How do you find out radius of earth?

Answer 1

The Greek mathematician and astronomer Eratosthenes calculated this in 200 B.C.. He knew that on a particular day in Syene, the sun would be directly overhead. He set up a stick in Alexandria 5,000 stadia away (800-900 kilometres) on this day, and measured the angle of the shadow that the sun formed at midday. He measured this to be 7.2°. He deduced that this is the same angle from the Earth's centre that the distance between the two cities represents. Since there are 360° in a circle, he worked out that 5,000 stadia is one-fiftieth of the circumference of the Earth. He therefore suggested the value of 250,000 stadia (40,000-46,000km). Astronomers recently measured it using satellite equipment and submitted the figure of 40,008 km. From this, using the geometry of circles, he could calculate the Earth's radius: Pretty good for a man who lived 2,200 years ago who only used only a stick!