Suppose that the boom of a gun is recorded by three microphones: Let’s call them A, B and C. Assume that the three microphones are spaced 386 meters (1,266 feet) apart. That is the distance sound travels through air in one second. The boom reaches microphone A first. One second later it reaches microphone B. One second after that it reaches C. Draw a circle with a radius equal to 386 meters centered on B and a circle with radius twice that distance centered on C. Now draw a circle that goes through the position of microphone A and that also touches the two other circles. The center of this circle is the position of the gun that fired.