This should start from ancient Greece in the 6th century BC. Students who have studied geometry all know Pythagoras, who discovered Pythagoras theorem. Pythagoras was the first person to put forward that the earth is a sphere, but he didn't prove it, but thought that the sphere is the most perfect shape in the universe, so the earth must be a sphere.
100 years later, Aristotle demonstrated the sphere theory, including: when there is an eclipse, the shadow of the earth on the moon is round; Walking at night, you will find that some stars rise from the horizon in front, and some stars disappear from the horizon behind; When you look at the seaside, you always see the mast first and then the hull. You see, the seemingly complicated and profound truth can actually be drawn through the observation of daily phenomena, but the premise is to think about the connection between things.
After Aristotle, the knowledge center of the world gradually shifted from ancient Greece to Alexandria, Egypt. This is because Aristotle taught an outstanding student-Alexander the Great. Alexander from Macedonia unified Greece, conquered Egypt, defeated the powerful Persian Empire, conquered the whole Middle East, invaded India, and even planned to attack China. After he conquered Egypt, he established the city of Alexandria where the Nile flowed into the Mediterranean, and built the library of Alexandria in the city. The goal of the library is to collect all the books in the world, which is not bragging, because Alexandria is the hub of Mediterranean trade, and merchant ships from all over the world have to stop here and every ship has to be searched. Whoever finds a book must copy it before releasing it.
Eratosthenes, the librarian of Alexandria Library, measured the circumference of the earth in the 3rd century BC. His method is similar to Aristotle's, which uses simple theorems and logic to solve seemingly inscrutable problems.
Eratosthenes heard that Thain (today's Aswan) in the south of Alexandria is a place with direct sunlight, and there is no shadow on the summer solstice every year, but there is a shadow in Alexandria on the same day. Eratosthenes realized that this is also evidence that the earth is a sphere, and the circumference of the earth can be calculated accordingly.
At noon on the summer solstice, he measured the shadow length of the Alexander obelisk. The height of the obelisk and the length of the shadow are two sides of a right triangle. Knowing these two sides, we can calculate the degree of the inner angle of the right triangle, where the upper angle is the angle between the sun and the obelisk.
The following is a wonderful geometric reasoning, you can use the diagram to ponder. α is the angle between the sunlight and the obelisk. Since the light rays are parallel, the angle θ in the figure is equal to the angle α. Eratosthenes asked people to find a spot in Thain. Assuming that the earth is a sphere, then the obelisk and Thain extend to the center of the earth, and the final intersection angle is equal to θ, which is still the logic based on the simple principle of parallel lines. The rest will be easy. Eratosthenes asked someone to measure the distance from Thain to the Alexandria obelisk, and divide the distance by the ratio of θ to 360 degrees to get the circumference of the earth: 39,690 kilometers. This is only a few hundred kilometers away from the accurate data of 40076 kilometers measured today.
For a little sense of ceremony, Eratosthenes used the obelisk to calculate the angle of sunlight. In fact, the same effect can be achieved with a wooden stick. In fact, Eratosthenes established a geographical system that is still in use today by measuring light with wooden sticks.
If you find direct sunlight, you will find the Tropic of Cancer and the equator. The Tropic of Cancer measured in Eratosthenes is only half a degree away from today's precision measurement results. By determining the Tropic of Cancer, we can deduce the latitude of the Arctic Circle in the Antarctic Circle, so Eratosthenes drew the latitude for the earth and divided it into five temperature zones: the North Cold Zone, the North Temperate Zone, the Tropical Zone, the South Temperate Zone and the South Cold Zone. The latitude of each temperature zone he measured is almost the same as that measured today. At the same time, Eratosthenes also drew a meridian for the earth, measured the distance between the sun, the earth and the moon, revealed the law that the length of day and night changed with the latitude season, expounded the principles of tides and ocean currents, and so on. These are written in his two books: Revision of the Earth's Size and Introduction to Geography. Eratosthenes established a discipline system with a wooden stick.
/kloc-the map of the world drawn by people in the 0 th and 9 th centuries according to Eratostheny's works has to make people admire the power of knowledge.
Compared with the ancient Greek and Macedonian empires in the same period, the achievements of ancient China in astronomy, geography and mathematics were relatively low. Why is this happening? I think one of the reasons may be related to the premature centralization of power in China. Things studied by Pythagoras, Aristotle and Eratosthenes, such as geometry, algebra, astronomy and geography, are of no practical use. They all study purely out of the pursuit of rationality and perfection. It is said that a student asked Pythagoras what was the use of learning geometry. Pythagoras said contemptuously, please give this gentleman a silver coin to leave. He wants to know what geometry is for!
Then who can do such useless things? Only nobles. They don't worry about food, clothing, leisure and money. Since the Spring and Autumn Period and the Warring States Period in China, every vassal state has carried out reforms, the goal of which is to strengthen the kingship and weaken the nobility. With the deepening of centralization, the aristocratic class in China became weaker and weaker, and gradually disappeared. It is not excluded that some emperors like this useless knowledge, but generally speaking, the emperor certainly hopes that everyone can do something useful. You are not paid to study metaphysics all day.
At the end of Han dynasty, the aristocratic society in Wei and Jin dynasties was a bit like aristocratic society, but it was not exactly the same. Typical aristocratic society is associated with feudalism. After China was unified from the Qin Dynasty, feudalism was no longer the mainstream.