Article by Capt. Mark Sampson
During the summer I doubt that a week goes by that someone aboard my boat doesn’t ask, “How far out do you have to go before you loose sight of land?” Often, before I have a chance to respond to the question, another member someone else will speak up and rattle off some kind of distance such as seven, twelve, or eighteen miles, and while I don’t like to bluntly contradict my clients, whenever I hear such a common mistake of the facts being passed around my boat I can’t help by being blunt and saying “no, you’re wrong about that,” and then go on to set the record straight about the subject.
The fact is there’s no “one” correct answer to the question of, “how far out do you lose sight of land?” because there are a number of variables that come to play that determine the distance in question. Visibility over the water, or over the land for that matter, is restricted by the curvature of the earth, and even though this ball we live on is so large that it might seem that the arch shouldn’t really affect visibility within the “relatively” short distance we might typically try to see across the water, it actually does make a difference.
Although distance of visibility due to the curvature of the earth can be very precisely calculated, there is no one answer to the question, “how far?”. The answer is so very dependent upon the height of the object being viewed as well as the height of the person doing the viewing. To better understand this one should imagine standing on the bank of calm lake looking at a piece of paper floating on the surface. From the bank you might be able to see the paper from 100-yards away, but if you were to get down into the water to where your eyes were just a couple inches off the surface you'd likely lose sight of the paper when it’s just a few feet away. Similarly, even with your eyes close to the water, if the paper was lifted up out of the water you would be able to see it from much farther away.
So for us boaters “how far” is a factor of how high our eyes are off the water and how tall or how high whatever it is we’re trying to see is at the time. Obviously how high up we are depends upon our vessel. The guy paddling around in a kayak is going to have nowhere near the view as the fellow riding 30-feet up in the tuna tower of a big sport-fishing boat. I’ve never been much for big tall boats, but when you’re trying to see something “height” is everything!
But depending upon what’s being viewed, the guy in the kayak might not be disadvantaged at all if whatever it is he’s trying to see is high enough above the water or land. From 20-miles out even a swimmer will be able to spot land if there are mountains on the horizon. To a mariner, knowing “how far” can be more important than just satisfying a curiosity. Look at any chart one will see that the height is given for many tall structures such as towers, water tanks, lighthouses, and other aids to navigation. Knowing the height and how far away they might be able to see these objects can help mariners figure out where they are (or at least where they “aren’t”) at a give time. If a lighthouse is 100-feet tall and you happen to know that for your given height off the water you must be within 15-miles to see a 100-foot object, seeing its light at night you would know that you must be at least within 15-miles of it.
How far something can be seen can be determined by using a “Geographic Range Table” which is basically just a couple columns of numbers that show how far something can be seen given the height of the eye and the height of the observed object. From this table I can tell you that if your eyes are 5-feet of the water you can see a 20-foot tall object from about seven and a half miles away. Or, to answer the question that so many of my clients ask “how far can you see land?” let’s assume you’re on a good size boat and your eyes are 10-feet off the water and looking for signs of Ocean City while heading home. Assuming that the tallest high-rise condos are about 200-feet tall, using the chart one can calculate that under ideal conditions from just about twenty miles out someone should be able to start making out the onshore structures. Of course that’s the buildings - not the land. If the tallest trees on or behind Assateague Island are 70-feet then nothing is going to be seen south of the inlet until that same boat comes within thirteen and a half miles.
But since the Geographic Range Table takes into account only the curvature of the earth, the ranges given assume perfect viewing conditions without allowances for hazy kind of weather we so often encounter on a typical hot summer day. Humidity alone can reduce visibility by more than half, add to that, illusions created by heat waves as well as each person’s unique vision impairments and it should be easy to see that there’s no stock answer to the question “how far?” §
