Look at the background: trees, buoys or any other fixed feature. If the other boat appears to be moving backward against the background, you will pass in front of it; if it's moving forward, it will pass you. If the other boat appears steady against the background, you are on a collision course and should probably take action, or at least keep a close eye out.

Not only useful to avoid accidents: when racing sailboats, you often want to run very close to your competitor to steal their air, especially at key points like turns. Watching the background helps you precisely figure your approach from a long way away.

It can also help you tell whether you will beat that locomotive through the railroad crossing.

So you're out on the lake in your dinghy, or lost on the Bonneville Salt Flats in your go cart, and you spy a dot on the horizon. You know it's not a piece of dirt on your spectacles; you're getting an ominous feeling that it's a cigarette boat or a rocket-propelled car that could slice your pedestrian craft in half.

Should you be worried? Assuming both of you are traveling at constant velocity, here's how to tell. Take a bearing on the approaching vehicle; wait a bit (how long depends on how close it is), and take another bearing. If the two bearings are the same, you've got a problem.

An easy way to do this, without resorting to a compass or other technological crutch, is to simply look at it for a while and see if you have to move your head (or eyes) to keep it in the same place in your field of vision.

The rule of thumb, therefore, is: Constant bearing implies collision

There are two self-evident exceptions to the rule:

  1. If you're motionless relative to the other (i.e., your velocities are the same, or you're both stationary (a special case of a special case))
  2. If the paths are colinear and the separation between the vehicles is increasing

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