Title

A Study on the Identification of the Pole-stars and the Meaning of “du” in Ancient Chinese Astronomy 

Author

Yi-long Huang 

Genre

Article  

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Abstract

    The effect of precession on the position of the celestial pole is considerable, causing it to move roughly 1.38s in a hundred years. This paper attempts to identify the pole-stars used by post-Han astronomers based on the various distance estimates between the pole and the pole-star in ancient literature. 

    Because few bright stars are located near the trajectory of the pole, the pole stars identified are sometimes quite faint. The pole-stars used in China before and after the Tang dynasty are HD 11112 (6.3mag) and HD 112028 (5.3mag), respectively. Even in the late 19th century, the role of HD 112028 was not replaced by the Polaris (2.0 mag; the present pole-star) which has been located closer to the pole during the Ch’ing dynasty. 

    In this study, it is also found that most ancient Chinese astronomers did not realize that the pole could move and that the pole-stars used by various dynasties might be different. The main reason for overlooking these phenomena could be due to the fact that there is no accurate enough star map or coordinate measurements available for the polar region stars in ancient China. 

    It is also argued in this paper that the unit du used in ancient Chinese astronomy had a totally different concept as “degree” in western geometry. While many researchers confuse that two terms mainly because they share the same character in modern Chinese and in some cases their values are quite close, we evince here that their values could be off by a large as ~100% in measuring the distance between the pole and the pole-star and in measuring the sizes of the moon and the sun.