Range of Lights

There are three distinct types of range quoted for navigational lights, each determined by different factors. Understanding which applies in any given situation is critical for safe pilotage at night.

Nominal range is the distance at which a light can be seen when meteorological visibility is 10 nautical miles. It is the range printed on charts and is based purely on the light's intensity — it does not account for the curvature of the earth or the observer's height of eye. A powerful light may have a nominal range of 25 miles, but that does not mean you will always see it at that distance.

Luminous range depends on the intensity of the light and the prevailing atmospheric transparency (visibility). In very clear conditions the luminous range exceeds the nominal range; in haze or rain it is significantly less. Luminous range diagrams (found in the Admiralty List of Lights and nautical almanacs) allow you to convert nominal range to luminous range for the actual visibility conditions.

Geographical range is determined solely by the height of the light above sea level and the height of the observer's eye. Because light travels in straight lines and the earth's surface is curved, a light eventually drops below the horizon regardless of its intensity. The geographical range sets an absolute maximum — you cannot see a light beyond its geographical range no matter how powerful it is.

Rising and dipping distance is the geographical range at which a light appears above or disappears below the horizon as you approach or leave it. Tables of rising and dipping distances (based on height of light and height of eye) are published in nautical almanacs and the Admiralty List of Lights. Observing a light 'dip' below the horizon provides a useful position line — you know your distance from the light at that moment.