Eratosthenes (276-195 BC) was the head of the Library at Alexandria, Egypt. In one of the most elegant experiments ever reported, he calculated the circumference of the Earth to be approximately 25,000 miles, what we know it to be today.

He used the following information:
1. He drew an imaginary line from the center of the Earth to both Alexandria and Syene.

   Image Source

2. Because of its distance, the light from the Sun travells in parallel lines toward the Earth.

   Image Source

3. The Sun, at noon, would be directly overhead a Well at Syene (present day Aswan). It would shine directly into the Well, casting no shadow. While at Alexandria, the same day and time (noon in Syene), the Sun would strike any vertical object, in this case an Obelisk, at a angle that could be measured. He measured the angle to be 7^o or 7^o/360 ^o; 1/50th of the circumference of the Earth. The 5000 stadia (500 miles) between Alexandria and Syene was, as well, 1/50th the distance around the entire Earth.

   Image Source

4. Since he knew that a line (a transversal: his lines from the center of the Earth) through two parallel lines (light striking the Earth) results in equal angles*, this 7^o angle also represented the angle formed by his imaginary lines drawn from the center of the Earth to both Alexandria and Syene.

   Image Source

   He calculated the circumference as: 7o / 360 o = 5000 stadia / circumference of the Earth; 1 / 50 = 5000 stadia / circumference of the Earth; Circumference of the Earth = 250,000 stadia (25,000 miles).

   *For a transversal that intersects two parallel lines, the alternate interior angles are equal.