Watchkeeping
Horizon and light range
How far the horizon is from your height of eye, and from how far a light can be seen over it: when to expect it to rise above the horizon.
From the chart or List of Lights
Nominal range, in clear weather
Enter your height of eye.
How it works
With standard refraction, distance to the horizon ≈ 2.08 √h nautical miles for a height h in metres. From an 18 m bridge the horizon is 8.8 nm away.
A light is first seen when its horizon meets yours: geographic range = 2.08 (√H + √h). It is seen at the lesser of that and its luminous range (from the nominal range in clear weather, less in poor visibility). Dip of the sea horizon ≈ 1.76 √h minutes of arc, used to correct sextant altitudes.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
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