Lights and their characteristics

Lights on the chart

In the last three lessons you met the buoys and beacons that mark safe water and dangers, and learned to recognise them by shape, colour and topmark. That works by day. At night, shapes and colours disappear, and each lighthouse and lit mark has to be recognised by its light alone. So every light shows its own pattern of light and darkness — its characteristic — and the chart tells you what to look for.

On an Admiralty chart a light is marked with a magenta flare, a small teardrop shape. A lighthouse is drawn as a star with the flare beside it, and its characteristic is printed alongside in a compact code. Gull Point lighthouse, on the eastern tip of the Isle of Kerran, is printed Fl(3)10s28m15M.

Lit buoys and beacons carry a flare too, usually with a shorter characteristic: just the rhythm, colour and period, such as Fl G 5s on No 1 buoy. Their lights are low and fairly dim, and the chart usually gives no elevation or range for them. Where a harbour has leading lights, the chart prints their bearing and characteristic along the line — at Port Kerran Ldg Lts 000° Oc 4s, which you used in Lesson 13.

The chart is only a summary. The Admiralty List of Lights and Fog Signals gives fuller details of lighthouses and other major lights: position, characteristic, elevation, range and a description of the structure, with remarks such as the bearings of any sectors and, often, the timing of each flash and eclipse. Yachting almanacs print the main lights of their area in a similar way. The spacing and punctuation vary — Fl(3)10s28m15M on the chart may appear as Fl(3) 10s 28m 15M in a book — but the order is the same.

Reading a characteristic

Every characteristic is written in the same order. Learn the order and you can read any light:

Two small things trip people up. A small m is metres (the elevation) and a capital M is nautical miles (the range). And because white is so common, charts usually leave the W out: no colour letter means a white light.

The rhythm: how the light flashes

The rhythm is the heart of the characteristic: it tells you how the light switches on and off. A period of darkness between appearances of light is called an eclipse. The rhythms fall into three families: lights that are mostly dark and flash, lights with equal light and dark, and lights that are mostly on and blink out.

Flashing: less light than dark. Isophase: equal. Occulting: more light than dark.

You have already met many of these on buoys. Lateral marks (Lesson 16) show red or green lights, and preferred-channel marks a composite group, Fl(2+1): two flashes, then one (try Fl(2+1) R 10s in the decoder above). Cardinal marks (Lesson 17) show white quick or very quick lights — continuous for a north cardinal, and groups of 3, 6 and 9, like a clock face, for east, south and west, with a long flash after the six of a south cardinal. The isolated danger mark flashes white in pairs, Fl(2); safe water marks use isophase, occulting, a long flash every 10 seconds or Morse A; and special marks show yellow (Lesson 18).

Colour and period

The colour comes straight after the rhythm: R red, G green, Y yellow and, rarely, Bu blue (the emergency wreck marking buoy of Lesson 18 shows blue and yellow flashes alternately). No letter means white. An alternating light, written Al, changes colour as it goes: Al WR shows white and red in turn.

The period is the time for one complete cycle of the pattern, in seconds. For a group, it runs from the start of one group to the start of the next — it is not the length of the darkness between groups. Fixed lights, and quick or very quick lights that flash without a break, have no period printed.

Elevation and range

The figure in metres is the light’s elevation: the height of the light itself — not the top of the tower — above MHWS, the same level the chart uses for heights of land (Lesson 8). Gull Point’s light is 28 m up; the front Port Kerran leading light only 6 m, and the rear one 14 m.

The figure in M is the nominal range: how far the light can be seen at night in clear weather — strictly, when the meteorological visibility is 10 nautical miles. It depends only on how bright the light is, and it is the range usually printed on Admiralty charts. What you actually see can be quite different:

• Visibility. How far a light can be seen in the visibility at the time is its luminous range. In haze, rain or fog it can be far less than the nominal range; in exceptionally clear air it can be more.
• The Earth’s curve. The sea curves away beneath your line of sight, so a light can drop below your horizon while it is still bright enough to see. The higher the light, and the higher your eye, the further off it stays in view — which is why major lighthouses stand tall. Working out that distance (the geographical range) comes later in the course.
• The loom. At night the glow of a powerful light in the sky — its loom — can often be seen before the light itself appears over the horizon.
• Background lights. Near a town, a navigation light can be hard to pick out among street and shore lights.

Sectored lights

Some lights show different colours in different directions. A sectored light often shows white over the safe approach, with red or green sectors either side to warn that you have strayed; its characteristic lists all its colours, for example Fl WRG 5s. The chart draws the sector edges and colours, and the List of Lights gives the edges as true bearings from seaward — the direction you look towards the light. A light may also be obscured, hidden by land, over some arcs.

If you are in the white sector and the light turns red or green, you have crossed a sector edge: check the chart to see which way to alter to get back into the white. The change of colour across an edge is not perfectly sharp, so keep well inside a sector rather than on its edge. The practice chart has no sectored lights, so you will meet them on real charts.

Identifying a light at night

Before dark, list the lights you expect to see from your planned track, with their characteristics and roughly where they should appear. Then, when a light shows up:

1. Note the colour.
2. Count the flashes (or eclipses) in each group, and judge the rhythm: short flashes, a long flash, quick flashes, or mostly on?
3. Time the period with a stopwatch, from the first flash of one group to the first flash of the next. Timing several periods and dividing gives a better answer than timing one.
4. Check the chart. Only one light near you should match. If you can, confirm it with the hand-bearing compass: its bearing should agree with where the chart puts it.

Sometimes two lights match on colour, count and period. On the practice chart, Gull Point lighthouse (Fl(3)10s28m15M) and the East Teeth buoy (Q(3)10s) both show three white flashes every 10 seconds. The buoy’s flashes are quick, about one a second, and it is a dim light low on the water, while the lighthouse is 28 m up and far brighter. They are also in different places, so a bearing settles it. When two lights could match, use everything you know.

The practice chart’s lights

Here is every light on the practice chart, decoded. Learning the lights on your chart before dark is the best preparation there is: when one appears, you will already know what it should look like.

Colour, count, period — then check the chart.

Buoys and lighthouses are not the only lights at sea: every vessel under way at night shows navigation lights too. After the steering and sailing rules in Lesson 20, Lesson 21 teaches you to read them.