By tieing one side of the battery to this and defining it as the common point, it does define polarity. If it had been on the other side of the battery the polarity would have been opposite. Think ± power supplies.
As with most low voltage circuits, the earth doesn't have anything to do with it, it merely defines the common point of measurement.
As with most low voltage circuits, the earth doesn't have anything to do with it, it merely defines the common point of measurement.