478. A lighthouse is 120 feet (36.6 meters) high and the light has a nominal range of 18 miles. Your height of eye is 42 feet (12.8). If the visibility is 11 miles, approximately how far off the light will you be when the light becomes visible?

The luminous range, determined using the luminous range diagram with a nominal range of 18 miles and a visibility of 11 miles, is approximately 19 miles. This value is less than the geographic range of about 20 miles, so the light will first become visible at a distance of 19 miles.