The Light List indicates that a light has a nominal range of 20 miles and is 52 feet high. If the visibility is 12.0 miles and your height of eye is 20 feet, at what approximate distance will you sight the light?
• Nominal range vs geographic range of a light • Formula for distance to the horizon based on height of eye and height of light • Effect of meteorological visibility on the distance a light can be seen
• First, compute how far you can see to the horizon from your height of eye, then how far the light can be seen to its horizon from 52 feet. What do you do with those two distances? • Compare the geographic range you calculated with the nominal range of 20 miles and the visibility of 12 miles. Which factor actually limits how far away you will FIRST see the light? • Think about whether you are limited by how bright the light is (nominal range) or whether the curvature of the earth and visibility stop you sooner.
• Be sure you use the correct geographic range formula for each height (observer and light) and then combine them properly. • Compare your combined geographic range to both nominal range (20 miles) and visibility (12 miles) and decide which one is the controlling limit. • Check that your final distance is not greater than any physical limit you identified (you can’t see farther than the earth’s curvature or farther than the light is rated/visibility allows).
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