The Twelve Western Zodiac Signs: Origins and Development in Astronomy and Astrology

The Western zodiac, composed of twelve signs, originated from Babylonian astronomy, which divided the ecliptic into twelve equal sections. This framework was transmitted to Classical Greece and Rome, where it crystallized into Hellenistic–Latin astrology and evolved into the “Tropical Zodiac,” fixing 0° Aries at the vernal equinox. The discovery of the precession of the equinoxes by Hipparchus (c. 2nd century BCE) and the systematic synthesis of Claudius Ptolemy in the Tetrabiblos (2nd century CE) established the foundations of Western astrological thought. This article traces the historical and astronomical development of the zodiac, explains its internal structure including the four elements, three modalities, planetary rulerships, twelve houses and aspects and provides the tropical zodiac calendar with date ranges for contemporary use.