In a country where the daytime heat is the enemy and travel and threshing happen after dark, the moon outranks the sun. That is the fact behind everything on this page. Nanna is his Sumerian name, often extended to Nanna-Suen; Suen contracts in Akkadian to Sîn, which is the form that turns up inside royal names — Sin-ahhe-eriba, whom the Bible calls Sennacherib, and Sîn-lēqi-unninni, the scribe credited with the standard Gilgamesh. He is the father of the sun and, in the astral arrangement of the family, of the morning star.
Ur, and a ziggurat you can still climb
His temple at Ur was the E-kishnugal, and the stepped platform Ur-Nammu raised over it around 2100 BCE is the best-preserved ziggurat anywhere: a mudbrick core in a baked-brick skin, three staircases converging on a first landing, drainage channels through the mass so the interior could dry. Nabonidus rebuilt it in the sixth century BCE, and the lower stages were reconstructed in the 1980s, so photographs of tourists on its steps show a structure whose bottom courses are original Third Dynasty work.
The temple's most consequential office was the entu, the high priestess, a royal daughter installed at Ur to serve the moon god. The best-known holder is Enheduanna — and it is worth being exact here, because popular accounts routinely make her a priestess of Inanna. She was Sargon of Akkad's daughter, she served Nanna at Ur, and the hymns she is credited with are addressed to Inanna, which is a different thing and arguably a more interesting one. Installing your daughter in this god's house was how a northern conqueror made himself legitimate in the south, and the practice continued for a thousand years; Nabonidus revived it for his own daughter in the sixth century.
The calendar was his
A Mesopotamian month began when the thin crescent was first sighted after sunset, so the calendar was an observational programme rather than a printed fact — and because twelve lunar months fall eleven days short of the solar year, an intercalary month had to be inserted by decree every few years to keep the barley harvest in the right month. Royal letters survive ordering exactly that. The moon god's visible behaviour is therefore the mechanism by which the state scheduled taxes, festivals and campaigns.
His symbol is the crescent, drawn lying on its back as it appears at that latitude — which is why the texts call it a boat, and why the god is imagined crossing the sky in one. He is also "the shining bull", with the crescent read as horns and a beard of lapis lazuli.
The eclipse, and the man who died in the king's place
Because he was the calendar, anything irregular he did was an emergency. Lunar eclipse omens fill long stretches of the celestial omen series Enuma Anu Enlil, and the standard reading of a bad one was a threat to the king's life. The response was the substitute king ritual, and it is documented not in myth but in working correspondence between Assyrian scholars and their rulers in the seventh century BCE. A man — sometimes a prisoner, in one case a gardener — was enthroned with the royal insignia and a substitute queen, and the omen was formally transferred to him. The real king withdrew and was addressed as "the farmer" while the ritual ran. At the end of the hundred days the substitute and his queen were put to death and buried with royal honours, and the danger was considered discharged. In at least one instance the substitute outlived the king and the throne passed to him, which the scholars recorded without comment.
Harran, and the last Babylonian king
Sin's other great house was the Ehulhul at Harran, far to the north-west in what is now Turkey, and its politics ended an empire. Nabonidus, who took the Babylonian throne in 556 BCE, was the son of Adad-guppi, a devotee of Sin at Harran whose memorial stele claims she lived to a hundred and four. He rebuilt the Ehulhul, promoted Sin above Marduk in his inscriptions, and then spent roughly ten years at the oasis of Tayma in Arabia while his son governed Babylon. The New Year festival, which required the king's presence, went unheld year after year. When Cyrus took the city in 539 BCE, Babylon's own priesthood had a well-developed grievance available, and Persian propaganda used it: the Verse Account of Nabonidus is a hostile Babylonian pamphlet accusing him of inventing a cult nobody recognised.
Nabonidus has a second reputation. He dug for the foundation inscriptions of earlier kings to establish how old temples were and who had built them, and reported his finds — behaviour that has earned him the label "the first archaeologist", with the usual caveat that he was doing it for legitimacy rather than for knowledge. The collection of older labelled objects excavated at Ur near his daughter's quarters is frequently called the world's first museum; the interpretation is contested and the label should carry that warning.
Remarkably, the moon cult at Harran outlasted Babylon, Persia, Rome and Byzantium. The city's pagan community, the Harranian Sabians, maintained recognisably ancient astral worship into the Islamic centuries and produced scholars who worked in Baghdad in the ninth and tenth centuries CE.
Three things people get wrong
Is the crescent on Islamic flags descended from Sin?
No. It is a claim that circulates widely and has no historical route. The crescent-and-star reached the Ottomans through Constantinople, where it had been a civic emblem in Byzantine and earlier Greek use, and it became associated with Islam only in the Ottoman period — many centuries and several religions after the last worshipper of Sin. Shared iconography of the commonest shape in the night sky is not evidence of descent.
Why is the moon a male god here and a goddess in Greece and Rome?
Grammatical gender in Sumerian and Akkadian does not work like the Indo-European systems, and the assignment simply differs by culture. What is worth noticing is the pattern: across the ancient Near East the moon is usually male and senior — Sin here, Yarikh at Ugarit, the South Arabian moon gods — while the Greek and Roman arrangement puts a goddess there. Neither is the "natural" reading of a rock in the sky.
What did the moon give to astrology?
Almost the whole framework of timing. Babylonian celestial omens were watched against a lunar calendar, the eclipse cycles were worked out to a predictive standard by the later first millennium BCE, and lunar position is one of the first things any birth chart records. The technical achievement here is real and independent of the theology: Babylonian astronomers derived the eighteen-year eclipse cycle, and the arithmetic that made prediction possible, out of centuries of records kept in this god's temples. What the sun contributed to the same tradition is on the Utu/Shamash page, and the tradition overview explains how the observational archives were kept.