Sextant Read online

  Coming off watch on the first night out, I curled up at the far end of my bunk trying to keep clear of the drips coming through the deck just above me. This is no fun, I reflected—no fun at all. I am cold and scared, and there are nearly three thousand miles of ocean ahead of us. What the hell am I doing here? Why did I agree to come on this trip? I kept thinking of all the comforts I had left behind, especially warmth and dry clothes. The leaks would eventually stop—more or less—but only when the sun-baked teak was thoroughly soaked and the seams had tightened up.

  The following day we crossed the edge of the continental shelf southeast of Sable Island, a menacing sliver of sand and grass that lies right in the shipping lanes—the scene of countless wrecks. Perhaps the first of these to be recorded (by the great compiler of accounts of early voyages, Hakluyt)1occurred in August 1583, when one of the vessels in a small squadron led by the Elizabethan adventurer Sir Humphrey Gilbert ran aground there and broke up. Her crew suffered terribly from thirst and hunger before they reached the coast of Nova Scotia in an open boat. There they were rescued by Basque fishermen (who were by then routinely taking cod from the Grand Banks), and they eventually managed to get back to Europe—unlike Gilbert, who, having coolly turned his back on his shipwrecked companions, was lost on the return voyage.

  Sable Island is a desolate spot, inhabited only by wild ponies and sometimes a few research scientists, with a lighthouse at either end. Colin—who relished a difficult pilotage challenge*—had been planning to land there, but even he did not feel like making the detour in these conditions. I was quietly relieved. The first accurate chart of Sable Island was published in 1779 by Cook’s mentor, DesBarres. From 1763 to 1773 DesBarres charted the heavily indented coast of Nova Scotia, and he devoted two summers to Sable Island alone. Apparently the surf often broke “mast high”on the two sandbars at either end of the main island, which were “strewn with wrecks for seven leagues” (twenty-one miles).2 It was difficult and dangerous work, but of vital importance, and DesBarres’s chart was one of the first to show longitudes (a word that Colin taught me to pronounce—naval fashion—with a soft “g”) based on the Greenwich meridian.

  We had now reached the real ocean, where the seabed plunges from a few hundred feet in depth to ten or twenty thousand. The “abyssal plain”stretched out gloomily beneath us. The Canadian Navy had promised us that no icebergs would drift this far south, so the only likely hazards from now on—apart from bad weather—were other vessels. Nevertheless, it was disconcerting to know that we were suspended over several vertical miles of water, kept afloat only by an inch or two of wood.

  Standing watch alone on the second night out, I was confronted by an overwhelming sight: half the visible universe, velvet black from horizon to horizon, filled with the brightest stars I had ever seen. Their brilliance was undimmed by the orange glow of man-made light that veils the skies over so much of the land, and they seemed infinitely numerous. Three stars, named by the old Arab astronomers, formed a brilliant triangle above us, with the Milky Way—a glowing river of light—running among them: Vega, the falling eagle; Deneb, the tail of the hen; and Altair, the flying eagle.3 The light from the nearest of them had taken sixteen years to reach my eyes. Very occasionally, a shooting star slid soundlessly across the blackness, momentarily animating a spectacle of timeless grandeur and serenity.

  As we sailed into the Atlantic, leaving the land farther and farther astern, I watched the night sky as I had never done before. I recognized some of the main constellations and, with the help of a star chart, picked out a few of the fifty-odd “navigational stars” whose coordinates are listed in the Nautical Almanac. Aldebaran, Alkaid, Alioth, Antares, Arcturus, Capella, Mirfak—as well as the three stars of the so-called Summer Triangle—were all in sight. The planet Jupiter shone brightly, low in the southeast, while Mars rose later, to be followed shortly before dawn by Saturn. On Colin’s advice I was reading Mary Blewitt’s classic introduction to celestial navigation for yachtsmen,4 and a solitary four-hour watch gave me time to watch how the heavens moved. Above me the entire night sky, with all its stars, was slowly revolving counterclockwise around a stationary Polaris. Stars lying closest to the northern celestial pole (the point vertically above the geographical north pole, or in its zenith) never touched the horizon, while the others rose at different points along the eastern horizon, just like the sun by day, climbing up in long majestic arcs before declining slowly in the west. The poet Homer had closely observed the same phenomenon early in the first millennium BCE, as this passage from The Odyssey reveals:

