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Light Lists, Lighthouses, and Visible Ranges

Concepts such as loom and range are but a few of the important aspects to consider when using lights to navigate.
When using aids to navigation there are several important things you need to know in order to use them safely. First, you need to have a copy of the Light List for your cruising areas. Of course Light Lists furnish complete information about all lights and other navigation aids, but these publications are only meant to supplement charts and sailing directions. To use the Light List properly, first look at your chart for the location and characteristics of all navigation aids along your intended route. Then consult the Light Lists to determine their detailed description and any important information that cannot be displayed on the chart. For example, the Light List may reveal that certain channel buoys are actually located 50 yards beyond the charted channel limits.

Secondly, you need to have the latest Notices to Mariners corrections applied to both your chart and the Light Lists. The Notice to Mariners corrects both. Corrections that have accumulated since the print date of a Light List are included in the Notice to Mariners as a Summary of Corrections. All of these summary corrections, and any corrections published subsequently, should be noted in the "Record of Corrections" section of the Light List book.

If you are using an electronic charting program, you can now automatically update these corrections to your electronic chart. Otherwise you must manually make the corrections to your paper charts. Either way you still need to make manual corrections to the Light List. One company, Nautical Technologies (the makers of The Cap'n), supplies the Light Lists on computer disks as an add-on option with the capability of automatic updates. This Light List database can be accessed from the electronic chart to show the detailed information.

Just in case you are not familiar with the Light List publications, here is everything you need to know. The US government publishes two different sets of publications. The US Coast Guard publishes the Light List in five volumes for lights in US territorial waters including the Intracoastal Waterway, Great Lakes and the Mississippi River; the National Imagery and Mapping Agency (NIMA) publishes the List of Lights in seven volumes for lights in foreign waters. Old timers may remember NIMA as the former Defense Mapping Agency (DMA). Within each volume, aids to navigation are listed in geographic order from north to south along the Atlantic Coast, from east to west along the Gulf Coast, and from south to north along the Pacific Coast. Seacoast aids are listed first, followed by entrance and harbor aids listed from seaward to the head of navigation.

Lighthouses    The most familiar aids to navigation, known by almost everyone, are lighthouses. Their physical size, characteristics and intensity of light, make them readily identifiable, even though no two are exactly alike in appearance. Most of them are located on the coast or on offshore islands and shoals. Lighthouses are usually the first aids seen when making landfall. They were designed to warn vessels of the proximity of land and to serve as aids to navigation at night. Lighthouses are marked with colored bands, stripes, or other patterns to assist in identification and to help them stand out in daylight against the background. At night they have pronounced light characteristics.

Many lighthouses use red sectors to warn of shoals and other hazards and these sectors are displayed on the chart. In some cases a light may be obscured in a particular sector and that will also be identified on the chart. You should take special note of lighthouses along your route and highlight any red or obscured sectors on the chart. I also calculate and plot a "not more than" or a "not less than" compass bearing line on these sectors to use while underway. A sector changes the color of a light, but not its characteristic. For example, a four-second flashing white light having a red sector will appear as a four-second flashing red light when viewed from within the red sector. Sectors may be only a few degrees in width or extend in a wide arc from deep water toward shore. Bearings referring to sectors are expressed in degrees true as observed from a vessel. In most cases, areas covered by red sectors should be avoided. The nature of the danger can be determined from the chart. In some cases a narrow white or green sector may mark the best water across a shoal, or a turning point in a channel.

All mariners owe a debt of gratitude to lighthouses and those who keep them operating.
Understanding Lights    In earlier articles I explained how to use aids to navigation by taking and plotting bearings from them. In addition to this method, a navigator also needs to know when he can expect to see the light. Whenever possible you should plan the route to pass within a light's visible range. While underway, if the light is not sighted when predicted, the vessel may be significantly off course and standing in danger. On a dark, clear night, the visual range of a light is limited by either luminous intensity, or the curvature of the earth. Regardless of the height of the eye, one cannot see a weak light beyond a certain range. This distance is called the luminous range.

Since light travels linearly, an observer located below the light's visible horizon cannot see it. The Distance to the Horizon table in Bowditch gives the distance to the horizon for various heights. The Light Lists also contain a condensed version of this table. By adding the distance to the horizon for your height of eye to the light's visible horizon you arrive at the total visible or geographic range for that light. Obviously the higher the light and the observer, the greater the geographic range will be. When a light is first sighted, you can determine if it is on the horizon by reducing your height of eye by squatting. If the light disappears and then reappears when you stand up, it is on the horizon. This is called bobbing a light. In high seas the same effect can be observed as you go from the crest to the trough of a wave.

Prudent navigators know that the condition of the atmosphere has a considerable effect upon a light's range. Sometimes lights are obscured by fog, haze, dust, smoke, or precipitation. On the other hand, refraction may cause a light to be visible farther than it is usually under ordinary circumstances. A light of low intensity will be easily obscured by unfavorable conditions. For this reason, the intensity of a light should always be considered when looking for it in thick weather. Haze and distance may reduce the apparent duration of a light's flash. In some conditions of the atmosphere, white lights may have a reddish hue. In clear weather green lights may have a more whitish hue. Lights placed at higher elevations are more frequently obscured by clouds, mist, and fog than those near sea level. The distance from a light cannot be estimated by its apparent brightness. There are too many factors that can change the perceived intensity. Also, a powerful, distant light may sometimes be confused with a smaller, closer one with similar characteristics. Every light sighted should be carefully evaluated to determine if it is the one expected. The presence of bright shore lights may make it difficult to distinguish navigational lights from background lighting. Lights may also be obscured by various shore obstructions, natural and manmade. If so, you should report these cases to the nearest Coast Guard station.

