The “3 basic lights” sidelights, red and green plus the stern-light. These were “the lights” in the old days of sail.
In 1840 in London, Trinity House drew up a set of regulations which were enacted by Parliament in 1846. They were updated in 1972 to replace the Collision Regulations of 1960 which were adopted at the same time as the 1960 SOLAS Convention.
So, why the strange angles?
Basically a full circle can be divided in to 360° or 32 points of the compass.
A traditional sailing vessel was assumed to be able to sail withing 45° or 4 points, either side of the wind. ie not directly into the wind.
A sailing vessel could “normally” steer 45° (4 points of the compass) either side of the wind.
It was possible to approximate the possible headings that a sailing vessel may be following and then deduct the headings that she could not be following as some headings would be “too close” to the wind and not steerable.
The angle of arc of the sidelights was visible from “right ahead” to two points abaft the beam.
Compass points vs angles
In the original “rules” the side lights were over an arc of 10 points of the compass, eg for a vessel heading North, this would mean that the starboard sidelight would be visible to another vessel who was any where “North to East South East” of the sidelight.
From the illustrations below, we can see that would be 112.5° of a full circle of 360°
Simple mathematics show that 10 points of the compass is equal to 112.5°
Each point of the compass is
360° / 32 = 11.25°
and so 10 points of the compass would be
Hopefully this simple explanation will allow you to spread the word and explain to your marine colleagues 🙂