Mesopotamian cosmology was essentially a mythological tale, and the tale has some similarities with that told in Egypt. The universe was ruled by three gods, each with their separate domain. Heaven was ruled by Anu, the Earth and the waters around and below it were the domain of Ea, and Enlil was the ruler of the air in between (the names here are those used by the Babylonians; the earlier Sumerian names were different). Although Anu was thought of as a kind of father god, he only ruled the universe as part of a triumvirate, together with Ea and Enlil. Not unlike the Egyptian cosmology, the gods were descended from a primeval chaos of waters, in this case a mingling of salt water and sweet, associated with the goddess Tiamat and the god Apsu, respectively. Again in conformity with the Egyptian myths, what came to be the domains of Anu and Ea were originally tied together and only became separated after Enlil moved heaven away from the Earth. The Mesopotamian universe also included an underworld, ruled by a god or a goddess.
It is well known that the Mesopotamian civilizations came to include a sophisticated scientific astronomy, more highly developed than that of the Egyptians. In view of this, it is remarkable that the world picture of the Babylonians remained mythological and that their mathematical astronomy had almost no impact at all on their cosmology. The clay tablets do not discuss the shape of the Earth, but it was evidently thought to be a flat disc. There are only a few glimpses of astronomical knowledge in the creation myth known as Enuma Elish, the earliest known version of which was composed around the middle of the second millennium BC but is based on material going further back in time. One of these glimpses relates to the Moon as a timekeeping device. The Moon is portrayed as a god wearing a crown which changes in shape through the month, corresponding to the lunar phases. The young warrior god Marduk, city god of Babylon, not only organized the calendar, but also ‘bade the Moon come forth; entrusted night to her.’He ‘made her a creature of the dark, to measure time; and every month, unfailingly, adorned her with a crown. “At the beginning of the month, when rising over land, thy shining horns six days shall measure; on the seventh day let half [thy] crown [appear]. At full Moon thou shalt face the Sun. . . .[But] when the Sun starts gaining on thee in the depth of heaven, decrease thy radiance, reverse its growth.”