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How to Compute the Visibility of a Light
Information
Frequently, the navigator will want to know at what time and position
on the ship’s track a given light might be sighted. This information is
especially important when the ship is making a landfall. Failure to sight
certain lights when expected could mean that a navigational error has
been made. The distance calculated is termed the computed visibility of
the light.
Rule: When you compute the visibility of a light, the computed
visibility will NEVER exceed the light’s luminous range.
Examples
The following examples illustrate the recommended procedure for
determining the visibility of a light. Bear in mind that computed
visibility cannot be greater than the luminous range.
Example 1: Determine the visibility of light Alpha for an observer with
a height of eye of 50 feet.
Solution: From the Light List, the nominal range is determined to be 20
miles; the height of light Alpha is 90 feet above the water. Determine
horizon distance from table 4-1.
Height of eye for 50 feet
8.3 miles
Height of light (80 feet)
11.1 miles
Computed visibility
19.4 miles
Nominal range
20.0 miles
Answer:
19.4 miles
Example 2: Determine the visibility of light Bravo for an observer with
a height of eye of 35 feet.
Solution: From the Light List, determine the nominal range (10 miles)
and the height of the light above water (80 feet). Determine horizon
distance from table 4-1.
Height of eye for 35 feet
Height of light (80 feet)
Computed visibility
Nominal range
Answer:
6.9 miles
10.5 miles
17.4 miles
10.0 miles
10.0 miles
4-11
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