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Lights
You forgot COLOR!!!!
Cheers Bart Senior wrote: 2 points to you Doug--an extra point for the lengthy detailed description. DSK wrote Bart Senior wrote: Good descriptive answers Doug, but not quick enough for points. Not enough information for the one below. No points awarded for it yet. Care to expand on it? Well, you can get quite dogmatic about determining a light's visible range. The two keys are height & brightness. Because the earth (and thus the surface of the sea) is spherical, the higher the light is, and the higher the observer's eye, the further away it will come over the horizon. There is a formula for determining this, adding the two heights and multplying by a factor to get whatever distance units you like. This is called the geographic range. I'm not entirely sure I understand your method. But you hit thed key points, height of eye for the observer, and height of the light are both limited by the curvature of the earth and the horizon--even if the light is luminous enough to be seen at greater distances. I typically calculate the horizon for my height of eye, and also for the light based on it's height and add the two together, and check this against the specs on the light, which is usually greater. The horizon is usually the limiting factor for major lights that are visible at greater ranges. Brightness is also specified for major & minor lights, this is the reason for the elaborate Fresnel lenses on lighthouses... to increase the effective brightness. Obviously a tiny bulb connected to a AAA battery is not going to be seen as far away as large halon floodlight connected to high amp 120VAC. This is termed luminous range, and is usually the limiting factor. It is affected very much by atmospheric conditions. In most nav references there are tables for determining the luminous range of a given light in given conditions. Fresnel lenses are amazing techonology. Have you seen one up close? The visible range of a light is the lesser of the geographic range and the luminous range under prevailing atmospheric conditions. If one is navigating towards a landfall, and expecting to see a light during the approach, it is good practice to work out the visible range of the light under current conditions, plot it against one's future DR, and tell the watch on deck when to expect to see it. If a long time goes by and no light, then it's time to work on an alternate approach... and a good excuse... Fresh Breezes- Doug King |
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