What the Sea Does at Night - Night Passage Observations

Reading the Sea at Night — Bioluminescence, Navigation Lights, and What Night Passage Reveals

Most of what you read by day is still there at night, in a different edition. Bioluminescence, the coded language of buoys and lighthouse beams, glitter paths off harbour lights, the sound of breaking water on a coast you cannot see. All of it available continuously, requiring no power, never losing signal. The skills built by day translate sideways.

I do not think most sailors understand what bioluminescence is until they have seen it. The first time is strange. You trail a hand in the cockpit and the water lights up around your fingers. A cold blue-green glow you have not switched on and cannot switch off. Every disturbance to the surface throws light. The bow wave is a sheet of it. The crest of a breaking wavelet is a brief explosion. A seabird disturbing the surface looks, for a moment, like it has set fire to the water.

The cause in British and European waters is single-celled. Noctiluca scintillans, sea sparkle. It produces light through a biochemical reaction when mechanically disturbed in the same way a glow stick produces light when you bend it. Warm water and calm nights mean a healthy bloom in late summer and early autumn when sea surface temperatures are highest and Noctiluca populations peak. The Bay of Biscay in August can throw a sheet of cold fire off a breaking bow wave. North Sea displays are usually more modest. The Baltic produces striking shows in the warmer enclosed southern waters in summer.

None of this is just spectacle. The Marshallese developed a navigation system based on it.

Phosphorescence as a navigation instrument

David Lewis records in We, the Navigators that Marshallese navigators read directional streaks of phosphorescence visible on the sea surface at night between islands. These streaks arise where deep underwater currents, deflected and shaped by submerged topography, disturb the water column enough to produce surface patterns in the bioluminescent layer. The relationship between underwater geography and surface light was precise and consistent enough that a trained navigator could determine which passage they were in, and where land lay.

This is one of the more localised elements of Pacific Island navigation. It required deep knowledge of a specific area built up over generations. It is not a transferable skill. But the underlying mechanism, that bioluminescence at the surface reflects subsurface conditions, is general. A sudden increase in phosphorescent activity in a previously dark patch of water can indicate a change in current, a turbulent mixing zone, or disturbed water over a shoal. Not a precision instrument. A signal worth noticing.

For the Channel or North Sea sailor the immediate value is more modest. In flat dark conditions with no visible horizon and a failing log, a glowing bow wave confirms you are moving through the water and gives a rough sense of speed from the width and brightness of the disturbance. It is also a useful collision-awareness aid. A vessel on a converging course with a glowing bow wave is visible at distances that would otherwise require you to be watching its navigation lights specifically.

The system of buoys and lights

There is a moment in becoming a competent offshore sailor where a harbour approach at night stops being a confusing array of blinking coloured lights and starts being a system that makes complete sense. It takes longer than it should because the logic is rarely explained whole.

Every light at sea has a colour and a pattern. Between them those two properties tell you exactly what the light is and which side of it you should be on. No colour combination and flash pattern is repeated in any given area. Every buoy, mark, and lighthouse is individually identifiable once you know what to look for.

Fixed lights are on permanently. Flashing lights are off more than on. Occulting lights are the reverse. Isophase lights are evenly split. The naming is not decorative. The flash pattern is something you count at three in the morning with your eyes streaming, and it needs to be unambiguous.

Lateral marks

Lateral marks are the most numerous lights you will encounter in harbour approaches. Red marks port. Green marks starboard. Always from the perspective of a vessel coming in from the sea. The IALA-A system used across Europe and most of the world has you keep red marks to port and green to starboard on approach. The Americas and Japan use the opposite convention, which is the kind of thing worth knowing before you arrive in a strange port in the dark.

Fixed laterals form a continuous line that can be followed like the cat’s eyes on a road in low visibility. Flashing ones pick out specific hazards or key turning points with unique flash characteristics, one flash then two then three, so a prepared skipper can confirm exactly which mark they are looking at before committing to a turn.

Cardinal marks

Cardinal marks point you toward safe water. The name refers to the cardinal compass points. A north cardinal means safe water lies to its north, a south cardinal to its south.

All cardinal-mark lights are white and flash to a clock-face pattern. East at three o’clock flashes three times. West at nine o’clock flashes nine times. North at twelve flashes continuously. South at six flashes six times, followed by a distinguishing long flash. Learn the clock-face rule once and you have it for life. This is the rare piece of navigational knowledge that stores cleanly and retrieves easily at 0300 in a Channel swell, and I have come to think it is the most quietly elegant thing in the IALA system. Five seconds of arithmetic and you have placed yourself.

Lighthouses and sectored lights

Lighthouses use the same characteristic system as buoys but operate at greater range and typically have longer flash periods. Two extra pieces of information appear on the chart: height above sea level, and nominal range. Height matters because it determines how far away you can raise the light from different deck heights. A skipper in a small boat raises a lighthouse considerably later than the navigator on a large ship.

Sectored lights add a layer of navigational information. A single lighthouse can appear red, white, or green depending on the bearing it is viewed from. Red is the danger sector. White is the safe channel. Green is another sector. The Needles light at the western end of the Isle of Wight uses this system, with colour sectors corresponding to the various approaches into the western Solent. It is one of the more instructive lights to study from a chart before a first night approach to that area.

