Learn more about Ur from the Easton’s Bible Dictionary

Light, or the moon city, a city "of the Chaldees," the birthplace of Haran (Gen. 11:28, 31), the largest city of Shinar or northern Chaldea, and the principal commercial centre of the country as well as the centre of political power. It stood near the mouth of the Euphrates, on its western bank, and is represented by the mounds (of bricks cemented by bitumen) of el-Mugheir, i.e., "the bitumined," or "the town of bitumen," now 150 miles from the sea and some 6 miles from the Euphrates, a little above the point where it receives the Shat el-Hie, an affluent from the Tigris. It was formerly a maritime city, as the waters of the Persian Gulf reached thus far inland. Ur was the port of Babylonia, whence trade was carried on with the dwellers on the gulf, and with the distant countries of India, Ethiopia, and Egypt. It was abandoned about B.C. 500, but long continued, like Erech, to be a great sacred cemetery city, as is evident from the number of tombs found there. (See [648]ABRAHAM.)
The oldest king of Ur known to us is Ur-Ba'u (servant of the goddess Ba'u), as Hommel reads the name, or Ur-Gur, as others read it. He lived some twenty-eight hundred years B.C., and took part in building the famous temple of the moon-god Sin in Ur itself. The illustration here given represents his cuneiform inscription, written in the Sumerian language, and stamped upon every brick of the temple in Ur. It reads: "Ur-Ba'u, king of Ur, who built the temple of the moon-god."
"Ur was consecrated to the worship of Sin, the Babylonian moon-god. It shared this honour, however, with another city, and this city was Haran, or Harran. Harran was in Mesopotamia, and took its name from the highroad which led through it from the east to the west. The name is Babylonian, and bears witness to its having been founded by a Babylonian king. The same witness is still more decisively borne by the worship paid in it to the Babylonian moon-god and by its ancient temple of Sin. Indeed, the temple of the moon-god at Harran was perhaps even more famous in the Assyrian and Babylonian world than the temple of the moon-god at Ur.
"Between Ur and Harran there must, consequently, have been a close connection in early times, the record of which has not yet been recovered. It may be that Harran owed its foundation to a king of Ur; at any rate the two cities were bound together by the worship of the same deity, the closest and most enduring bond of union that existed in the ancient world. That Terah should have migrated from Ur to Harran, therefore, ceases to be extraordinary. If he left Ur at all, it was the most natural place to which to go. It was like passing from one court of a temple into another.
"Such a remarkable coincidence between the Biblical narrative and the evidence of archaeological research cannot be the result of chance. The narrative must be historical; no writer of late date, even if he were a Babylonian, could have invented a story so exactly in accordance with what we now know to have been the truth. For a story of the kind to have been the invention of Palestinian tradition is equally impossible. To the unprejudiced mind there is no escape from the conclusion that the history of the migration of Terah from Ur to Harran is founded on fact" (Sayce).