From Korea study of Cheonsang-yeolcha-bunyajido (天象列次分野之圖)
mean is -0.39 du(古度）
delta ( 1 SD) = 3.9 du
365.26 du = 360 degrees
some says 古度 is 渾儀上"定極環" 長度距離, cf 黃一農
The bigger errors in some asterisms may be due to the astronomical instruments, eg armillary sphere in use. The rings may obstruct accurate alignment , especially polar distance of 96 to 100 du
From Xiaochun Sun, Suzhou Astronomical Planisphere (苏州石刻天文图)of the thirteenth century, the error is less than 1.5 degrees
大統通占，用Fourier transform, 計算系統誤差：
一二四七年（淳祐七年）黄图在乎江府复刊，即现存的苏州天文图。崇宁年间观测到的记录，部分载入纪元历内，所测二十八宿距度星的平均误差绝对值只有0°.15
reference
極星箕古度考 - 黃一農
sun 孫小淳
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