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This uses a quote from Wikipedia; not from any AFTTP, etc. Pilots learn that two aircraft on a collision course have no apparent motion in the cockpit. So planes that appear to be moving aren't going to hit you without a course change, while stuff that will hit you will appear fixed or nearly fixed, blending in with cultural lighting, stars, etc. This is why deliberate visual scanning still remains important. The same principal is used to automatically compute appropriate lead in some Air to Air missiles. Rather than use an eleaborate lead computing algorithm that depends on a processor and rapid communication to relevant servos, some missiles make course adjustments by change in relative bearing to target rather than relative bearing itself. Pretty neat, eh? This is why you should hug an engineer! (Engineers hate hugs, though.) "The Sidewinder is not guided on the actual position recorded by the detector, but on the change in position since the last sighting. So if the target remained at 5 degrees left between two rotations of the mirror, the electronics would not output any signal to the control system. Consider a missile fired at right angles to its target; if the missile is flying at the same speed as the target, it should "lead" it by 45 degrees, flying to an impact point far in front of where the target was when it was fired. If the missile is traveling four times the speed of the target, it should follow an angle about 11 degrees in front. In either case, the missile should keep that angle all the way to interception, which means that the angle that the target makes against the detector is constant. It was this constant angle that the Sidewinder attempted to maintain. This "proportional pursuit" system is very easy to implement, yet it offers high-performance lead calculation almost for free and can respond to changes in the target's flight path,[7] which is much more efficient and makes the missile "lead" the target."