Navigation and Seamanship

4.3.10. Visibility of lights

The visibility of light is a specific distance, in nautical miles, a navigator is expected to see a lighthouse or beacon. The following are the different terms used to express the visibility of lights used for navigation purpose.

Horizon distance (Distance of visibility): T he distance, expressed in nautical miles from a position above the surface of the earth along the line of sight to the horizon.

Nominal range – is the maximum distance at which a light may be seen in clear weather (meteorological visibility).

Luminuous range – is the maximum distance at which a light may be seen under the existing visibility conditions. This luminous range does not take into account of the elevation of the light, the observer’s height of eye, the curvature of the earth, or interference from background lighting.

Geographic range – is the maximum distance at which a light may be seen under the existing visibility conditions, limited only by the curvature of the earth.

Computed range – is the geographic range plus the observer’s distance to the horizon based on his height of eye. This is determined for a particular light, taking into account its elevation, nominal range, height of eye of the observer, and the curvature of the earth.

Light visibility may be restricted due to fog, rain, snow, smoke etc,.

Bobbing a light

When a light is first seen on the horizon, it will disappear if the observer tries to sight it from a point several feet or one deck, lower and reappear when he returns to original position. This is called bobbing a light and this will be helpful to estimate the distance of light .