Eratosthenes discovered that you could measure the circumference of the Earth by looking down a well.
Eratosthenes was a Greek mathematician and astronomer who lived in the 3rd century BCE. He is credited with many important discoveries, including the concept of latitude and longitude and the measurement of the Earth's circumference.
Eratosthenes was curious about the size of the Earth and had heard that at noon on the summer solstice, the sun was directly overhead in the Egyptian city of Syene (now Aswan). He also knew that at the same time, in his hometown of Alexandria, the sun was not directly overhead but was at an angle.
Eratosthenes realized that the difference in angle between the two locations could be used to determine the Earth's circumference. He knew the distance between Alexandria and Syene was approximately 800 km.
Eratosthenes then measured the angle of the sun's rays in Alexandria at noon on the summer solstice and found it to be about 7.2 degrees from vertical. He assumed that the rays of the sun were parallel, and using simple trigonometry, he calculated that the circumference of the Earth was about 39,375 km, which is very close to the actual circumference of the Earth.
To understand how Eratosthenes was able to use a well to make this discovery, you must first understand that on the summer solstice, the sun is directly overhead in Syene. This means that if you look down a well at noon, you will see the sun's reflection in the water at the bottom of the well. Eratosthenes used this fact to measure the angle of the sun's rays in Syene.
By comparing the angle in Syene with the angle in Alexandria, Eratosthenes was able to calculate the Earth's circumference. His method was simple yet incredibly accurate, and it is a testament to his ingenuity as a scientist and mathematician.
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