Originally Posted by Ahboon
IF THE ANGLE DOES NOT CHANGE YOU WILL ARRIVE AT THE INTERSECTION AT THE SAME TIME.
This is a very old sea/boating navigation technique that is still used today in many different ways.
To put it another way, the visual technique for small boats is to observe the motion of the background/horizon in relation to the boat in question. If that boat is moving faster than the horizon appears to moving (from your viewpoint in another moving boat), then that boat will cross your path before you get there. If it appears to be moving slower than the horizon appears to be moving, you will pass in front of him. If it appears to be locked with the horizon, ie no differential movement between the two, you are on a collision course.