There were reasons for this: first, as tenmondo was then considered to be a special skill and was only passed down among family members (hereditary learning), the Imperial Court allowed it in order to preserve divination techniques, and second, a hereditary government office system had been established.
これは当時天文道が特殊な技術と考えられ、その学術が親子間の相伝（家学）となり、朝廷も天文占術技術の維持を目的としてそれを許容したこと、官司請負制の定着によるところが大きい。