Lists of lights have been in existence for about 150 years. These consist of tables, published in book form, giving a simple listing of:
The light shown;
Its geographical position in latitude and longitude;
a description of the light shown;
A description of the structure from which it is shown to help with identification during daytime.
Other information such as the height of the light above the sea might also be given. This would help in determining the distance from which the light could be seen - the range.
Early lists were published by Alexander Findlay from the 1860s onwards. (A facsimile is available from us.) Later, the British Admiralty, through its hydrographic department, took on the task of compiling detailed listings of navigational aids, as well as charts. These lists are very important even today and are called the Admiralty List of Lights (ALL). They cover most of the world and are divided into a number of volumes according to the geographical region. Regions not covered are those outside international waters, most importantly the Great Lakes of the USA and some other inland waterways.
In the present volumes of ALL, each light is given a unique number for identification purposes. The number has a single letter prefix that defines the volume in which the light is listed. Thus, the famous Eddystone lighthouse has the ID A0098 which means that it is to be found as light number 0098 in Volume A of the Admiralty List of Lights. These ID numbers are internationally recognised, however, it must be pointed out that other countries adopt other designation systems.
The sequence of the numbers generally follows the order in which the lights occur along the coastline, although there are inevitably breaks in the numbering as the geography changes. Thus, the numbering begins in volume A at 0001 - the farthest extremity west of the British Isles and proceeds in an anti-clockwise direction (roughly speaking) around the coastline.
Once a number has been assigned to the site of a navigational light, it remains allocated there. If a new light is created it is given a new ID, inserted into the numerical sequence with a previously unallocated value, and going to decimal values if necessary. For example, suppose a light exists at A as D0658 and another at B as D0659. A new light is created which is geographically located between A and B. The new ID might be D0659.5, for example. The actual number allocated is decided by the Hydrographer of the British Royal Navy.
If a light has been allocated a number and subsequently becomes disused, the ID number is not reallocated. For example, the lighthouse at St Mary's Island in the north-east of England has the international number A2748, but it was taken out of service in the 1980s and neither the lighthouse nor the number appear in the current volume A of the ALL.
A standard practice is used for leading lights as follows: if two leading lights are created there is always a front and a rear light. The rear light is always given the same number as the front light but with 0.1 added. For example, the front light of the two leading lights at Brighton Beach, PE, Canada is denoted by H1012, whilst the rear light is given H1012.1