Genesis 12: Why leave Ur?

In my previous post, I discussed how we seldom think about the real-life implications of Abraham's (actually, at this point he is still called 'Abram', because God hasn't changed his name yet) obedience to God. He had to leave everything that was familiar and set out to follow God to an unknown place. If you have searched out information about Ur, where Abraham had lived, you have seen that he was leaving quite a significant city. Why did God make him leave in the first place? Aside from the obvious test of obedience that this action entailed, in the thinking of people in those times, a 'god' was usually associated with a place and a person or group of people -- like, say, the god of a certain city, or of the mountain, or of the desert, or of a certain family. Ur, for example, was known as the city of the moon 'god'. In this case, God is calling Abram to be the head of a people who will worship Him as the only true God, in a place which God will claim for His own. This would make perfect sense to the people who held such a mindset. It didn't make it any easier to leave, mind you, but Abram would know that in order to follow this God, he would have to go to wherever God commanded him to go. Of course, God is the creator and God of the whole world, but He would in some way be especially connected to the land where He was leading Abram. So Abram goes. He begins to follow the light which God has given him, and steps out of his idolatrous past and into the beginning of what God has promised him.