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Double the angle off the bow.

You are sailing on a southerly course at ten knots off the coast of San Diego, CA.  You observe the Point Loma Lighthouse twenty degrees off the port bow.  Holding course and speed constant for thirty minutes, the lighthouse is now forty degrees off the port bow.  How far are you now from the lighthouse?

Ans: 5 nautical miles.

This is an ancient navigation trick known as “double the angle off the bow”.  If you hold course constant while the angle off the bow doubles, the distance traveled and the new distance to the lighthouse are equal.  (They form the equal sides of an isosceles triangle.)