The 12 Zodiac Constellation System. The 12-zodiac system in Mesopotamia traces its development from an earlier 18-constellation system (found in MUL.APIN) to a uniform 12-part zodiac by the late 5th century BC. The 12-sign system emerged from merging astronomical observation, a 12-month schematic year, and mathematical division of the ecliptic. This Babylonian mathematical zodiac, later adopted by Greek astronomers, was refined over centuries, with the final 12-sign format solidifying in the Neo-Babylonian period.