Chapter 12 Section 2 Sanyuan twenty-eight nights

The position of the stars is not "constant", but because they are so far away from the earth, their position changes are difficult to detect for hundreds of years or even longer.Therefore, when the ancients explored the laws of motion of the sun, moon and the five major planets, they took the "constant" starry sky background as a coordinate reference system. To establish this frame of reference, the characteristics of the distribution of stars must be clarified.Usually, the stars are divided into several constellations, called star officials, similar to the constellations we call now.The number of stars in each star official is different, ranging from one to dozens of stars. According to the shape of their composition, they are given similar names, for example, "chu" [chu Chu] three stars and "jiu" [jiu old ] Four stars, very similar to the real thing (see Figure 10).

Star officials with names are easy to remember, but there are too many star officials in China. The 1,464 stars summarized by Chen Zhuo belong to 283 star officials, which is still not easy to identify, which requires a higher level of division. "Historical Records·Tianguan Shu" once divided the visible starry sky into five major sky areas, called the Five Palaces.The middle palace refers to the starry sky near the North Pole. The sky other than the middle palace is based on the observation at dusk on the day of the vernal equinox. It is divided into four palaces according to east, west, south and north. Each palace has seven constellations, a total of twenty-eight constellations. , all star officials are included in the middle palace and the twenty-eight mansions, and become small units under the big unit.Although the number of star officials and stars has changed a lot after Sima Qian, the basic framework has been formed.The table below is based on Chen Zhuo's star list.

When the sun appears, due to the scattering effect of the atmosphere on the earth's surface, its brightness covers all the stars, making it impossible for people to judge its position.The ancients noticed that the phases of the moon essentially showed the positional relationship between the moon and the sun.For example, when the moon is full, the sun is opposite to the moon, and when the sun sets in the west, the moon rises from the east; during the first quarter moon, the phase difference between the sun and the moon is 90°, and when the sun sets, the moon should be overhead.Observing the position of the moon among the stars can reverse the position of the sun. Therefore, ancient China attached great importance to the study of the laws of the moon's movement.

How did the twenty-eight mansions form?Why is it twenty-eight and not some other number?Ancient India also had a history of dividing the yellow equatorial sky area with 28 parts, called 28 monthly stations.The word "Su" has the same meaning as "Yuezhan". The original meaning of Erbasu should be the twenty-eight camps in the moon's movement.In fact, the moon's sidereal cycle is 27.32 days. Assuming that the moon walks once a day, it cannot be said that it is not in line with the speculation.However, the distance values ​​of the twenty-eight mansions in China are not equal, the largest is 33°, and the smallest is only 1°. This unequal regulation obviously violates the meaning of lunar stations.However, traces of the division of the twenty-eight mansions at equal intervals can be found in the pre-Qin literature.What caused the unequal spacing of the twenty-eight mansions has always been a mystery.

The Zhonggong was later divided into three districts, namely Ziweiyuan, Taiweiyuan and Tianshiyuan.Yuan means wall.Since these three sky areas have star officials like walls, they are named this way. Allocating the starry sky to the twenty-eight mansions in the whole sky started from the "Historical Records·Tianguan Shu", but the division of the three circles and the twenty-eight mansions was not completed at one time, until the astronomical enlightenment book "Dan Yuan Zibu Tiange" in the Tang Dynasty, It is the first time to comprehensively summarize the visible starry sky throughout the day with the twenty-eight mansions of Sanyuan.