Current difficulty is at 55 589.518126. What is this a measure of? Is this directly related to the # of leading zeros in the SHA256 block hash we are working on? How is it possible to adjust difficulty only marginally every 2016 blocks?
The difficult is a multiple of the minimum difficulty level, which is just an arbitrary 64 bit binary integer that the hash must be less than to qualify. The leading zeros thing is an approximatation of the method, but not entirely accurate. If the clients were just counting leading zeros, then the difficulty could only increase as a log of 2 ( each additional leading zero doubles the difficulty). Basicly, the 'target' is a 64 bit integer that the binary interpretation of the sha-256 hash must be less than, and the target number is adjusted up or down by a process similar to a 'fuzzy logic' type of system; always seeking it's balance. In this case, the balance point is 6 block per hour, and every two weeks or so, the system makes a calculated guess as to how much the target must be raised or lowered based upon the average blocks per hour seen during the prior two week span. All of the clients make this calculation independently, but because it's a deterministic calculation based upon a shared set of data, they all come up with the same answer. The clients don't use the 'difficulty' at all, but use only the 'target'. The 'difficulty' is just a metric devised to allow us humans to make sense of it all, because for us to look at the targets, it would be difficult for us to draw useful information from the target or it's changes over time. Since the difficulty is expressed as a ratio (number of times more unlikely for a given hashing attempt will produce a hash below the target) it makes it
easier for humans to look at the difficulty number and prior difficulty numbers and derive useful meaning from it.
