Distance to the horizon: how far can you see at sea?

The earth is round, so your view across the water ends at the horizon. How far can you see at sea? That depends on your height of eye, and whether a light comes into view depends on its elevation and its power. Set both: the boat in the picture closes the lighthouse, and the distance to the horizon is shown below.

The formula behind it

Distance to the horizon in nautical miles is roughly 2.075 times the square root of the height of eye in metres, normal refraction included. With 2 metres of eye height in the cockpit that is just under 3 miles, from 9 metres up the mast a good 6.

For a light both sides count: your horizon distance plus the light’s, from its elevation. When a light rises exactly on the horizon you know your distance off, and with a bearing you have a fix.

What nominal range means

The nominal range on the chart applies to a meteorological visibility of ten nautical miles. In haze you see a light later, in very clear air sometimes earlier. Weak lights are limited by the lamp, tall lighthouses almost always by the curve of the earth.