STEVEN JEDAEL. Counting days in ancient Babylon: eclipses, omens, and calendrics during the Old Babylonian period (1750-1600 bce).(Under the direction of DR. JOHN C. REEVES) Prior to the sixth century BCE, each lunar month of the Babylonian calendar is believed to have been determined solely by direct observation of the new moon with the insertion of intercalary months arbitrarily dictated by the king and his advisors. However, lunar eclipse omens within the divination texts of the Enūma Anu Enlil, some which date to the second half of the Old Babylonian period (ca. 1750-1600 BCE), clearly indicate a pattern of lunar eclipses occurring on days 14, 15, 16, 20, and 21 of the lunar month—a pattern suggesting an early schematic structure. In this study, I argue that observed period relations between lunar phases, equinoxes, and solstices as well as the invention of the water clock enabled the Babylonians to become aware of the 8-year lunisolar cycle (known as the octaeteris in ancient Greece) and develop calendars with standardized month-lengths and fixed rules of intercalation during the second millennium BCE.