Statistical Estimation of Observation Dates for the Kitora Star Map and Relating Chinese Star Catalogs

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  • キトラ古墳星図および関連史料の成立年の数理的再検討
  • キトラ コフン セイズ オヨビ カンレン シリョウ ノ セイリツネン ノ スウリテキ サイケントウ

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Abstract

A star map with an elaborate appearance was discovered in 1998 on the ceiling inside the Kitora burial mound of Nara Prefecture (hereafter we call it the Kitora star map). From archaeological evidence, this ancient tumulus is considered to have been constructed between the end of the 7th century and the beginning of the 8th century. We had a chance to make anew positional measurements of the 28-xiu constellations depicted on this circular star map, with the purpose of inferring their observed time epoch. As sky positions of stars are subject to change due to the precession, we can estimate when the stars in the Kitora star map were observed. We adopted a statistical least-squares approach to minimize the mean positional shift for all the 28-xiu standard stars, where the positional shift of a star means the difference between its measured position and the one calculated using precession theory. For a confidence level of 90%, the confidence interval of the observation year for the Kitora star map was found to be [123BC, 39BC], or approximately 80BC ± 40. This obviously indicates that the star map is of Chinese origin. Thus, in order to investigate the relationship of the Kitora star map to ancient Chinese star catalogs, we made a statistical analysis of Shi-shi's Star Catalog, the oldest star catalog in China. By applying to it the same analysis technique as the one adopted for the Kitora star map, we obtained the confidence interval to be [65BC,43BC]. Comparison of this interval with that for the Kitora star map strongly suggests that the latter was drawn based on the Shi-shi's Catalog. A similar analysis of the Korean star map stone-inscribed in the 14th century, Ch'onsang Yolch'a Punyajido, also showed nearly the same time epoch. Finally we discuss the political and social background why the Kitora star map was produced in such an early time in Japan.

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