Night Grammar

The grammar

A characteristic is a sentence in a fixed order: how the light moves, what colour it is, how long before it repeats, how high it stands and how far it reaches. Most of the time only the first three matter to you.

Fl(2)rhythm Wcolour 10speriod 15mheight 18Mrange

Rhythm is the pattern of light and dark, with a group count in brackets. Colour is W, R, G, Y or Bu; when no colour is written, the light is white. Period is the time for the whole pattern to run once. Height is the light's elevation in metres (in US light lists, feet). Range is in nautical miles, a capital M: how far the chart says it carries. Sources for each rule are at the foot of the page.

Lit less than dark: flashing

In a flashing light, the light is on for less time, in total, than it is off. A long flash lasts two seconds or more. Flashes can come in groups, and a group can be composite: Fl(2+1) is two flashes, a pause, one flash, then the long dark. The gap between groups is at least three times the gap inside one, which is how the eye sorts a run of flashes into words.

Fl G 3s 0 s3 s One green flash every 3 seconds.
LFl 10s 0 s10 s One long white flash every 10 seconds. A long flash lasts at least two seconds.
Fl(3) R 12s 0 s12 s Three red flashes close together, then darkness; the group repeats every 12 seconds.
Fl(2+1) G 15s 0 s15 s Green flashes in groups of two and one, three in all, the whole sequence repeating every 15 seconds.

Lit more than dark: occulting and isophase

Turn a flashing light inside out and you have an occulting one: on for longer, in total, than it is off, so what you notice are the eclipses. Groups work the same way. An isophase light is the balance point between the two: light and dark of exactly equal length.

Oc 6s 0 s6 s A white light that is on more than off, going dark once every 6 seconds.
Oc(2) R 8s 0 s8 s A red light that is on more than off, going dark twice in quick succession every 8 seconds.
Iso G 4s 0 s4 s Green light and darkness in equal halves, one full cycle every 4 seconds.
Iso 8s 0 s8 s White light and darkness in equal halves, one full cycle every 8 seconds.

Too fast to count: quick, very quick, ultra quick

Past a certain speed, flashes stop being things you count and become a texture. The Canadian Hydrographic Service's Chart 1 sets the bands by flashes a minute: quick, 50 to 79 (usually 50 or 60); very quick, 80 to 159 (usually 100 or 120); ultra quick, 160 or more. Here they are drawn at 60, 120 and 240. Written alone, Q runs without a break; with a bracket and a period, it comes in groups; with an I in front, its run of flashes is broken by a regular dark pause.

Q 0 s4 s Quick white flashing, about 60 a minute, without a pause.
VQ 0 s4 s Very quick white flashing, about 120 a minute, without a pause.
UQ 0 s4 s Ultra quick white flashing, about 240 a minute, without a pause.
IQ G 12s 0 s12 s Interrupted quick green flashing: runs of flashes at about 60 a minute, broken by a regular dark pause, every 12 seconds.

Spelled out: Morse

Mo(A) flashes the letter A: a short flash and a long one. Safe-water marks, which usually mean open, deep water lies ahead, often use it.

Mo(A) 8s 0 s8 s The letter A in Morse code (· –), in white, every 8 seconds.
Mo(U) 15s 0 s15 s The letter U in Morse code (· · –), in white, every 15 seconds.

Several colours: alternating and sectored

Two colours written together mean one of two things. With Al in front, the light changes colour as it runs, so everyone sees both colours. Without it, the colours are sectors: the lantern shows a different colour in different directions, and the colour you see tells you which arc you are standing in. Choose a sector below to see what a ship in it would see.

Al.Fl WR 10s 0 s20 s One flash every 10 seconds. Its colour alternates: white, then red.
Fl WRG 5s 12m 15M 0 s5 s One flash every 5 seconds. It shows white, red or green depending on where you stand: a sectored light. The light stands 12 m above high water. Its charted range is 15 nautical miles.

A worked set: the cardinal marks

A cardinal mark is a buoy or beacon that tells you which side of a hazard to pass: a north mark has the safe water to its north. Their lights are white, quick or very quick, and they count like a clock face. East is three flashes, as three o'clock is; south is six; west is nine. North flashes without stopping. South adds one long flash to help tell it from west in poor conditions. Each comes in a quick and a very quick version, so two marks near each other can differ.

NorthVQ 0 s4 s Very quick white flashing, about 120 a minute, without a pause.
EastVQ(3) 5s 0 s5 s Three very quick white flashes, then darkness, repeated every 5 seconds.
SouthVQ(6)+LFl 10s 0 s10 s Six very quick white flashes and then one long flash, then darkness, repeated every 10 seconds.
WestVQ(9) 10s 0 s10 s Nine very quick white flashes, then darkness, repeated every 10 seconds.
North, quickQ 0 s4 s Quick white flashing, about 60 a minute, without a pause.
East, quickQ(3) 10s 0 s10 s Three quick white flashes, then darkness, repeated every 10 seconds.
South, quickQ(6)+LFl 15s 0 s15 s Six quick white flashes and then one long flash, then darkness, repeated every 15 seconds.
West, quickQ(9) 15s 0 s15 s Nine quick white flashes, then darkness, repeated every 15 seconds.

Sources

Definitions of flashing, occulting, isophase and long-flashing, and the quick, very quick and ultra quick rates: Canadian Hydrographic Service, Chart 1, section P, light characters. Group spacing, composite groups, Morse, alternating lights, the implied white and the order of the abbreviation: Light characteristic, Wikipedia. Cardinal mark rhythms and the clock-face rule: Cardinal mark, Wikipedia. Safe-water marks and Morse A: Safe water mark, Wikipedia.