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The Structure of Lighthouses

When we talk of a lighthouses, we are may be thinking of a light station, a lighthouse or the lantern room and perhaps it would be helpful if we were to look at what each part is, and what it's used for.

On this page we are looking at the physical structure, arrangements of lighthouses, daymark and its positioning. Details on the methods of powering the lights, messages its gives and the optics used can be found in Lighthouse Lights. 

If you don't understand what a lighthouse is used for and why its still required at least for now, see our article What are Lighthouses For.

A light station, is the lighthouse and all its outbuildings and land, so includes light keepers cottages or living accommodation, fuel stores, boat houses as well as the fog horn, or other sounding device and the main lighthouse tower or room.

Aerial photos above and below by  Marinas.com

St Mary's Lighthouse  Northumberland

Lighthouse, can often be used to mean light station, and I often use it this way as more people understand the term lighthouse than light station. The lighthouse is the structure than includes and supports the lantern room where the light shines from, its not always a tower. Some define a lighthouse as a structure that people can get inside, while many others just think of it as a source of light for navigation and warning for those using floating craft of all types, often referred to as vessels, although again I often just lump them all together as ships, as everyone knows what a ship is.

Tower lighthouse, Tower station  or Rock station, is a lighthouse that has all of the light station within its tower, for example a tower out at sea where the living accommodation and all other parts are within the tower structure.

Image by Peter Jordan

Bishop Rock Lighthouse

Lantern Room, is the place where the light in a lighthouse shines from, usually its a dome structure with some, if not all, sides being constructed of glass. Older designs needed to have storm proof ventilators to allow fumes to escape, and have a lightning rod on top to take lightning strikes down to earth so the lighthouse is not damaged.

Watch room, usually below or behind the lamp room. This was used for storing supplies but also where the light keepers would stand watch for the night so the lights did not go out. It was also the preparation room where work ahead and cleaning of some parts was undertaken. Many have windows allowing those on watch to look out for ships that may be in difficulty. Where bells were fitted to lighthouse balconies, it was often the watch room that they were rung from.

Galleries, is the outside area around the top of a tower lighthouse or in front of a lower one. In some cases there are two galleries a lantern or main gallery, and a watch gallery below, accessed from the watch room. Some of the main galleries are in practice also accessed from the watch room. The initial function of the main gallery was to allow the windows to be cleaned and any repair work undertaken.

Photo by Scott Rimmer

Flamborough Head Lighthouse  with two galleries

Daymark. In the daytime when the light is not lit, the lighthouse is still a navigational and warning aid, and so as to avoid confusing one with another, they are different shapes or decorated in deferent ways, or where out at sea and this is not practical, having red bands or other bands around them. Some may also have stripes or others special features. Lighthouses are also often coloured so that they will stand out from the background.

Leading lights or range lights. Two lights that are lined up to identify a safe passage. The front light, leading light or low light is nearer to the ship, while the rear range or high light sits further back. In the daytime you often have two distinct matching marks to line up, such as the two red vertical lines on the pair at Burnham-on-Sea. In some complex ways into harbours, there can be a series of these leading lights, so you go on a course with one until you pick up the track from the next. With leading lights whichever side of the front the rear is, tells you that you are to far out in that direction. So if the rear light is to the left, you are also too far to the left.

Twin tower lights. Historically before lighthouses had lights that flashed or rotated, often twin towers or twin lights were used to help differentiate one lighthouse from another. The Mumbles Lighthouse  in Glamorgan had two lights, fires on two platforms one above the other. Once lights could be made to change, you could identify a light by its signature pattern.

Photo by Gordon McKinlay

Lizard Lighthouse   originally had two working towers

Lamp height. We need to place the lamp high enough so that it will be seen by those at sea some way off, but also low enough that when there is fog or low cloud, it is still visible. This is why you see very short lighthouses on cliffs, and tall ones stood on sea level rocks. In the deep ocean where the waves can be much larger you may also need to get your light higher so that the light is still visible for all of the time, rather than just at certain parts of the waves peaks, or you can't read its light pattern. Some tall lighthouses have secondary lights at a lower level. In some cases you will see lighthouse stations cut into the side of hillsides or cliffs, this is for the same reason to get the optimum height.

Visible range.  The earth is curved and over a distance you loose sight of items due to this. If either you stand on a tall ship or on a tall cliff then you can see further. There are recognised means to calculate the distance a lighthouse is said to be visible, assuming the person on the ship is at a set height. A light, particularly a powerful one, can be seen at a greater distance than the distance of line of sight, as some of the light is refracted or bounced within water globules within the atmosphere, which is known as the loom of the light.  To get the distance a light can reach, they take a standard height for the observer to be stood 15ft above the water, and when they take a 100ft lighthouse, this way they end up with a visibility of around 18 miles. The distance also varies with the amount of mist or fog, and look up tables were used to determine the range in different weather conditions.

Distance to Horizon. The distance you can see can be calculated as the square root of the height, in feet, your eyes are at and multiplying this by 1.17. This gives the distance of the horizon in nautical miles. So if you were 100ft up then the square root would be 10, multiplying this by 1.17 gives you a distance you can see to the horizon of 11.7 nautical miles, while at 16 feet up, the Sq root is 4 so the horizon would be at 4.7 nautical miles. Logic to me would say that you could add the 100ft result for lighthouses with the 16ft result for the boat and they could see each other at 16.4 miles, as long as there was no waves. Both a modern boat deck and the crows nest of an old sailing ship is much higher so they would have greater visibility.