Chapter 42 Section 2 Yu Xi discovers precession

Yu Xi discovered the precession, but due to the fierce opposition at that time, no direct documents were left.We can only know a little bit through the narration of Tang Dynasty astronomers and his party and Song Dynasty astronomer Zhou Cong.It turned out that in 330 AD, Yu Xi observed that the evening star at the winter solstice was the east wall, which is the wall constellation in the twenty-eight constellations.From this, he thought of the "short star Pleiades on the winter solstice in Yao's time" recorded in "Shangshu·Yao Dian", that is, the star in the evening of the winter solstice in Yao's time was Pleiades.That is to say, during the period from Emperor Yao to the Eastern Jin Dynasty, the winter solstice evening star has gone from Pleiades, passing through Weisu 14 degrees, Lousu 12 degrees, Kuisu 16 degrees, and retreated to Bisu 9 degrees, a total of 51 degrees.And Yu Xi calculated that he was 2,700 years away from Yao at that time, and from this, we can get about 53 years of precession by 1 degree.Therefore, he asserted that "the days in the ancient calendar have constant degrees, and the week of the sky is the end of the year, so the star degrees are related to the solar terms." If things go on like this, errors will occur. For the year", set up the precession, so that it is in line with the sky.

Yu Xi's discovery may have been inspired by the fact that the winter solstice has receded from the first day of the Altair to 21 degrees of Dousu, because the positions of the evening star and the sun are synchronized, but Yu Xi did not explain it.