court astronomer/astrologer (ritsuryō system)
Historical official title under the ritsuryō system, responsible for astronomy, astrology, and calendar-making. Archaic; no modern equivalent.
The tenmon hakase was responsible for calendar-making and astronomical observation under the ritsuryō system.
陰陽博士 was a similar ritsuryō-era official specializing in onmyōdō (yin-yang divination), while 天文博士 focused on astronomy and calendar-making.
Compound of 天文 (astronomy) and 博士 (learned person, expert; here a court title). The reading はかせ is an older form of はくし, preserved in historical titles.