astronomy (n.)

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.