An is the Sumerian word for 'heaven,' and is the name of the sky god who is the prime mover in creation, and the distant, supreme leader of the gods. He took over heaven when it was separated from earth (Ki), creating the universe as we know it.
In the theory of the three superimposed heavens, An occupies the topmost heaven. The 'way of Anu (Babylonian An)' is the vertical band of the eastern horizon, between the ways of Enlil and Enki, which lies to its north and south respectively.
Although in almost all periods one of the most important of Mesopotamian deities, An's nature was ill-defined and, as he is seldom (if ever) represented in art, his specific iconography and attributes are obscure. His symbol is sometimes represented by the horned cap, but that has become an attribute of all the deities.
(mostly taken from 'Gods, Demons and Symbols of Ancient Mesopotamia' by Jeremy Black and Anthony Green. University of Texas Press, 1997)
The Power of the Sky
An ranked highest among the gods. His name, borrowed by the Akkadians as Anum, is the Sumerian word for “sky”, and inherently An is the numinous power in the sky, the source of rain and the basis for the calendar since it heralds works and celebrations. Originally, one may surmise, An belonged to the herders’ pantheon since he is often visualized in bovine form.
An’s spouse was the earth, Ki, on whom he engendered trees, reeds, and all other vegetation. A late Akkadian incantation refers to this when it says: “As the sky impregnated earth (so that) vegetation became plenteous,” and in the myth Lugal-e, which dates from the end of the third millennium, the opponent of Ninurta, Azag, king of plants, was so engendered:
...An impregnated the verdant earth (Ki) and she bore him one unafraid of the warrior Ninurta, Azag.
Another name for Ki -probably an early loan from Akkadian- was Urash, “The tilth.” as father of Enki, god of flowing waters, An is paired in the list of gods with the goddess Nammu, who seems to be the power in the riverbed to produce water. There also seems to have been a tradition that saw the power in the sky as both male and female and distinguished the god An (Akkadian Anum) from the goddess An (Akkadian Antum) to whom he was married. According to that view the rains flowed from the sky goddess’ breasts, or (since she was usually envisaged in cow shape) her udder- that is, from the clouds.
An had not only engendered vegetation, he was father and ancestor of all the gods, and he likewise fathered innumerable demons and evil spirits. Frequently he was envisaged as a huge bull. One of his epithets is “Fecund Breed-Bull,” an apt personification of the overcast skies in the spring whose thunder recalls the bellowing of a bull and whose rain engenders vegetation far and wide. As an older form of the god himself we should probably consider the “bull of heaven” which belongs to him and is killed by Gilgamesh and mourned by Inanna and her votaries.
Source of Authority
The view of An as a major source of fertility, the “father who makes the seed sprout,” engenderer of vegetation, demons, and all the gods, led naturally to the attribution of paternal authority to him. As a father he presides over the assembly of the gods, his children.
With the developing of social differentiation and the attitudes of growing respect and awe before the ruler, a new sensitivity to the potential in the vast sky for inducing feelings of numinous awe seems to have come into being. The sky can, at moments when man is in a religiously receptive mood, act as vehicle for a profound experience of numinous awe, as may be instanced in our own culture.
To the ancient Mesopotamians what the sky might reveal was An, its own inner essence of absolute authority and majesty- might reveal, but would not necessarily reveal, for in everyday moods the sky would be experienced apart from the numinous power in it and would recede into the category of mere things.
The absoluteness of the authority divined in An may be seen clearly in statements that make him the fountainhead of all authority and authoritative commands, whether parental, lordly, or royal, in the myth of the elevation of Inanna the gods address him saying:
What thou hast ordered (comes) true!
The utterance of prince and lord is (but)
what thou hast ordered, doest agree with.
O An! thy great command takes precedence,
who can gainsay it?
O father of the gods, thy command,
the very foundation of heaven and earth,
what god can spurn (it)?
The passage as we have it probably dates from the second rather than the third millennium BC and so may conceivable bring the powers in An to a sharper point than older materials. Yet it is clearly a piece with them. What it says is that all authority, that of prince or lord, derives from An; he is its source, it carries out his will.
Since human society is not the only structure based on authority and command (the natural world as well), all things and forces in the polity that is the universe conform to An’s will. He is the power that lifts existence out of chaos and anarchy and makes it an organized whole. As a building is supported by and reveals in its structure the lines of its foundation, so the ancient Mesopotamian universe was upheld by and reflecting An’s ordering will. His command is “the foundation of heaven and earth.”
(taken from "The Treasures of Darkness" by Thorkild Jacobsen, Yale University Press, 1976)