Ur (biblical, Ur of the Chaldees), ancient city of Mesopotamia. Its ruins are approximately midway between the modern city of Baghdâd, Iraq, and the head of the Persian Gulf, south of the Euphrates River, on the edge of the Al ajarah Desert. The site of Ur is known today as Tall al Muqayyar, Iraq. In antiquity the Euphrates River flowed near the city walls. Controlling this outlet to the sea, Ur was favorably located for the development of commerce and for attaining political dominance.Ur was the principal center of worship of the Sumerian moon god Nanna and of his Babylonian equivalent Sin. The massive ziggurat of this deity, one of the best preserved in Iraq, stands about 21 m (about 70 ft) above the desert. The biblical name, Ur of the Chaldees, refers to the Chaldeans, who settled in the area about 900 BC. The Book of Genesis (see 11:27-32) describes Ur as the starting point of the migration westward to Palestine of the family of Abraham about 1900 BC.