Nanna / Sîn, the Moon-God
The moon that measures the months — wise elder of the gods, and the cause of a king's downfall.
Also: Nanna · Sin · Sîn · Suen · moon god · Ur · Nabonidus · crescent
The moon-god Nanna — Sin in Akkadian — was the great deity of the city of Ur and a god of the first rank. As the moon, he measured out the months and regulated the calendar, and he was honored as the wise elder who fathered the sun-god and the goddess Inana/Ishtar. His crescent was his emblem. Centuries later the last king of Babylon, Nabonidus, controversially elevated Sin's cult above Marduk's, a move that estranged Babylon's priesthood. The moon-god's long prominence shows how a city's patron could shape the religion of an empire.
In short
The moon-god Nanna (Sumerian) / Sîn (Akkadian), patron of Ur, measurer of months and time, father of the sun and of Inana/Ištar, prominent in the late piety of Nabonidus.
Where it’s found
'Nanna-Suen's journey to Nibru' (ETCSL 1.5.1, in corpus); Nabonidus' inscriptions for the late cult (akkadian-pending)
How the idea developed across Sumer, Babylon & Assyria
Nanna of Ur and Akkadian Sîn are syncretized strata. His cult peaks early (Ur III) and again strikingly under Nabonidus (r. 556–539 BCE, Middle Chronology era dates firm), whose promotion of Sîn at Babylon and Harran alienated the Marduk priesthood — a documented historical episode.
Nabonidus' elevation of Sîn (r. 556–539 BCE) is historically attested; his 'Harran' and dream texts are royal apologetic — read their claims as claims.