Although lead is the third of the three fundamentals required to hit a moving target with a shotgun, and even though the first two, head on the gun, eye on the target, are much more important to our conscious plan, lead is still a necessary ingredient to the mix.
Lead is necessary because modern day target ammunition has a muzzle velocity of only 1200 feet per second. As the shot string moves through the air, it meets resistance in the form of air molecules, slowing its velocity and losing energy. So by the time a one ounce load of number nine shot travels twenty yards, its velocity has dropped to 800 feet per second. I find this energy loss astounding, but nevertheless, 800 feet per second is still very fast!
But so is fifty miles per hour, which is the velocity that a normal American Skeet target flies. So unless you have shotgun shells that go the speed of light, we need to lead our targets, that is, shoot in front of the moving target, to allow time for the shot string to intercept the flight path of the target.