How Far to the Horizon…

We have heard it before, “No land as far as the eye can see”. How do we know how far that actually is? What can this information be used for?

The best example of how to use this information is wanting to know when you will be able to see a fixed land based object or know when someone will be able to see you. Using this information you will be able to show on your charts when and where you should be able to see land.

Lets set this up in on an excel sheet.

A B C
1    Height Above Sea Level – Eye Level 25 Feet
2    Distance to Horizon 5.9 Nmiles
3    Height of Fix 81 Feet
4    Visible at 16.4 Nmiles

Cell B1 is the height of the observers eye above sea level. You have to take into account the height of the deck you are standing on. The formulas after that are fairly simple.

Cell B2 find the square root of B1 and multiply by 1.17. The excel formula will look like this:

=(1.17*(SQRT(B1)))

This tell us how far we can see or our effective visual horizon.

Cell B3 contains the height of the object fix. This usually found on nautical maps, like lighthouses, or by topographical information found for the landmass, like mountain heights.

Cell B4 shows us at what distance we will be able to see the object. We find this by using how far we can see to the horizon plus the like information for the fix object. The formula will look like this:

=((1.17*(SQRT(B3)))+B2)

We can  show this final data on a chart so that we can get a rough estimate as to when we will be able to see land and ensure we are on the right course.