In view of astronomy's potential as a tool of political intrigue, in the Tang and Song dynasties China's rulers made it a crime to leak astronomical knowledge to the general populace, in a bid to prevent commoners from telling the fortunes of the imperial house. Astrology books were banned, the position of imperial astronomer became hereditary, and the appointment gradually lost importance.
天象作為政爭工具的結果，唐、宋之後，為防止民間預測皇家命運，乾脆阻止天文知識外流，星占專著成為禁書，天文官改為世襲，天文官興趣、能力因此逐步退化。