Fun Science Idea

Eratosthenes, a Greek geographer (about 276 to 194 B.C.), was able to calculate the size of the earth. At Alexandria, Egypt was a great library. There Eratosthenes read about a well in the city of Syene. On a certain day of the year and at a certain time of day, the entire bottom of the well was lit up with sunlight. At that time the sides of the well cast no shadows on the well's bottom. Eratosthenes reasoned that the sun must then be exactly over the well. While in Alexandria, Eratosthenes placed a pole in the ground so that it was exactly vertical. He measured the angle between the pole and the pole's shadow at the time the well's bottom was covered with sunlight. From that experiment Eratosthenes was able to accomplish his amazing feat.

From geometry Eratosthenes knew that the angle he measured was the same as the angle between the pole and the well. The angle was 1/50 of one full revolution. Alexandria was about 500 miles from Syene. So 500 miles would be 1/50 the distance around the earth.The distance around the earth would be 50 times 500 which equals 25000 miles.

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