  [Calypso] gave him a warm, fair wind, and Odysseus joyfully spread his sail before it, while he sat and steered skillfully with the stern oar. He never allowed sleep to close his eyes, but kept them fixed on the Pleiades, on Arcturus that sets so late, and on the Great Bear . . . which revolves, keeping watch on Orion, alone never dipping into the stream of Okeanos—for Calypso had told him to keep it on his left.5

  I knew, of course, that this celestial motion was in a sense unreal—that it was the earth that turned while the distant stars maintained their imperturbable stillness—but the illusion is too powerful to resist. The earth remains firmly at the center of the sailor’s universe, just as it did for the Greek astronomer Ptolemy in the second century CE. It is easy to understand how difficult it was for people of the sixteenth and seventeenth centuries to adapt to the heliocentric view of the universe, and for the purposes of the working navigator the Copernican Revolution might never have occurred.

  Day 3: Came on watch at 0400. A fabulous dawn with scarcely a cloud in sight. On a close reach under full main and genoa making 5–6 knots. The seas were much calmer and there was a striking change of color. Instead of a dull green, the water is now an amazing sparkling azure. Also much warmer—the Gulf Stream. Alexa is feeling better. The sun shone brightly all day and we had bread and cheese for lunch with beer.

  Took a meridian altitude with the sextant—my first. Latitude 43°17' N.

  Colin cooked another excellent stew for supper and we all had some whisky. Everyone cheerful.

  Though I was very sleepy, the brilliant light and warmth instantly lifted my spirits. The wind had shifted into the south and we were sailing fast, surging smoothly across the waves rather than crashing through them. At last the decks were dry and we could do without oilskins.

  We had crossed the “Cold Wall,” which marks the division between the frigid, soupy Labrador Current that pours southward out of the Arctic and the vast body of crystalline, lapis-lazuli-blue water that surges out of the Gulf of Mexico between Cuba and Florida and sweeps northward off the east coast of the United States. The volume of the Gulf Stream is so huge that it retains its separate character until well out into the Atlantic, and its vital warming influence is felt across the whole of northern Europe.

  A hundred yards ahead of us, dozens of shearwaters were diving on a shoal of fish, and suddenly the surface of the water exploded in their midst: for a startled moment I thought a missile had been launched from a submarine. An enormous streamlined shape emerged, rising at least ten feet in the air. Turning and catching the sun, its flank flashed silver, before it crashed clumsily back into the sea in a colossal shower of spray. I had never seen such an enormous fish—it was a tuna, perhaps half a ton in weight, a seaborne sprinter that could keep pace with a cheetah. The astonishing spectacle lasted only a couple of seconds. Soon the small fry that had attracted the predators had been consumed, and all was still again.

  As the sun approached its highest point above the southern horizon, Colin appeared in the main hatch, his silver hair sticking out in all directions, wearing an old guernsey jumper that was full of holes. In his hands was the sextant that had, until now, lain unused in the cabin down below, firmly secured in its square wooden box. Before handing it to me Colin warned me with unusual solemnity never, ever to drop it. “Care of the sextant” was a serious matter: it was a precision instrument and our lives depended on its accuracy. My firs
t lesson in celestial navigation was about to start.

  There are many kinds of sextant, and they come in many different sizes—from pocket ones just a few inches across to heavyweight models on a much grander scale. And many materials have been employed in making them, from brass and steel to plastic and even cardboard. The essential design, however, has varied little since the eighteenth century, and a good sextant has the reassuring heft and feel of something really well made. With familiarity comes the recognition that this is an instrument perfectly adapted to its purpose: a solution to a practical problem so elegant and efficient as to be quite simply beautiful. But although I had studied a diagram, the sextant now in my hands was bafflingly unfamiliar. Attached to a triangular black steel frame with a wooden handle on one side were two mirrors, several dark shades, a small telescope, and an index arm with a micrometer drum that swung along a silvered arc marked in degrees. Colin showed me how to hold it, with the handle in my right hand and my eye to the telescope.

  Fig 1: Principles of the Meridian Altitude.

  I had to measure the height of the sun above the horizon just as it reached its highest point in the sky due south of us—as it crossed our meridian. Colin first adjusted the shades on the sextant, then, looking through the telescope, moved the index arm until the sun’s lower edge (or “limb,” in astronomical jargon) was more or less on the horizon. I then took his place in the main hatch, braced against the slow roll of the boat, and, gripping the handle firmly, peered tentatively through the telescope at the southern horizon.

  Fig 2: Diagram illustrating the sun’s varying declination.

  (G.P. is the geographical position: see Glossary.)

