Measure the Circumference of the Earth

How Do You Measure the Circumference of the Earth with a Stick and a Ruler

Aristotle, 400 B.C. Knew that Earth must be round because he saw that the planet cast a curved shadow on the Moon during an eclipse.

Greek, Erastosthenes (276 – 196 B.C.)
Erastosthenes heard of a well in Syene (what is now Aswan, Egypt) in which the Sun’s reflection could be seen on June 21, the longest day of the year. He figured that meant the Sun was directly over the well on that day. He also knew that Syene was pretty much due south of the city of Alexandrea. By measuring the shadow cast by an obelis (a tall carved stone) in Alexandria on June 21, he computed the length of two sides of a right triangle—the length of the shadow and the height of the obelisk. Then he figured the angle of the remaining side of the triangle—about 7 degrees, or 1/50 of a full circle—and knew that was how far the Sun was from being directly overhead in Alexandria.

Erastosthenes learned that it took a camel fifty days to travel from Alexandria to Syene. Since the average well-fed camel could cover 10 miles a day. He figured the distance between the two cities is 500 miles. Multiply that by 50, he came up with a circumference of 25,000 miles.

Fact: The Earth’s actual circumference is 24,901 miles at the equator.
Only 0.4% error.