The wise ancient Greek measured the circumference of the earth two thousand years ago. How did he do it? If you are interested, come and have a look with me. You are welcome to read.
How did ancient Greek mathematicians calculate the circumference of the earth
Ancient Greek scholars have several different views on the shape of the earth: some people think it is flat, disc-shaped or rectangular; But many people correctly believe that it is a circular sphere, because they judge that the arc shadow on the moon during the lunar eclipse is the projection of the earth. The famous mathematician eratoseni, who was then the curator of the Alexandria library, first tried to calculate the circumference of the earth with strict mathematical methods.
Eratoseni learned that at noon on the summer solstice every year, the sun shines directly at the bottom of a deep well in SAIN (now Aswan, Egypt); At the same time, in Alexandria, 5000 miles north of SAIN, the sun's rays have an included angle of 2 π / 50 radians (7.2 °) with the vertical line of the ground. Assuming that the sun's rays are parallel, according to the calculation formula of the circumference of the circle, the circumference of the earth along the meridian passing through the north and south poles is
Perimeter = 5000 × 2 π ÷ 2 π / 50 = 250000 Greek lire.
According to the unit of length of Athens, one Greek mile is equal to 185 meters, so the circumference of the earth is more than 46000 kilometers. According to the Egyptian unit of length, one Greek mile is equal to 157.5 meters, and the circumference of the earth is about 39000 kilometers.
Now people have accurately measured that the meridian circumference of the earth is 400008 kilometers. It can be seen that eratoseni's calculation was relatively accurate more than 2000 years ago. Eratoseni sieve method
Another important mathematical contribution of eratoseni was the invention of an effective method for screening prime numbers - later called "eratoseni sieve method".