How can you tell where you are by looking at a lighthouse?

Answer 1

A bearing taken over the compass will give you a Line Of Position (LOP) on your chart. Your position must be somewhere on this line. If you happen to notice exactly when the lighthouse hoves into view, you can tell from your chart how far away from the lighthouse you are. From that, you can plot a Circle of Position (COP) showing distance from the lighthouse. And, if you've done this right, it means your boat is somewhere on that circle. Combine the two and you have a True Fix.

Alternately, if you didn't notice the lighthouse (or at night it's light) coming over the horizon, you can still plot the same circle knowing you're not on the circle but inside it. This can help you determine exact position.

You can also advance a running fix from the first LOP.