Stories and famous sayings of mathematicians

The careful Eratosthenes, the man who calculated the circumference of the earth, discovered: The city of Thain (now Egypt) is about 800 kilometers away from Alexandria. (near Aswan), the summer noon sun can shine all the way to the bottom of the well, so all upright objects on the ground should have no shadows at this time.

However, the upright objects on the ground in Alexandria cast a short shadow.

He believed that the shadows of upright objects were caused by the angle between the sunlight in Alexandria and the upright objects.

Starting from the two premises that the earth is a sphere and sunlight travels in a straight line, two straight lines are drawn from the imaginary center of the earth to the city of Thain and Alexandria. The angle between them should be equal to the angle between the sunlight in Alexandria and The angle formed by upright objects.

According to the proportional relationship of similar triangles, if the distance between two places is known, the circumference of the earth can be measured.

Eratosthenes measured the included angle to be about 7 degrees, which is one-fiftieth of the Earth’s circumferential angle (360 degrees). From this, it is estimated that the Earth’s circumference is about 40,000 kilometers. It is almost the same as the actual circumference of the earth (40076 kilometers).

He also calculated that the distance between the sun and the earth is 147 million kilometers, which is surprisingly close to the actual distance of 149 million kilometers.

This fully reflects the teachings and wisdom of Eratosthenes.

Eratosthenes was the first to use the name "geography". From then on, he replaced the traditional "local chronicle" and wrote a three-volume monograph.

The book describes the shape, size and distribution of land and sea on the earth.

Eratosthenes also used graticules to draw maps, and was the first to combine the principles of physics with mathematical methods and create mathematical geography.