page twenty-eight

The Lights

Twenty-seven rooms, and every one of them started by showing you the whole thing: the whole map, the whole board, the whole tower. This one starts in the dark. You are on a small boat off a coast that today's number drew, it is night, and the electronics have gone dead. All you have is the chart, a hand-bearing compass and a black horizon with a few lights winking on it. Every lighthouse on that chart flashes in its own rhythm. Time them, work out which is which, take their bearings, and find out where you are.

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On deck

This is the whole horizon, all the way round, with the compass bearing along the bottom. Click a light to put the stopwatch on it. The strip underneath records the last twenty-four seconds of it, so you can count the flashes and time the gap. When you're ready, take a bearing on it.

No light selected. Click one on the horizon.

Night
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Move over the chart for a position. Click to mark your fix.

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B takes a bearing on the light you're watching, N moves the boat somewhere else, and the left and right arrow keys walk to other nights. Every night is a different coast.

Tonight's light list

Every light is written down twice: once on the chart in shorthand, and once in an official list with more detail. The shorthand gives the rhythm, the colour, the period, the height and the range, in that order, so Fl(3) 15s 38m 19M means three white flashes every fifteen seconds, from a lamp thirty-eight metres above high water, seen nineteen miles off on a clear night. The strips show one full period of each, with light as bright and dark as dark.

LightChartOne periodHeightNominalTonightDips at
Date
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Visibility
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Height of eye
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Ship's head
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Lights in direct sight
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Lights you can't see
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What is real here and what is not

The rules are real, the coast is not. Nobody should take a boat anywhere near this chart.

  1. 01

    The rhythms are real

    Every light carries a characteristic from the international system: flashing, group flashing, long flash, occulting (more light than dark), isophase (equal light and dark), quick, and Morse. Neighbouring lights are given different ones on purpose so a navigator can tell them apart. That is the whole point of them. The cardinal buoys here flash the real codes for north, east, south and west of a danger.

  2. 02

    The ranges are real

    A light's nominal range is how far it carries in ten miles of visibility. Tonight's range comes from Allard's law with the IALA threshold for night, run against tonight's visibility. Whether the lamp is above your horizon at all comes from the standard geographic range rule, 2.03 × (√height + √eye) miles. Beyond that you may still see its loom, the beam sweeping the haze above the horizon, which is real and much less exact to take a bearing on.

  3. 03

    The fix is real

    Each bearing puts you somewhere on one line through that light. Two lines cross at a point, and three nearly never meet exactly. They leave a small triangle, the cocked hat, and its size tells you how much to trust the fix. A bearing plotted from the wrong light doesn't warn you. It just draws a confident line in the wrong place, and that is why timing the light comes first.

  4. 04

    The sector light is real

    The harbour light shows white only in a narrow slice along the safe way in, red to one side and green to the other. On this chart, coming in from seaward, red is on your left. The colour you see tells you which slice you are in before you've taken a single bearing.

  5. 05

    The rest is invented

    The coast, its heights, its names and every light on it are made up by the day's number, and so is the dead reckoning position. The compass has no deviation, and each bearing you take is off by about a degree, as a hand-bearing compass on a moving deck would be. The land in the view is drawn several times taller than it would really look so that it can be seen at all.

Twenty-eight rooms, and one number doing all the work. Tonight it draws a coast, puts a lamp on each headland, gives every lamp its own rhythm so nobody could mistake one for another, and then leaves a boat out in the dark and doesn't tell you where.