Page 198 - NAVAL SCIENCE 3 TEXTBOOK

204 NAVAL SKILLS

The most important navigation lights are usually either flash-
ing or occulting, since these patterns are the most easily identified.

Visibility of a Navigation Light

The visibility of a light is the distance, in nautical miles. a mari-
ner can see the light at night. The distance at which a given naviga-
tion light may be seen depends on its intensity and height above sea
level, the height of eye of the observer. and the existing lIIeteorolog-
im/llisibiiit)' collditiollS (weather). Information about the intensity
and height of navigation lights can be found in publications called
light lists. A navigator will usually know his or her height of eye
at different positions on the vessel from which he or she normally
makes observations, and can estimate the meteorological visibil-
ity conditions in any of several ways, \Vith this information, the
navigator can compute the exact visibility of a light using various
tables, A good approximation of the distance at which a light may
be seell is printed on nautical charts beside the light symbol. This
chnrted mllge is the distance a light will shine in clear weather and
is about the distance a mariner in a small boat can usually expect A navigation light with white, red, and green sectors, The bearings
to see the light. shown at the bottom are those from the light. In a light list the bear-
ings given Vlould be the reciprocals, as would be seen from a vessel
proceeding clockwise around the light. In this case, the description
Lighthouses and Light Towers of the light would read obscured from land to 315°, red thence to
358°, green thence to 050°, white thence to land.
There are many lighthouses along the coasts of the United
States and the Great L1kes and along many interior waterways.
The)' are placed wherever a powerful light, called a prill/my light,
A lighthouse may have fog-signaling and radio-beacon equip-
may be of assistance to navigators. or where veq' dangerous water
ment in addition to the light itself. At one time, most such lights
requires a warning beacon of long-range visibility. Since the range
were run by keepers who lived at the lighthouse. Now, however,
of visibility of a light increases with its height, the main purpose of
the lights in lighthouses are mostly automatic. with no keepers
a lighthouse is to increase the height of a light above sea level.
required, The towers of lighthouses are usually painted distinctive
colors and patterns to make them easier to identif)' in the daytime,
These may be solid colors, bands, stripes, or squares.
At some locations, primary navigation lights are placed atop
large structures of girders, painted similarly to lighthouses, Some
powerful ofl-shore primary lights are mounted either on large
buoys or a tower on stilts embedded in the ocean bottom. These
light towers often house a crew. and they have a powerful light,
radiobeacons, oceanographic research equipment, and a helicop-
ter platform. Like lighthouses ashore, their increased height allows
their lights to be seen for greater distances, and their crew ensures
that these important lights will always be on, Secondary, minor,
and automatic lights are located in structures ranging in size from
towers to a single piJe supporting a battery box and the light.
Some lights have sectors of red glass placed in their lanterns
to show danger bearings. Danger bearings on a chart show a ves-
sel when it is in danger of running aground on rocks. shoals. or
some other hazard, Arcs over which the red light shows are the
danger sectors, The red color shows only within the danger zolle;
other light characteristics remain the same, Some lights also show
The Ambrose Offshore Light Tower, 7 miles east of Sandy Hook, New
Jersey, Its primary light has a high intensity of 6 million candlepower a green sector, which indicates a turning point or the best water
with a charted range of 18 miles. across a shoal. All sector bearings are true bearings to the light.