Two thousand years ago, the wise ancient Greeks measured the circumference of the earth. How did ancient Greek mathematicians calculate the circumference of the earth? There are several different views about the shape of the earth. Some people think that the earth is a round disk or rectangle, but many people correctly think that it is a sphere. Because they judge that the circular shadow on the moon during the eclipse is the projection of the earth, they regard the earth as a whole, not a simple building. Eratostheny, a famous mathematician who was then the curator of Alexandria Library, first tried to calculate the circumference of the earth with strict mathematical methods. If the sunlight is parallel, then the circumference of the earth along the meridian passing through the north and south poles is =/50 = 250,000 Greek miles, which is calculated according to the formula of circumference. Egyptian unit 1 Greek plum equals 157.5 meters, which is about 39,000 kilometers around the earth. Now, people have accurately measured the meridian circumference of the earth as 40008 kilometers, in which most of the planets on the earth are located. Thus, Eratosthenes's calculation more than 2,000 years ago is relatively accurate. Another important mathematical contribution is the invention of an effective method for screening prime numbers, which is later called "Eratosthenes screening method".