this because the shadow of someone looking down a deep well at that time in Syene blocked the reflection of the Sun on the water . He then measured the Sun 's angle of elevation at noon in Alexandria by using a vertical rod , known as a gnomon , and measuring the length of its shadow on the ground . 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 . This turned out to be about 7 ° , or 1 / 50th the circumference of a circle . Taking the Earth as spherical , and knowing both the distance and direction of Syene , he concluded that the Earth 's circumference was fifty times that distance . </P>
<P> Eratosthenes calculated the circumference of the Earth without leaving Egypt . He knew that at local noon on the summer solstice in Syene ( modern Aswan , Egypt ) , the Sun was directly overhead . ( Syene is at latitude 24 ° 05 ′ North , near to the Tropic of Cancer , which was 23 ° 42 ′ North in 100 BC ) He knew this because the shadow of someone looking down a deep well at that time in Syene blocked the reflection of the Sun on the water . He then measured the Sun 's angle of elevation at noon in Alexandria by using a vertical rod , known as a gnomon , and measuring the length of its shadow on the ground . 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 . This turned out to be about 7 ° , or 1 / 50th the circumference of a circle . Taking the Earth as spherical , and knowing both the distance and direction of Syene , he concluded that the Earth 's circumference was fifty times that distance . </P>
who measured the earth with sticks and shadows
who measured the earth with sticks and shadows
Eratosthenes
Eratosthenes