Sin was the name of the lunar god in Babylonia and Assyria. He was also known as Nanna, the "illuminer."

The two chief seats of Sin's worship were Ur in the south, and Harran to the north. The cult of Sin spread to other centres, at an early period, and temples to the moon-god are found in all the large cities of Babylonia and Assyria.

He is commonly designated as En-zu, i.e. "lord of wisdom,". This attribute clings to him throughout all periods. During the period (c. 2600-2400 BC) that Ur exercised a large measure of supremacy over the Euphrates valley, Sin was naturally regarded as the head of the pantheon. It is to this period that we must trace such designations of the god as "father of the gods," "chief of the gods," "creator of all things," and the like. We are justified in supposing that the cult of the moon-god was brought into Babylonia by the Semitic nomads from Arabia.

The moon-god is par excellence the god of nomadic peoples. The moon being their guide and protector at night when, during a great part of the year, they undertake their wanderings. This is just as the sun-god is the chief god of an agricultural people. The cult once introduced would tend to persevere, and the development of astrological science culminating in a calendar and in a system of interpretation of the movements and occurrences in the starry heavens would be an important factor in maintaining the position of Sin in the pantheon. The name of Sin's chief sanctuary at Ur was E-gish-shir-gal, "house of the great light". At Harran it was known as E-khul-khul, "house of joys." On seal-cylinders he is represented as an old man with flowing beard with the crescent as his symbol. In the astral-theological system he is represented by the number 30, and the planet Venus and his daughter by the number 15. The number 30 stands obviously in connection with the thirty days as the average extent of his course until he stands again in conjunction with the sun.

The "wisdom" personified by the moon-god is likewise an expression of the science of astrology in which the observation of the moon's phases is so important a factor. The tendency to centralize the powers of the universe leads to the establishment of the doctrine of a triad consisting of Sin, Shamash and Ishtar, personifying the moon, sun and the earth as the life-force.