How Far Can You See?

Periodically someone calls me from a condominium on the beach, asking how far out on the Gulf Stream they can see a ship. And thereby hangs a tale!

The distance at which one can detect an object is directly related to the curvature of the earth. Even “Flatlanders” find it hard to explain why a ship, well offshore, appears to have no hull; only kingposts, wheelhouse and smokestacks. As the old salts say, “She’s hull down,” that is, over the horizon.

PICTURE OF VESSEL HULL DOWN

It should be immediately apparent that both the height of the observer and the height of the object being observed enter into line-of-sight calculations. Furthermore the distance that an object can be seen increases very dramatically with height. A swimmer, diver or man overboard has almost no distance to their horizon! It is so bad that such a person cannot see a small boat 1\2 mile away, much less a floating life ring. This is a good argument for a four foot staff and flag on a rescue buoy. A small boat with a standing helmsman has a substantially improved distance to their horizon but it does not even compare to the horizon from a tuna tower or from the spreaders of a sailboat. The higher you are the farther you can see.

The table reproduced below would be good for condominium dwellers to clip and post alongside their telescope or on the refrigerator door. It gives the distance to the horizon from an object of a given height. A 100 foot high ship’s mast can just be seen from the horizon at 11.4 nm. The view to the horizon from 40 feet is 7.2 nm. The sum of the distances in the illustration is 7.2 + 11.4 nm, or 18.6 nm.

Example of visibility vs curvature of the earth!

Hillsboro Light, 136 feet high can just be seen from sealevel at a distance of about 13.3 nm but from the tuna tower of a sport fisherman about 20 feet above sealevel, it can just be seen at 13.3 + 5.1 or 18.4 nm.

The Bowditch American Practical Navigator has a table of Horizon Distances in it that will give you the results above. Here is an abbreviated copy of the table for you condo dwellers starting with your eye 40 feet above sealevel (4 stories) and every story to 20 stories high. Remember that you have to estimate the height of the portion of the ship you are looking at and add it to your elevation, then pick the distance off the table.

By the way, Bowditch is loaded with valuable information for mariners and is available from the US Government Printing Office in Washington, D.C. I do not have the latest price but since you pay for part of the costs it should be a lot cheaper than a similar publication from private sources.

Remember to use the DISTANCE to the horizon from your eye level elevation PLUS the distance to the horizon from the highest part of the ship or object you are viewing! Do NOT add the two ELEVATIONS and then look up the sum of the elevations in the table!

Height Above Sea Level in feet Distance to horizon in nautical miles Height Above Sea Level in feet Distance to horizon in nautical miles 40 7.2 150 14.0 50 8.1 160 14.5 60 8.9 170 14.9 70 9.6 180 15.3 80 10.2 190 15.8 90 10.9 200 16.2 100 11.4 300 19.8 110 12.0 500 25.6 120 12.5 1000 36.2 130 13.0 1500 44.3 140 13.5 2000 51.2

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