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