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Rather than shining in all directions, lighthouses usually produce a directed beam of light that continuously sweeps in a circle. This allows:

  • the light to be more concentrated and so appear brighter.
  • the flashing to be more obvious than a continuous light in bad viewing conditions.
  • the sweep frequency to be distinctive, eliminating possible confusion by identifying itself (e.g. one lighthouse flashes every 3 seconds, while another one a few miles away flashes every 5 seconds).

In poor visibility, such as at night, navigators can tell which direction the lighthouse is from the ship, but it doesn't help them know which direction the ship is relative to the lighthouse.


Were any lighthouses designed to also provide directional information (similar to aircraft VOR navigation)?

Specifically, a second light, say a different colour light located at the top of the lighthouse, briefly flashes every time the main beam faces north.

Navigators could then measure the delay between the two flashes to determine their line of direction from the lighthouse.

For instance, assuming a standard clockwise rotation as seen from above, if the sweep frequency is every 4 seconds and the main beam appears 1.5 seconds after the north flash, the navigator will know that their ship is currently positioned at (360° × 4s) ÷ 1.5s = 135°, or directly south-east of the lighthouse.

Being able to do this calculation with two lighthouses would allow the ship's position to be accurately determined.

It seems like a useful and obviously better system, but I've never heard of its actually being used.

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    I don't understand why this would be necessary: if you can see the lighthouse, you can use a compass to take a bearing to it. – Pont Sep 26 at 16:43
  • Convenience. Many sector lights will show white in the middle and red/green each side. Makes it very easy to tell where you are in relation to a channel or other hazards. – jhch Sep 26 at 17:13
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    That being said, the idea that "navigators can tell which direction the lighthouse is from the ship, [but not] which direction the ship is relative to the lighthouse" is off. Just add or subtract 180 from the bearing. – jhch Sep 26 at 17:16
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    @JohnHughes Yes, sector lights and leading lights can be more convenient than a compass bearing (provided that they happen to provide the bearing that you're interested in), but timing lighthouse flashes to 0.1s accuracy and performing angular arithmetic on the results, as suggested here, would be less convenient. – Pont Sep 26 at 20:09
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    The big difference between lighthouse beams and VOR signals is that radio receivers are almost omnidirectional (by design), but light receivers (our eyes) are very directional. So VOR solves a problem that just doesn't exist for lighthouses. – Toby Speight Sep 27 at 12:34
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Well, firstly, there's generally no need, given that any reasonable navigator carries a compass, thereby allowing the direction to be determined. With two or more known lights in sight, a simple resection gives the vessel's position.

However, some lights are set up to give rapid indication of whether the vessel is on the right heading in one of two ways:

  • Leading lights which are in the same direction when approaching from the correct direction (e.g. into a harbour), and
  • sector lights which show different colours according to direction, allowing vessels to correct a course which would intercept an obstacle.

No timing is required for either of these to work.


The presupposition to the question is probably that VOR solves a problem for aircraft navigation, so there ought to be a solution to the same problem in marine navigation.

The flaw in that reasoning is that there is no marine equivalent problem: radio receivers are almost omnidirectional (by design), but light receivers (our eyes) are very directional. So, the problem that VOR solves is one that simply doesn't exist for lighthouses.

  • I used range (leading) lights on the St-Lawrence both during the day and at night, and they are amazing. You can pretty much rely on them exclusively to navigate when in the very narrow parts of the seaway channel as every time you need to turn, there's another alignment telling you when you align on the next leg. – Gabriel C. Sep 27 at 16:31
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As already pointed out in a comment, your question is based on a misconception.

In poor visibility, such as at night, navigators can tell which direction the lighthouse is from the ship, but it doesn't help them know which direction the ship is relative to the lighthouse.

If you know one of these bearings, you automatically know the other: it's just the same bearing line, looked from the other end. So take a bearing from the vessel to the lighthouse (using a compass), flip it 180°, and now you have the bearing from the lighthouse to the vessel.

5

There is a of type navigational aid that partially provides some bearing information, and that is a range marker. Such markers occur in pairs, positioned close enough together to both be seen at the same time, yet far enough apart to be visually distinct from each other. Further, the two markers are placed at different heights, so that one always appears above the other.

photograph of a pair of range markers in a harbor source: BoatUS

By day, range markers appear as signs having three vertical stripes of two contrasting colors, and most are lit at night. These markers are placed such as to be aligned along the center of a preferred channel. If you are in the channel, the higher, farther away marker should appear to be directly above the lower, closer marker. If the markers are not aligned, you have strayed off-center. In the photo above, the vessel is to the port side of the channel.

example nautical chart having range markers

source: 13th Coast Guard District

When range markers are displayed on a nautical chart, such as the pair on the island in this fictitious example, there is also a dotted line drawn between them. Often this line is labeled with the true (as opposed to magnetic) bearing.


EDIT: I didn't know this before, but thanks to @Gabriel C., I learned that much of the above information is only applicable to the United States, where our Coast Guard maintains a standardized system of navigation aids. Apparently, in Canada, range markers look like this (trapezoid shapes):

range markers as they appear in Canada

source: safeboat.ca

In other nations, range markers may follow other standards or no standards at all.

It might also be worth mentioning that some objects can form a range without being marked as such officially, for example two unique buildings, one near shore and another on a hill just inland. Or a water tower and a bridge support. Many such things are shown on charts and serve as unofficial landmarks even though not meant for precise navigation. Draw a line between them on your chart, slide over to the compass rose to get the bearing between them, and when you see those objects lined up, you have your bearing.

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    These appear to be the same as leading lights as mentioned in my answer, but much better described than I managed. And vastly better for being illustrated - thank you! – Toby Speight Sep 30 at 8:03
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    Bear in mind, range lights don't necessarily always have the vertical stripe patterns. In the St-Lawrence seaway, they are orange trapezes, and elsewhere in the world, there are examples of zero uniformization. – Gabriel C. Sep 30 at 12:59
  • Until 1891 the Palmer Island Light and Fairhaven Bridge Light were used as range markers, an example of a lighthouse in a directional aid. – Matthew Gauthier Oct 2 at 12:27

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