'Astronomy' or 'astrology': a brief history of an apparent confusion

The modern usage of the words 'astronomy' and 'astrology' is traced back to distinctions that are largely ignored in recent scholarship. Three interpretations of celestial phenomena (in a geometrical, a substantialist and a prognostic form) co-existed during the Hellenistic Period. From Plato to Isidore of Seville, the semiotic contrast is evidenced, and its later developments are sketched. The concept of astronomy is found to be rather constant and distinct from changing views about astrology.