578. The Light List indicates that a light has a nominal range of 18 miles and is 38 feet high. If the visibility is 6 miles and your height of eye is 15 feet, at which distance will you sight the light?

The geographic range, calculated from the observer's and light's heights, limits visibility. The formula d = 1.17√h (height in feet, distance in nautical miles) yields a geographic range of 11.7 nm, which is less than the light's nominal range of 18 nm. Therefore, the light will be sighted at a distance of 11.7 nm.