How Eratosthenes calculated the Earth's circumference

in the mid 20th century we began launching satellites into space that would help us determine the exact circumference of the earth forty thousand thirty kilometers but over two thousand years earlier in ancient Greece a man arrived at nearly that exact same figure by putting a stick in the ground that man was Eratosthenes a Greek mathematician in the head of the Library of Alexandria Eratosthenes had heard that in Syene a city to the south of Alexandria no vertical shadows were cast at noon on the summer solstice the Sun was directly overhead he wondered if this were also true in Alexandria so on June 21st he planted a stick vertically in the ground and waited to see if a shadow would be cast at noon it turns out there was one and it measured about seven degrees now if the sun's rays are coming in at the same angle at the same time of day and a stick in Alexandria is casting a shadow while a stick in Syene is not it must mean that the Earth's surface is curved and Eratosthenes probably already knew that the idea of a spherical earth was floated by pythagoras around 500 BC and validated by Aristotle a couple centuries later if the earth really was a sphere Eratosthenes could use his observations to estimate the circumference of the entire planet since the difference in shadow length is seven degrees between Alexandria and Syene that means the two cities are seven degrees apart on earth's 360 degree surface Eratosthenes hired a man to pace the distance between the two cities and learn there were 5000 stadia apart which is about 800 kilometers he could then use simple proportions to find the Earth's circumference 7.2 degrees is one fiftieth of 360 degrees so 800 kilometers times 50 equals 40,000 kilometers and just like that a man 2,200 years ago found the circumference of the entire planet with just a stick and his brain you

Channel: Business Insider

Views: 639,304

Keywords: Business Insider, Earth's circumference, measurements, Earth, science, experiment, Eratosthenes, calculation, circumference, Ancient Greece

Id: Mw30CgaXiQw

Length: 2min 2sec (122 seconds)

Published: Sat Sep 10 2016