Low lying clouds, haze, fog, and other atmospheric conditions can influence a light's effectiveness.

A light's loom is seen through haze or the reflection from low-lying clouds when the light is beyond its geographic range. Only the most powerful lights, such as the ones used on aerodromes and in lighthouses, can generate a loom. The loom may sometimes be sufficiently defined to obtain a bearing. If not, an accurate bearing on a light beyond geographic range may sometimes be obtained by ascending to a higher level where the light can be seen.

On a chart, a circle centered on a light with a radius equal to the visible range of the light usually defines the area in which a light can be seen. If a light is not sighted within a reasonable time after prediction, a dangerous situation may exist. Conversely, the light may simply be obscured or extinguished. The ship's position should immediately be fixed by other means to determine any possibility of danger.

Range and Bearing at Initial Sighting    A light's luminous range is the maximum range at which an observer can see a light under existing visibility conditions. This luminous range ignores the elevation of the light, the observer's height of eye, the curvature of the earth, and interference from background lighting. It is determined from the known nominal range and the existing visibility conditions. The nominal range is the maximum distance at which a light can be seen in weather conditions where visibility is 10 nautical miles. The Light List usually lists a light's nominal range. Use the Luminous Range Diagram shown in the Light List, or in Bowditch, to convert this nominal range to luminous range. Remember that the luminous ranges obtained are approximate because of atmospheric or background lighting conditions.

"The geographic range is the maximum distance at which the curvature of the earth permits a light to be seen from a particular height of eye without regard to the luminous intensity of the light."
The geographic range is the maximum distance at which the curvature of the earth permits a light to be seen from a particular height of eye without regard to the luminous intensity of the light. A height of eye of 15 feet is often assumed when tabulating a lights' geographic range if the geographic range is printed on the chart for a particular light.

A light's geographic range depends upon the height of both the light and the observer. You can calculate the observer's distance to the horizon based on his height of eye and the light's geographic range. For example, for a light 150 feet above the water, Table 12 (Table 8 in the older Bowditch), Distance of the Horizon, in Bowditch yields a value of 14.3 nautical miles. Within this range, if the light is powerful enough and atmospheric conditions permit, it is visible regardless of the height of eye of the observer. Beyond 14.3 nautical miles, the geographic range depends upon the observer's height of eye. Thus, by the Distance of the Horizon table, an observer with height of eye of five feet can see the light on his horizon if he or she is 2.6 miles beyond the horizon of the light. The geographic range of the light is therefore 2.6 + 14.3 or 16.9 miles. For a height of 30 feet, the distance is 6.4 + 14.3 or 20.7 miles.

To predict the bearing and range at which a vessel will initially sight a light, determine first the light's geographic range. Compare the geographic range with the light's luminous range. The lesser of the two ranges is the range at which the light will first be sighted. Plot an arc of this distance centered on the light. Extend the vessel's track until it intersects the visibility arc. The bearing from the intersection point to the light is the light's predicted bearing at first sighting. The ETA to this intersection point is the expected time of sighting. Not sighting a light when predicted does not always indicate the vessel is farther from the light than expected. It could also mean that atmospheric conditions are affecting visibility.

If it is not known from a weather broadcast, estimate the meteorological visibility by the Meteorological Optical Range Table, reproduced below. Next, enter the Luminous Range Diagram from Bowditch, or in the Light List, with the nominal range on the horizontal nominal range scale. Follow a vertical line until it intersects the curve or reaches the region on the diagram representing the meteorological visibility. Finally, follow a horizontal line from this point or region until it intersects the vertical luminous range scale.

Example 1: The nominal range of a light as extracted from the Light List is 15 nautical miles.

Required: The luminous range when the meteorological visibility is (1) 11 nautical miles and (2) one nautical mile.

Solution: To find the luminous range enter the Luminous Range Diagram with nominal range 15 nautical miles on the horizontal nominal range scale; follow a vertical line upward until it intersects the curve on the diagram representing a meteorological visibility of 11 nautical miles; from this point follow a horizontal line to the right until it intersects the vertical luminous range scale at 16 nautical miles. A similar procedure is followed to find the luminous range when the meteorological visibility is 1 nautical mile.

Answers: (1) 16 nautical miles; (2) three nautical miles.

In predicting the range at which a light can be seen, one should first determine the geographic range to compare with the luminous range. If the geographic range is less than the luminous range, the geographic range is the limiting range. If the luminous range is less than the geographic range then the luminous range is the limiting range.

Example 2: The nominal range of a navigational light 120 feet above the chart datum is 20 nautical miles. The meteorological visibility is 27 nautical miles.

Required: The distance at which an observer at a height of eye of 50 feet can expect to see the light.

Solution: The maximum range at which the light may be seen is the lesser of the luminous or geographic ranges. At 120 feet the distance to the horizon, by table or formula, is 12.8 miles. Add 8.3 miles, the distance to the horizon for a height of eye of 50 feet to determine the geographic range. The geographic range of 21.1 miles, is less than the luminous range of 40 miles.

Answer: 21 nautical miles. Because of various uncertainties, the range is rounded off to the nearest whole mile.

Meteorological Optical Range Table
Code No.WeatherDistance
 0Dense fogLess than 50 
 1Thick fog50 - 200 yards
 2Moderate200 - 500 yards
 3Light fog 500 - 1000 yards
 4Thin fog .5 - 1.0 nm
 5Haze1.0 - 2.0 nm
 6Light Haze 2.0 - 5.5 nm
 7Clear5.5 - 11.0 nm
 8Very Clear 11.0 - 27.0 nm
 9Exceptionally ClearOver 27.0

Note: The above examples and table are taken from Pub. No. 9 American Practical Navigator by Bowditch.

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