The word Astronomy is used to denote the study of stars, planets and space. The word astrology is derided as the corrupted and non-scientific study of the effect that stars and planets have on human activities. The etymologies and histories show an interesting link. Both use the Ancient Greek word aster (plural: astron) meaning star. Astronomy adds the suffix nomos meaning law, custom or arrangement. Astrology adds the suffix more typically used to indicate the study of a subject: logos meaning word or thought.
How did one come to indicate science and one get relegated to the comics pages of newspapers? Astronomy is both the older word and the word that early on had its modern meaning. Largely synonymous during the Dark Ages, astronomy was the study of stars and space while astrology adopted the definition of the effect of the stars on human activity. From the 14th through the 17th centuries the words slowly diverged until by the mid-17th century astronomy stood alone as the study of stars and space.
Image of the Pillars of Creation courtesy NASA Hubble Program.