Classification of Lights

When discussing lighted aids to navigation, there are two classes of lights that are used: "Major Lights" and "Minor Lights." Major lights are further broken down into "Primary Seacoast Lights" and "Secondary Lights."

Major Lights

A “Major Light” is a high-intensity light exhibited from a fixed structure or a marine site; they are never located on floating aids to navigation. Major lights include Primary Seacoast Lights as well as Secondary Lights.

Primary Seacoast Lights

Primary seacoast lights are major lights of very high intensity established for making landfall from sea and/or making coastwise passages from headland to headland.

Secondary Lights

Secondary lights are major lights established at harbor entrances and other locations where high intensity and reliability are required.

Minor Lights

Minor light’s usually display a light of low to moderate intensity and can be either fixed or floating aids to navigation (ATON’s.) They are established on entrance jetties, in harbors, along channels, rivers, and in isolated locations. They usually have numbering, coloring, and light and sound characteristics that are part of the lateral system of buoyage.


Lighthouses are typically primary seacoast lights. They are placed on prominent headlands, capes, isolated dangers, harbor and port entrances, or other significant points of use to the mariner.

Point Arena Light
Point Arena Light - California Fastnet Light
Fastnet - South Coast of Ireland

Lighthouses are also found on large bodies of inland waters such as the Great Lakes like the Rock of Ages Light on Lake Superior or the Ostrov Chechen' Light on the Caspian Sea. They are also used as secondary lights such as Miah Maull Light in the Delaware Bay or the Thomas Point Light on the Chesapeake Bay.

Cape Hatteras Light
Cape Hatteras Light - North Carolina Boston Harbor Light
Boston Harbor Light - Massachusetts

Some Primary Seacoast Lights may also serve as secondary lights as well. The primary seacoast light being so located as to mark the general vicinity of harbor entrances. A good example of this would be St. Johns Light just south of the entrance to the St. Johns River and the approaches to Jacksonville, FL.

Lighthouses often have fog signals, radiobeacons, or RACONS included in their construction. They also vary greatly in shape and size. This along with their paint patterns assist the mariner in identifying the specific light during daylight hours while their light color and light characteristics allow identification at night.

Visual Range of Lighted ATON’s

There are basically (4) different visibilty ranges of any lighted aid to navigation, they consist of the Nominal Range, the Geographic Range, the Luminous Range, and the Visible Range.

Nominal Range

The Nominal Range is the maximum distance at which a given light may be seen in “clear weather.” (Clear Weather being defined as a meteorological visibility of 10 nautical miles.) It can also be defined as being equal to the Luminous Range (explained below) in a homogenous atmosphere with a meteorological visibility of 10 NM. The Nominal Range is shown on and can be read directly from nautical charts as well as Light Lists.

Geographic Range

The Geographic Range is function of the curvature of the earth and is determined solely from the heights above sea level of the light itself and of the observer’s eye.

Luminous Range

A light’s Luminous Range is an approximation of the maximum range at which an observer can see a light under existing meteorological conditions. This luminous range ignores all other considerations that may affect the visibility of the light such as but not limited to: background lighting, the elevation of the light, and the observer's height of eye. It is calculated by the use of the Luminous Range Table from the known Nominal Range and the existing meteorological visibility.

Visible Range

This is the distance that YOU will actually see the light. It is ALWAYS equal to the lesser of the Luminous Range or Geographic Range.

Chart Notations of a Primary Seacoast Light

To give some idea of what information can be expected from the nautical chart, let’s take a look at “Seguin Island Light” on the coast of Maine.

Approaches to Kennebec River, ME, VA

An examination of the excerpt of NOAA chart #13293, shown above, provides us with the following information about Seguin Island Light:

F 180ft 18M HORN(MRASS)

Calculating the Range of the Light

The Nominal Range

This one is easy, since the nominal range is given as part of the chart notation as 18 NM. Keep in mind that this is the range that the light would be visible with a meteorological visibility of 10 NM.

Geographic Range Calculations

The geographic range of a light, as mentioned above, is the distance that you will be able to see the light based on the height of the light and the height of the observer’s eye above sea level. For the purposes of this exercise, we will use a Height of Eye of 10’.

There are a number of methods to solve the geographic range of a light. We will show the two most common in use.

  1. Geographic Range Tables or Distance of the Horizon Tables.
  2. Performing the computations yourself using the formula: "d = 1.17 * √h."
Solution #1 - The Use of Tables

There are many Geographic Range Tables available for use, providing varying degrees of accuracy. However, most errors will be small and generally not noticeable at these ranges.

Since any serious cruising boat should have a copy of Bowditch's "American Practical Navigator" onboard, you will find using Bowditch (Table T-12 or T-13) “Distance of the Horizon or Geographic Range Table” to be the best alternative when resorting to tables for your solutions. My personal preference, when using tables, is Table T-12 as it provides the greatest accuracy and ease of use.

