The Sumerian sign for him is a star: four strokes crossed, drawn 𒀭 in cuneiform. It reads an and means "sky", and it also reads dingir and means "god" — and it is placed silently before the name of every deity in the writing system as a marker that what follows is divine. So the sky god's own sign sits in front of Enlil, Inanna, Marduk and every other name in this tradition, unpronounced, doing grammatical work. Nobody designed that as theology. It is an accident of how the script developed, and it happens to describe his position exactly.
An in Sumerian, Anu or Anum in Akkadian. His consort is Uraš or Ki, the earth, in the Sumerian material and Antu in the Akkadian — Antu being a straightforward feminine form of his own name, a goddess generated by grammar.
Authority without activity
He is the head of the pantheon and he almost never does anything. Sovereignty in his case is the power to legitimate, not the power to act: kingship among the gods descends from him, offices are held by his grant, and the abstract noun anūtu, "Anu-ship", is used for supreme rank as a transferable commodity. When Babylon promotes Marduk, Enuma Elish has the assembly confer anūtu on him — the office is separated from the person who originally held it, which is only possible because Anu had already become a title as much as a character.
The most revealing thing he does in any narrative is refuse. In Gilgamesh Tablet VI, Inanna comes to him demanding the Bull of Heaven and he objects on economic grounds — the bull's arrival means seven years of empty husks and no grazing, so has she stored seven years of grain? She threatens to break down the gates of the underworld and he gives in. A sky father who can be talked round by a daughter's tantrum is not a weak characterisation; it is a consistent one. He grants, he does not enforce.
In Atrahasis the universe is divided by lot at the outset: Anu draws heaven and goes up to it, Enlil takes the earth and the winds, Enki takes the freshwater deep. That is the last we hear of him for most of the poem. In Enuma Elish he is sent out to confront Tiamat, gets close enough to hear her, and turns back — one of two failed embassies that make Marduk's terms acceptable.
Uruk, twice
His temple was the Eanna at Uruk, whose name means "house of heaven", and the awkwardness there is that Inanna also held it and eventually held it alone. Sumerian material sometimes gives her the temple as something she acquired, and there is no version in which An objects effectively. For most of Mesopotamian history his active cult is thin: theophoric names invoking him are rare compared with Sin or Shamash, and his festivals are unspectacular.
Then, in the last centuries of cuneiform, Uruk did something remarkable and put him back. Under Seleucid rule, from the third century BCE, a new temple complex called the Bīt Rēš was built for Anu and Antu, and the local priesthood reorganised the pantheon with Anu at its head and Inanna's position reduced. Ritual tablets from that revival survive, describing the New Year procedures in detail — a golden canopy, a torchlit night vigil, precise offering lists. It is one of the very few documented cases of a deliberate archaising religious reform in the ancient Near East: scholars in a Greek-ruled city reconstructing what they took to be the oldest and purest form of their own religion. The Mesopotamian family tree draws both arrangements.
The Anunnaki, and what the word actually means
The name usually understood as "offspring of An" belongs to the great gods as a collective. In Sumerian texts the Anunna are the senior divine assembly; in the Descent of Inanna they are the seven judges of the underworld who fix the goddess with the eye of death. Akkadian usage drifts, and by the first millennium the Anunnaki are often specifically the gods of the earth and underworld while the Igigi are the gods of heaven — a division that is not stable and that earlier texts do not observe. In Atrahasis it is the Igigi who go on strike over canal labour while the Anunnaki are the senior gods who profit from it, which reverses the arrangement some later readers assume.
What the word does not mean is a species of visitors from another planet. That reading, and the associated claim about a planet called Nibiru, is set out at length on the Enki page along with what the astronomical tablets actually say about the term. Here the relevant fact is simply grammatical: it is a plural collective noun for gods, formed from this god's name, and it is used the way an English text might say "the powers".
A band of sky, and a road for the stars
Babylonian astronomers divided the sky into three paths named for the three great gods. The Way of Anu is the central band, straddling the celestial equator, through which the sun, moon and planets appear to travel; Enlil's way lies north of it and Ea's south. The star catalogue known as MUL.APIN lists the constellations of each path, and the Way of Anu's list is a direct ancestor of the ecliptic band that later became the zodiac — the machinery underneath every horoscope is a Babylonian filing system with this god's name on the middle drawer.
His emblem in art is the horned crown, usually shown alone on a pedestal or shrine rather than worn, and the number of horn-pairs on a crown indicated rank throughout Mesopotamian iconography. A symbol standing in for a god too remote to depict is an apt piece of shorthand for a deity whose defining quality is distance.
Questions about a god who barely appears
If he outranks Enlil, why is Nippur more important than Uruk?
Because rank and function came apart very early. Anu confers legitimacy; Enlil exercises it, and it was Enlil's temple at Nippur that Sumerian and Akkadian kings needed to endorse them. Practically, a ruler seeking recognition made offerings at the Ekur, not to the sky. The theology is untroubled by this because a delegated authority is still his authority — but it does mean that the most powerful god in the diagram receives comparatively little of what religion actually consists of.
Is he ever overthrown, like Uranus or Kronos?
No, and the contrast is instructive. Greek succession myth requires each sky father to be violently displaced; Mesopotamian succession does not. An is never castrated, imprisoned or killed. He recedes. Even Marduk's promotion is staged as a legal grant from the assembly rather than a coup, and Anu is among those granting it. A tradition in which supreme power changes hands by vote and precedent rather than by patricide is telling you something about how its authors thought institutions worked.
Does Inanna's parentage really depend on which city you ask?
It does, and it is one of the clearest demonstrations that this pantheon was never centrally standardised. Some texts make her An's daughter, which suits a goddess whose house is called the house of heaven; others make her the daughter of the moon god Nanna, which makes her the sun god's sister and puts her in a coherent astral family. Enheduanna's hymns use both framings for rhetorical effect. The god-lists compiled by scribes tried to reconcile such cases and frequently just recorded the alternatives side by side, which is why the Mesopotamian overview treats regional variation as the normal state of this material rather than as corruption of a lost original.