Around 3000 BC, Babylonians astronomers made methodical astronomical observations of the heavens. Around 2000 BC, the Babylonians created the system of the zodiac to describe the positions of the planets. In the 8th century BC, they defined the day as starting when the Sun was at the highest point in the sky.

However, the Babylonian calendar remained chaotic throughout most of the first millennium BC due to the irregular insertion of random months. The Babylonian year apparently consisted originally of 12 months of 30 days, but sometimes made use of sightings of the crescent moon to name the beginning of a month. Under Nabonassar (747-734 BC), a fixed-length month of 30 days was used. The Babylonians finally systematized a strictly lunar calendar which began with the first visible crescent moon around 500 BC.