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Although we originally started this blog to document the steps we would take on our way to becoming live-aboard cruisers, I can’t wait for the days when we can actually write about our travels. Sadly, for at least the next 80 days or so, we will be landlocked and separated from our boat. Until we are reunited with Katana our Blog readers will continue to be subjected to less-than-fun posts like the one that is to follow. Sorry kids. I promise there will be a lot more bikinis and beaches in the future!

Navigation for Newbies… episode 3:

At 0400 you leave your harbor on route to a destination some 10 miles away. Your destination is marked by a light with a 15 mile range. At what point would you be able to see the light?

This is similar to a portion of an exercise that we did in last week’s seamanship class. It would be easy to assume that we could see the light right away but the truth is there are multiple variables which could limit our ability to do so. Inclement weather such as fog or rain would limit the range, as would any pieces of land that could be jutting out in between our position and the light. Even if we did have an unobstructed view of the light area, and the weather was clear, our ability to see the light could still be limited due to the curvature of the earth. This is known as the Geographical Range of the light.

The diagram below shows the formula that you would use to calculate the range. We would need to know both the height of the light (this information, along with the lights characteristics, would be found in the List of Lights) and our height above the water surface (shown on the diagram as the height of eye). Make sure that both of these measurements are in the same units, either feet or meters (both formulae are shown).

Who would have thought that math would be so important. Stay in school boys and girls. 🙂


  1. Waiting for the sailing season to start can be a drag. Being away from the boat is bad,but sometimes being with it is worse. I went out to clean the snow off my boat yesterday and a big ice chunk clocked me on the forehead.
    You should see the faces when people ask what happened.
    “Boating accident” I tell them.
    Please let it be Spring very soon…..

  2. mmm! Did you derive these formulae from first pricipals? I get 1.93 (instead of 2.1) and 1.07 (instead of 1.17). I wonder what assumptions you are putting into the proof.

  3. I hadn’t taken into account atmospheric refraction which leads to the larger numbers given in the published formulae. It is nice to learn something every day.

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