When discussing lighted aids to navigation, there are two classes of lights that are used: "Major Lights" and "Minor Lights."
A "Major Light" is a very high-intensity light exhibited from a fixed structure or an offshore marine site. They are never located on floating aids to navigation.
Major Lights are further broken down into two classes: "Primary Seacoast Lights" and "Secondary Lights."
Primary Seacoast Lights are built and located to aid the mariner in either making landfall from sea and/or making coastwise passages from headland to headland.
Secondary Lights are major lights established at harbor entrances and other locations where high intensity and reliability are required.
Some Primary Seacoast Lights also serve as Secondary Lights as well. The Primary Seacoast Light being so located as to also mark the vicinity of entrances, inlets, and passes.
Charleston Light, above, is a Primary Seacoast Light for making landfall and also serves as a Secondary Light marking the entrance to Charleston Harbor.
Lighthouses come in many shapes, sizes, paint patterns, and colors for a reason; to assist the mariner in identifying the specific light during daylight hours. Their light color and light characteristics allow for easy identification during the hours of darkness. Additionally, they often have fog signals, radiobeacons, or RACONS included in their construction.
Typically being Major Lights, they are most often placed on prominent headlands, capes, offshore isolated dangers, harbor and port entrances, or other navigational important points for the use of the mariner.
Lighthouses are also found on large bodies of inland waters such as the Great Lakes, like the White Shoal Light on Lake Michigan below or the Ostrov Chechen' Light on the Caspian Sea. They are also used as secondary lights such as Brandywine Shoal Light in the Delaware Bay or the Smith Point Light on the Chesapeake Bay.
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.
There are basically (4) different visibility ranges of any lighted aid to navigation, they consist of the "Nominal Range," the "Geographic Range," the "Luminous Range," and the "Visible 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) when in a homogenous atmosphere a meteorological visibility of 10 NM exists.
The Nominal Range of any given light is shown on and can be read directly from nautical charts (see Chart Notations below) and is also given in the USCG Light Lists as well as the NGA List of Lights.
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.
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.
The "Visible Range" of a light is the distance that YOU will likely see the light. It is ALWAYS equal to the lesser of the Luminous Range or Geographic Range.
To give some idea of what information can be expected from the nautical chart, let’s take a look at “Seguin Island Light” off the coast of Maine.
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)
This one is easy. Using the example of "Seguin Light" above, the Nominal Range is given as part of the chart notation (18M), remember this is NM’s. Keep in mind that this is the range that the light would be visible with a meteorological visibility of 10 NM.
The Geographic Range of a light 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.
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.
Important: 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 183’. You cannot solve for 180’ and then solve for 3’ and add them together. It will produce significant errors!
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 (no interpolation needed) 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.
Important: 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 are done with the "Luminous Range Diagram" shown below.
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.
We know the Nominal Range of Seguin Island Light is 18 NM. NOAA WX Radio is reporting light haze with a visibility of 5 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 (5 NM.) Now moving straight left to the left border, read the Luminous Range of the light (≈ 11.0 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.
If you find the Luminous Range Diagram to be a PITA 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.”
Starpath School of Navigation
So again using the formula and example from above we can solve for a close approximation:
RL = ((Cv / 10) * Rn) + 1
RL = ((5 / 10) * 18) + 1
RL = ((0.5) * 18) + 1
RL = (9) + 1
RL = 10.0 NM
At this point you have done all of the above calculations the Geographic Range (19.4 NM) and the Luminous Range (≈10.0 to 11.0 NM) of Seguin Island Light.
At this point you have to determine at what range YOU are likely to actually see the light.
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 (10.0 to 11.0 NM.)
Simple observation shows that the Luminous Range is less than the Geographic Range (10.0 NM as compared to 19.4 NM). Thus the "Visible Range" that you will likely see this light is at (10.0 to 11.0 NM.)