Yuzhao dingzhen jing 玉照定真經

Yuzhao dingzhen jing 玉照定真經 "Classic of determining the truth by jade reflection" is a book on astrology written by an anonymous author. It is traditionally attributed to the Jin-period 晉 (265-420) Daoist Guo Pu 郭璞 (276-324) and the commentary to Zhang Zhuan 張篆 or Zhang Dun 張礅.

The book is neither mentioned in Guo Pu's biography in the official dynastic history Jinshu 晉書, nor is it mentioned in any bibliography earlier than the Yuan period 元 (1279-1368). A stylistic analysis of the text and its commentary proves that both were written by the same person. It might, therefore, be that Zhang Zhuan was the author of both and, for "marketing" purposes, pretended that the main text was written by a famous authority as Guo Pu. There is virtually nothing known about Zhang Zhuan.

The Yuzhao dingzhen jing discusses the rules of the year (nianyi 年儀), the rules of the moon (yueyi 月儀), the six harmful days (liuhai 六害), the three extraordinary combinations (sanqi 三奇), the three junctions (sanjiao 三交), and the four apparitions (sixiang 四象), all with the purpose to use these phenomena for prognostication of good and bad luck. The literary quality of the book is not very high, and the composition of the text crude and not cohesive. It is nevertheless of a specific value for the study of the theories of ancient Chinese astrologers.

The original text was lost and was only reconstructed for the imperial series Siku quanshu 四庫全書 on the base of quotations in the Ming-period 明 (1368-1644) encyclopaedia Yongle dadian 永樂大典. Is is also part of the Siku quanshu zhenben 四庫全書珍本.