The rapid development of space technology enables people to obtain quite accurate data about the size of the earth. 1979, according to geodesy, Geophysical Association decided to use the following data to represent the size of the earth:
Equatorial radius of the earth (a): 6378.137 km;
Polar radius of the earth (b): 6356.752 km;
The equatorial circumference of the earth (2 π r): 40,075.7 kilometers;
Earth surface area (4 π R2): 5 10 100934 km2.
Understanding the size of the earth has also gone through a long process. With the continuous development of science and technology, people's understanding of the size of the earth is becoming more and more accurate.
Eratosthenes in ancient Greece was the first astronomer in the world to measure the earth's size data by scientific methods. He believes that the earth is a sphere. At noon, parallel sunlight shines on different parts of the same meridian, which will form different angles with the ground. In this way, the radian and arc length of the meridian of the earth can be measured.
More than 200 years BC, Eratosthenes directly calculated the size of the earth. He chose Thain (near Aswan today) and Alexandria, which are roughly located on the same meridian, as observation points to observe the noon sunlight from summer to sun. In Thain, the sunlight can directly hit the bottom of the deep well, which means that the sunlight shines vertically on the ground at this time, that is, the angle between the sunlight and the local vertical line is 0; However, in Alexandria, at the same time, the sunlight does not shine vertically on the ground, and there is an included angle of 712' between the sunlight and the local vertical line. Obviously, 7 12' is the latitude difference between Thain and Alexandria. And this angle (latitude difference between the two places) is exactly equal to 1/50(360) of a circle. Eratosthenes measured the actual distance between the two places. Then, according to these measured data, the circumference and radius of the warp circle are obtained by mathematical calculation. Eratosthenes thinks that the earth is a regular sphere, so the radius of this meridian is also the radius of the earth. According to this radius, data such as the surface area of the earth can be calculated.
Eratosthenes estimated that radius of the earth was about 6,200 ~ 7,300 kilometers more than 2,000 years ago. This data is very similar to modern measurement results. The basic methods and principles he used in measurement have been used by later generations. Until modern geodetic work, this basic principle is still used, but when measuring latitude difference, the observed celestial body is not the sun, but a star farther away from the earth, and the actual distance between the two places on the ground is measured by triangulation. Other stars in the universe are much farther away from the earth than the sun. From the earth's point of view, the sun is a round surface, while other distant stars are just a point of light. Therefore, it is more realistic to regard the light of very distant stars as parallel light than to regard sunlight as parallel light. Coupled with sophisticated observation instruments and advanced calculation technology, the calculated results are naturally much more accurate than those of the ancients.