Chapter 24 The fourth section explores the "Three Schools of Words and Heaven" that explore the structure of the universe

Our ancestors have been concerned about the structure of the universe for a long time, and have been exploring, and put forward various ideas in an attempt to explain the astronomical phenomena we see. The story of "Worries" reflects people's concern and an answer to this problem. The story of "Worries about Heaven" is recorded in "Liezi". The moon and stars will fall, destroying crops and killing people, and even the earth will sink.For this reason, he was so worried all day long that he couldn't eat or sleep, and didn't know what to do.A well-meaning man was worried that he would be ill, so he took him to ask someone who understands the principles of heaven and earth, and explained to him that the sky is made of gas, and everything is full of gas.The sun, moon, and stars are also made of gas, but they emit light. Even if they fall, the gas will still fall into the gas, and there will be no damage.But the ground is solid, filled everywhere, so it won't break.As a result, the unscrupulous people turned their worries into joy.

What is said here is actually a view on why the sky does not fall and why the earth does not sink.Regarding this issue, there were still many opinions in the pre-Qin period, such as the myths and legends about four turtles supporting the sky and sea turtles carrying the ground, and the water floating theory, air lifting theory, and sports theory put forward by various scholars.In the Han Dynasty, "Three Schools of Yantian were formed: one is called Gaitian, the other is called Xuanye, and the third is called Huntian" ("Book of Jin Astronomical Records"). Gaitian theory was put forward in the Western Zhou Dynasty, and it was believed that "the sky is round like a cover, and the earth is flat like a chess game" ("Book of Jin Tianwen Zhi").That is to say, the sky is round and the earth is square. The layout that people see in the Beijing Temple of Heaven is a reflection of this idea of ​​a round sky and a place.During the Warring States period, the theory of Gaitian was revised to "the sky is like a hat covered, and the law of the earth covers the plate" ("Book of Jin Tianwen Zhi"), that is, the sky is like a bamboo hat covered on it, and the earth is like an upside-down plate.In the Han Dynasty, Gaitian theory had further development.The "Zhoubi [bibi] Suanjing" written in the 1st century BC is the representative work of this theory.It uses complicated numerical calculations and the Pythagorean theorem to prove that the sky is semicircular and the earth is arched. Mathematical.

The origin of Xuan Ye's theory is unknown.This theory holds that "the sky has no substance, and looking up at it, it is far and infinite", "the sun, the moon and the stars are naturally floating in the void, and they must move and stop with breath" ("Book of Jin · Tianwen Zhi") .In other words, there is no tangible sky shell, the sky is formless and boundless.The distribution and movement of the sun, the moon, and the stars in the vast and boundless space are natural. They are not attached to tangible things, but according to their own characteristics, under the action of air, they are suspended in the air and run in the air. .This theory contains the idea of ​​the infinite universe, and the movement picture of the sun, moon and stars it depicts is relatively close to the universe we know today. Therefore, the influence on later generations is far less than that of Hun Tian.

The theory of Huntian originated in the Warring States period, and it developed greatly in the Han Dynasty, and it was gradually accepted by people.Zhang Heng, a scientist in the Eastern Han Dynasty, was the master of the theory of the armillary sphere, and his book "Notes on the Armillary Sphere" is the representative work of the theory of the armillary sphere.The book pointed out: "The sky is like a chicken. The celestial body is round like a bullet, and the earth is like a yellow chicken. It lives alone inside. The sky is big and the earth is small. There is water in the sky and the sky is like a yellow shell. Riding on the air to stand, carrying water to float." That is to say, the sky is regarded as a round shell, and the earth is a sphere in the middle of the sky shell.The Huntian theory also believes that the sun, moon and stars are attached to the celestial shell and rotate with the celestial shell around the axis passing through the north and south celestial poles, which is a theory of the universe structure centered on the earth.These three cosmic structure theories have been inherited and developed later, but the Huntian theory has the greatest influence and occupies a dominant position in ancient cosmic theories.