I Can See The Lighthouse!
Marilyn has been criticized for claiming that you could see a 150 foot tall lighthouse from anywhere on a five mile long, five mile wide desert island. Her critics say that this is not necessarily so Wiskit. What Marilyn is referring to is the extent to which the curvature of the Earth obscures distant objects. A very convenient approximation, and one that is used extensively by both mariners and pilots, is that the distance to the horizon in nautical miles is equal to the square root of the height of the observer in feet. This approximation even incorporates the conversion between feet and nautical miles. If an island is five miles on a side the maximum distance is corner-to-corner and is just a little over seven miles. From the top of the lighthouse the distance to the horizon would be 12 1/4 nautical miles and thus, absent tall obstructions, a person at the top of the lighthouse and a person at the opposite corner could easily see each other. As far as obstructions go, most lighthouses are built on the highest ground available. San Diego's Point Loma lighthouse would be an exception.San Diego, California's old Point Loma lighthouse was built on such high ground that it often was obscured by fog. It was replaced by one built near sea level.
In calculating the distance at which a tall object can be seen, the height of the object and the height of the observer are cumulative, so a six-foot man could see the top of the lighthouse from as far as 14 1/2 nautical miles away.