Chapter 6 Section 2 Equatorial Coordinate System

The astronomical equator is not the earth's equator, it is a large ring formed by the extension of the earth's equatorial plane and the intersection of the celestial sphere, called the celestial equator.Ancient Chinese astronomers divided a wide range of star belts including the celestial equator into 28 sky areas from west to east. These sky areas have a special term called the constellation (xiuxiu), with a total of twenty-eight constellations. Each night has a standard star for measuring other stars, called the distance star, so the starting point of right ascension in the traditional Chinese equatorial coordinate system is not one but 28.Since it is a standard star, the right ascension difference between the distance star and the adjacent distance star, also called the distance value in ancient times, must be measured first.In the astronomical annals of the Twenty-Four Histories, there are records of the measured values ​​of the twenty-eight constellations, but there are some discrepancies between the data of each generation.After excluding the factors of the improvement of measurement accuracy, it can be found that the precession is the root cause of the unidirectional change of the distances of each station.Because the ancients did not understand this truth, when they found that there was a significant gap between the original record and the new observation value, they could only passively change the new standard value.

Before the invention of the telescope, ancient people of course observed with naked eyes, so most of the distance stars they selected were bright and eye-catching. The name is Kei Suyi), and the distance star from the place is λ Orion (the middle name is Xisuyi). In the Chinese equatorial coordinate system, the position of a celestial body is expressed by the two equatorial coordinate components of depolarization and innocence.The depolarization refers to the angular distance from the celestial body to the north celestial pole, and the occlusal degree refers to the difference in right ascension between the celestial body and the first distant star on its west side (see Figure 3).For example, there is a sentence in ancient books that "Vega (α Lyra) enters Dou five degrees", which means that Vega is in the Dousu sky area, and the difference in right ascension from Dousu (φ Sagittarius) is 5 degrees.