Table T-12 (Distance of the Horizon)
  1. Enter the Table (T-12) in the “Height Feet” column with the height of the light (180’) above sea level.
    (In the event the height of the light is not listed, not a problem. Simply interpolate between the two nearest entries to obtain your solution.)
  2. Moving right, locate the “Nautical Miles” column and read the distance of the light (15.7 NM.)
  3. Now repeat steps #1 and #2 for your height of eye (10’.)
    and read the distance (3.7 NM.)
  4. Now sum the two distances (15.7 + 3.7) and you have the Geographic Range of the light (19.4 NM) for your particular circumstances.
Table T-13 (Geographic Range Table)
  1. Enter the Table (T-13) in the “Object Height” column with the height of the light (180’) above sea level.
    (In the event the height of the light is not listed, simply interpolate between the two nearest entries to obtain your solution.)
  2. Moving right or left, locate the “Height of eye of Observer” column corresponding to your height of eye, and read the Geographic Range of the light (19.4 NM) at the intersection of the column and row.

In the event the height of the light or height of eye is not listed, Interpolate Only. Do not break the number apart, such as an object height of 184’. You cannot solve for 180’ and then solve for 4’ and add them together. It produces significant errors!

Solution #2 - Manual Calculations

Most navigators will have a calculator at their chart table. If so, then I have found this to be the easiest, fastest, and most accurate method of determining the Geographic Range of a light and is the method I most often use.

The calculations are done using the formula (d = 1.17 * √h). Or the “Distance to the Horizon in nautical miles (d) = the constant (1.17) multiplied by the square root of the height of the object in feet √h.” The calculation should then be repeated for the observers Height of Eye and the results summed to obtain the Geographic Range of the Light.

So using the formula d = 1.17 * √h and substituting the height of Seguin Island Light, we get:
d = 1.17 * √180
d = 1.17 * 13.416
d = 15.697 NM or rounded to one decimal place, 15.7 NM.

Repeating the calculation for the observers height of eye we get:
d = 1.17 * √10
d = 1.17 * 3.162
d = 3.699 NM or rounded to one decimal place, 3.7 NM.

Now sum the two distances (15.7 + 3.7) and you have the Geographic Range of the light (19.4 NM) for your height of eye.

Do not sum the height of the light and height of eye and enter the tables or perform manual calculations with the result. It produces significant errors!

Luminous Range Calculations

Solution #1 - Luminous Range Diagram

Luminous Range calculations are done with the "Luminous Range Diagram" shown below.

Luminous Range Diagram

This diagram allows the mariner to determine the approximate Luminous Range of a light when the Nominal Range and the prevailing meteorological visibility are known.

Using the scenario from above:

We know the Nominal Range of Seguin Island Light is 18 NM. NOAA WX Radio is reporting haze with a visibility of 2 NM in the area. Determine the Luminous Range of Seguin Island Light.

The diagram is entered at the top with the known Nominal Range (18 NM.) Moving straight down follow a line to where the Nominal Range intersects the appropriate visibility curve (2 NM.) Now moving straight left to the left borderor, read the Luminous Range of the light (≈ 5.5 NM.)

More often than not, you will need to use points between the existing visibility curves. Keep in mind that the Luminous Range of any light is only an approximation since atmospheric transparency (visibility) may vary between the observer and the light.

Solution #2 - Luminous Range Calculation

If you find the Luminous Range Diagram to be a PIA to use, Mr. David Burch of Starpath School of Navigation offered a simple formula that provides a reasonable alternative:

Luminous Range in NM = ((current visibility / 10) * the nominal range) + 1

“This Luminous Range Formula is a simple approximation of a complex table. It is correct within 20% or so, which is good enough, since you will never know the visibility any better than that.”

David Burch
Starpath School of Navigation

So again using the formula and example from above we can solve for a close approximation:

Luminous Range in NM = ((current visibility / 10) * the nominal range) + 1

RL = ((Cv / 10) * Rn) + 1
RL = ((2 / 10) * 18) + 1
RL = ((0.2) * 18) + 1
RL = (3.6) + 1
RL = 4.6 NM

Visible Range of a Lighted Aid to Navigation

At this point you have to determine at what range you are likely to see Seguin Island Light.

You have already completed all of the necessary calculations, now you must choose the right answer. This is simply nothing more than comparing the Geographical Range and the Luminous Range and selecting the lesser of the two.

We know that:
The Geographic Range of the light is (19.4 NM) and
The Luminous Range of the light is about (5.5 NM.)

Simple observation shows that the Luminous Range is less than the Geographic Range (5.5 NM as compared to 19.4 NM.) Thus the "Visible Range" that we will likely see this light is at (5.5 NM.)


Distance of the Horizon Tables
Geographic Range Tables
Luminous Range Diagram

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