An created Heaven and Earth and was the supreme god at the beginning of times. When the younger gods took over, An’s temple in Uruk was taken over by Inanna and the old god was only rarely worshipped. The Sumerian horoscope is based on the fact that the gods communicate their prophesies for mankind through signs and planetary constellations. These things required mathematical and astronomical knowledge which came up long after An lost his position in the Sumerian pantheon.
After the earth had been created, An was overthrown by his son Enlil, who split heaven and earth. He withdrew to the ether with his dangerous creatures, the Bull of Heaven and Asag. Asag was horrible in his appearance and so powerful that he caused fish to boil alive in the rivers. The monster was finally beaten by Ninurta and banished to Irkalla. Soon afterwards Inanna blackmailed An to hand over the Bull of Heaven, which she let lose against Gilgamesh, who killed the monster. Ashamed of his loss of power, the Lord of Constellations withdrew into the vastness of space. Since then he has been languishing his existence as a comet wandering through the universe. And since then he has been waiting for his chance to come to power once more.
The last time An was seen, he withdrew from the spheres of the Gods and mankind alike to dedicate his further existence to the only thing he really loves, the power and the harmony of the universe. He had always been a rather remote ruler, relying more on the power of his reputation than on a physical show of strength. But one should never forget that An embodies a rampant force which can manifest itself as marvellous creation and as total destruction alike. He can be unpredictable and cunning - at least that's what he was when he was at the height of his power. Who can say what thousands of years of loneliness do to the mind, even if it’s the mind of a god?