c. 1200, "astronomy, astrology, scientific or occult study of heavenly bodies," from Old French astrenomie "astronomy, astrology," from Latin astronomia, from Greek astronomia, abstract noun from astronomos, literally "star-regulating," from astron "star" (from PIE root *ster- (2) "star") + nomos "arranging, regulating; rule, law," from PIE root *nem- "assign, allot; take." Perhaps originally with reference to mapping the constellations or movements of planets.
Þer wes moni god clarc to lokien in þan leofte, to lokien i þan steorren nehʒe and feorren. þe craft is ihate Astronomie. [Layamon, "The Brut," c. 1200]
In English, astronomy is earlier than astrology and originally included the senses now distributed over both words; the gradual differentiation was complete by late 17c. and astronomy came to mean exclusively "the scientific study of the heavenly bodies." See astrology.