Chapter 13 The position of the stars in the third section

Books that record the positions of stars are called star catalogs.The earliest known star catalog in China is preserved in the "Kaiyuan Zhanjing". "Kaiyuan Zhanjing" is a work of the Tang Dynasty, and the star list it contains was measured by Shi Shen and his disciples during the Warring States Period, and there are 121 stars in total. The measurement of star positions was a routine work for ancient Chinese astronomers.As a standard star to measure other stars, the degree of entry and de-extreme of each star official's distance from the star has become the focus of each measurement.As the azimuth of the twenty-eight mansions, which are the coordinates of astronomical observations, astronomers of all dynasties paid more attention to its precise measurement.Li Chunfeng’s observations during the Zhenguan period of the Tang Dynasty (627-649 A.D.) found that the distance values ​​of the twenty-eight mansions were different from those of the previous dynasties, but out of some concerns, he still used the data from the Taichu calendar of the Han Dynasty. More than 100 years later, Yixing encountered the same problem. He did not doubt his measurement results and decisively adopted new data.Observations during the reign of Song Huangyou (1049-1054 A.D.) recorded the entry and de-extreme degrees of 345 star official distance stars, which was the most existing star catalog before the introduction of Western star catalogs in the early Qing Dynasty.In the Song Dynasty, Yao Shunfu made an observation during the Chongning period (1102-1106 AD) in order to compile the "Jiyuan Calendar". The observation accuracy was very high, the measurement error was only 0.15°, and the distance of the twenty-eight mansions was updated again.In the Yuan Dynasty, Guo Shoujing was proficient in both instrument manufacturing and astrometry. His observation accuracy was one step higher than that of Yao Shunfu: the average measurement error of the distance of the twenty-eight mansions was less than 0.1°.

After Chen Zhuo, astronomers of all generations paid little attention to other stars other than 1464 stars, but Guo Shoujing made systematic observations of these unknown stars and compiled them into star catalogs.Unfortunately, his star catalog has been lost for a long time.It was not until the past 10 years that someone found out a book titled "Sanyuan Lieshe Rusuo Quji Ji" from the Ming manuscript "Tianwen Huichao" in the rare book library of the Beijing Library, without the author's name.After research, it was confirmed to be Guo Shoujing's star catalog.Zhao Youqin, who was a contemporary of Guo Shoujing, created a new method of star observation, that is, using the time difference of transit to calculate the difference in right ascension of stars, which is completely consistent with the modern principle of meridian observation.

During the Southern and Northern Dynasties, Zu Chongzhi's son Zu Wei (geng Geng) discovered that the Polaris was not at the North Celestial Pole, but was more than one degree away from the North Celestial Pole. After 600, Shen Kuo measured more than three degrees. Shen Kuo attributed the difference between these two numbers to the inaccurate observation of Zu Yun. In fact, whether it is the change in the distance of the twenty-eight constellations or the polarity of the North Star, it is all caused by precession.Although ancient astronomers have discovered this phenomenon, and have tirelessly corrected, observed, and corrected again, no one has given any explanation for it, and no one has explored the real reason for this change.

A star map is a visual record of the positions of stars.There are roughly two types of ancient Chinese star maps. One is a schematic star map, which is often drawn on the inner walls of ancient buildings or tombs, such as the top of the tomb of Qian Yuanguan (guan Guan), the king of Wenmu in the Wuyue Kingdom of the Five Dynasties, and the Wuliang Temple in the Han Dynasty. Stars are engraved on the steles.This kind of star map is only for decoration, so it is relatively rough and simple, and it is not accurate at all.The astrology is often incomplete, and some are only part of the sky, or even one or two star officials.Another type of star map is used by astronomers, drawn to find and calculate the positions of the stars.Therefore, this kind of star map has high accuracy and the recorded astrology is relatively complete.However, judging from the known star maps, the second type of star map is far less than the first type, and there are almost no star maps before the Tang Dynasty.

The "Yueling Zhangju" written by Cai Yong (yong Yong) in the Eastern Han Dynasty describes the general structure of the Han Dynasty star chart. According to this text, the astronomical star chart at that time can be restored (see Figure 11).The star map is circular, centered on the North Celestial Pole, and the three red concentric circles outward are the inner gauge, the equator and the outer gauge.The inner gauge is equivalent to the declination circle at 55° north latitude, which means that the sky area within the inner gauge is always above the horizon and can be seen all year round.Outer gauge is equivalent to the declination circle at 55° south latitude, which means that the sky area outside outer gauge is always below the horizon and cannot be seen throughout the year.From the analysis of the degrees of the inner rules and outer rules, this star map was once used in the Central Plains.

The celestial sphere is a three-dimensional body.But ancient China has not yet mastered the technology of projecting it onto a two-dimensional plane.In Figure 11, the ecliptic, which is not equidistant from the North Celestial Pole, should be an ellipse, but it is drawn as a perfect circle.This distortion is more pronounced in circular star charts depicting astrology south of the equator.Around the Sui Dynasty, there appeared a long star map projected with rectangular coordinates, called a horizontal map.On the horizontal chart, although the astrology near the equator is close to reality, the astrology around the celestial pole is distorted.The best way to solve this problem is to draw them separately: use a horizontal map to show the astrology near the equator, and use a circular map to show the astrology near the celestial pole. A set of star maps drawn by Su Song in the Song Dynasty is a representative work of this method (see Fig. 12).This set of star maps comes from Su Song's "Xin Yi Xiang Fa Yao". The distance values ​​of the twenty-eight mansions marked in the map are the same as his observation records during the Yuanfeng period (AD 1078-1085), which shows that this star map is based on his actual Observations drawn.

The star map of the Qing Dynasty expanded the sky area to the vicinity of the South Pole. There are 277 star officials with 1319 stars in it, and 23 new star officials with 130 stars.Most of the newly added stars cannot be seen in China, and they are supplemented according to Western star catalogs.