The habit worth building is a brief pilotage plan before any night arrival. The expected sequence of lights. The unique characteristic of each critical mark. The bearing on which you should first see each one. This turns the approach from anxious improvisation into a sequence of confirmations.

Glitter paths after dark

The glitter path does not disappear at night. Its source changes. Harbour lights, lighthouse beams, the moon, bright planets and stars. The reading principles are the same. Wider where the water is rougher, narrower where it is calm.

Gooley describes watching the glitter path from Falmouth Harbour broaden in the middle where a tidal current was running, making a rough patch visible at night purely through this effect. This is one of the few clues available to what the current is doing in the approaches at night, when the surface eddies and ripples that give it away by day are invisible. A skipper noticing that the glitter path from the eastern pierhead is wider in one section than another, and who knows the tidal pattern of that harbour, can infer that the ebb is running stronger in the fairway than on the shoaler edges. Polarised sunglasses suppress glitter paths almost completely. Worth remembering if you are trying to use them. The day-time use of the same effect is in Beyond the Blue.

What you can still hear

The skills built through this series do not all go dark when the sun does. The sense of swell direction felt through the hull is more accessible at night, when visual distractions are removed and the boat’s motion becomes the primary input. The longer reading on swell is in The Ocean’s Long Memory. The back-bearing habit described in What Moving Water Tells You works at night, provided there are identifiable lights ashore to use as transit marks.

Sound returns as a useful navigation input at night in ways it is often neglected during the day. Breaking water over a shoal or a rock is audible at useful distances in calm conditions. The echo of the boat’s own sounds off a cliff face gives an approximate bearing. A surf beach at night is heard well before it is seen. The character of that sound, fine sand versus shingle versus cobble producing distinctly different pitches, has been used by Cornish fishermen for fog approaches to known beaches. The same principle is in What the Foreshore Tells You.

The stars are now available in a way they are not by day. Covered in The Sidereal Compass and in Finding Your Latitude Without Instruments, where the Polaris method requires nothing more than a clear northern horizon and a rough sense of what ten degrees looks like against an extended fist. Not emergency tools. Backup instruments that work continuously and require no power.

Night passages in European waters

The Channel in settled summer weather is about as well-lit a piece of open water as exists. Traffic separation lanes are marked on every chart. AIS makes large-vessel positions visible. The lighthouse network from Ushant to the Downs is comprehensive. The bewildering night approach to a French Channel port, Cherbourg or Dieppe or Fécamp, is bewildering only because you have not yet built the habit of reading it. The harbour lights, the flash patterns of the approach buoys, the sectored leading lights into the entrance, all in the almanac and on the chart, predictable, entirely readable once you have sat down before departure and drawn it out on paper. The lights do not move. You do.

The North Sea at night offshore is darker and emptier. The East Anglian sandbanks, Outer Gabbard and Sunk and Long Sand Head, are marked by light vessels and cardinal buoys whose characteristics are in the almanac, and which confirm your position against a DR plot in a way that a GPS fix cannot. The GPS tells you where the electronics think you are. The light character tells you where you actually are, in a tradition that goes back through every chart correction and wreck commission to the first keeper who climbed a lighthouse in the dark.

There is something clarifying about confirming a position by counting the flashes of a light you have never seen before and finding them exactly where the chart said they would be.

What I have not done enough of is night work in small open craft, where the eye level is lower, the platform is less stable for fine work, and different lights become visible in different sequences than from a yacht cockpit. The reading translates. The boat-specific calibration is its own piece of work, and I am not going to write it until I have done the hours.

For somewhere to start the after-dark calibration in sheltered water, the Hithe Finder is a community register of slipways, hards, and beaches. An hour at a launching site after dark, counting flashes and watching sound carry across calm water, builds the habit before it matters.

References

Gooley, T. (2016). How to Read Water: Clues and Patterns from Puddles to the Sea. Sceptre. Available via Google Books. Includes the chapters on bioluminescence and on glitter paths, including the Falmouth observation that informs the night-current section above.

Lewis, D. (1994). We, the Navigators: The Ancient Art of Landfinding in the Pacific, 2nd edition. University of Hawaii Press. Available via Google Books. The canonical account of Pacific Island navigation, including the Marshallese tradition of reading directional phosphorescence streaks between islands.

Lane, C.D. (1942, reprinted 2011). The Boatman's Manual: A Complete Manual of Boat Handling. Available via Anna's Archive and Internet Archive. Practical small-craft references for night handling and approach.

Trinity House (current). IALA-A buoyage system reference. The authoritative UK source for the lateral and cardinal mark systems described above.

Related notes in the series: tidal currents at the surface in What Moving Water Tells You; the wave-energy context in The Beaufort Scale and What It Actually Looks Like; the audible character of different shores in What the Foreshore Tells You. The complementary night skills — star compass bearings and latitude from Polaris — are in Series 2: Traditional Navigation Techniques. The full Reading the Sea series index is at Reading the Sea the Old Fashioned Way.

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