Definition
As a light rises above or dips below the sea horizon, geometry fixes the range for a given charted elevation and height of eye. The standard approximation is range in nautical miles equals 2.08 times the sum of the square roots of the two heights in metres, allowing for standard terrestrial refraction. For a light 40 metres high and an eye height of 3 metres this gives about 16.7 nautical miles. Anomalous refraction can shift this appreciably.
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