Before 2000, it used simple measuring tools to calculate the circumference of the earth. This man was Eratosthenes in ancient Greece (275- 194 BC).

Eratosthenes is a man of many talents. He is not only familiar with astronomy and geography, but also a poet, historian, linguist and philosopher. He was the curator of the Alexandria Museum.

Careful Eratosthenes found that about 800 kilometers away from the city of Thain in Alexandria (now near Aswan, Egypt), the sun always shines at the bottom at noon in summer, so when all people stand upright on the ground, there should be no shadow. However, Alexander standing on the ground has a short shadow. The angle formed by the sun Alexander and the shadow of an upright object. On the premise that the ball and the sun follow a straight line, the two imaginary geocentrics lead to two straight lines pointing to Thain and Alexander respectively, and the angle should be equal to the angle formed by Alexander's sunlight shining from the upright on the earth. According to the proportional relationship between similar triangles, knowing the distance between the two places can measure the circumference of the earth. The angle measured by Eratoseni is about 7 degrees, which is 50 degrees (360 degrees). It is estimated that the circumference of the earth is about 40,000 kilometers, which is almost the same as the actual circumference of the earth (40,076 kilometers). He is still between the sun and the earth1470,000 km, which is strikingly similar to the actual distance1490,000 km. This fully embodies Eratosthenes's theory and wisdom.

Eratosthenes was the first to use the name of "geography" instead of the traditional "local history" and wrote a three-volume monograph. This book describes the shape, size and distribution of the earth's land and sea. Eratosthenes also calibrated the physical principles and mathematical methods in the earliest stage of maps and established mathematical geography.