  All I could see at first was a circle divided vertically between a light half and a dark: the left-hand side was the direct view through the plain glass side of the horizon mirror, and the darker right-hand side was the reflected view of the sky above us through the heavily shaded index mirror. Then I found the horizon and, scanning to left and right, caught a glimpse of a brilliant white disc floating just above the dark line of the sea. In a moment it had gone, but then I caught and held it, fascinated to see it moving steadily upward, the gap between the disc and the horizon widening all the time. It was the sun, and I was watching the earth turn.

  If I rocked the sextant from side to side, the sun swung in an arc across the sky. By adjusting the micrometer, I brought the disc slowly down until, when the sextant was held vertically, its lower limb was just kissing the horizon. The sun was still moving upward, but much more slowly now as it neared our meridian. After a minute or two, the white disc paused at the top of its arc. Taking the sextant away from my eye, I looked at the scale and read off the angle: 64° on the main scale and 41 (60 minutes to one degree) on the micrometer. This was the sun’s meridian altitude, or “mer alt.”

  Colin took the sacred instrument from me and confirmed the reading. I looked up the sun’s declination in the Nautical Almanac and made a few corrections to the observed angle. In a few minutes, to my astonished delight, I completed the simple addition and subtraction sums that yielded our latitude.6 We were somewhere on the parallel of 43°17' North, and—as Colin observed—I was now as well equipped to find my way safely across an ocean as any European mariner before the time of Captain Cook.

  Chapter 3

  The Origins of the Sextant

  Day 4: Woken at 0400 and watched a perfect sunrise at 0535. Still reaching at a good 5 to 6 knots on course of about 110°. Have covered 330 miles. Beginning to feel very grubby but there’s no fresh water to spare for washing.

  Another hot, clear, calm day with wind SSW force 2–3. Passed a bulk cargo ship going the other way. Started reading Slocum1 sitting in the sun, then took a nap from 10–12. Then did another mer alt—42°58' N.

  Our course—approximately 120° magnetic—is meant to take us clear of the Tail of the Bank,* where there are likely to be many fishing boats. Slocum sailed on just this route when he set off on his round-the-world voyage.

  Alexa saw some dolphins. I could hear them down below. Everyone in very good spirits.

  The heavens have always fascinated people, and we have long looked to them for guidance, though we were not the first animals to do so. Many different species use the sun, moon, and stars to help them reach their destinations—whether these are nests a few yards away, or breeding grounds on the other side of the world. The magnificent monarch butterfly, for example, relies on an internal sun compass to find its way at the end of every summer from the eastern United States south to the mountains of central Mexico, where vast numbers pass the winter clinging to the trees. On a more modest scale, dung beetles have recently been shown to use the orientation of the Milky Way to help them roll dung balls back to their nests by the shortest route,2 and honeybees use polarized sunlight to navigate to and from their hives on foraging trips.3 Mystery still surrounds the exact nature of the homing pigeon’s skills, but they seem to involve a magnetic sense, coupled with a kind of sun compass, and the ability to hear low-frequency sound, such as that produced by the breaking waves that mark the line of the coast.4 Some migrating birds rely on Polaris, and seals too can steer by the stars.5

  Perhaps our pre-human ancestors wondered about the sun, moon, and stars before our own species appeared a couple of hundred thousand years ago. Certainly the earliest humans must have realized that most “heavenly bodies” (the old term is irresistible) moved in regular and predictable ways, and that these motions were linked to vitally important seasonal variations in the activities of plants and animals, as well as the length of the days, the weather, and the tides. The structures left behind by our prehistoric ancestors present many puzzles but they do at least reveal that their builders had an excellent grasp of the motions of the sun and moon. At dawn on the shortest day of the year (the winter solstice, when the sun stands vertically above the Tropic of Capricorn), the first light still strikes through a carefully placed stone aperture above the entrance to the great passage grave at Newgrange, in the Boyne valley in Ireland. Shooting down a long, low corridor of massive stones it briefly illuminates the burial chamber at the heart of the huge mound. Stonehenge may be a little younger than Newgrange—perhaps only 4,500 years old, rather than 5,200—but the behavior of the sun and moon clearly mattered deeply to the designers of this elaborate array of standing stones. The sun on the longest day of the year (the summer solstice, when the sun stands vertically over the Tropic of Cancer), when observed from the center of the stone circle, rises just over the top of a single, lonely stone at the perimeter (the Heelstone), as does the full moon closest to the winter solstice.6

  More recent than these Neolithic monuments, a mere 3,600 years old, is the spectacular Nebra Sky Disc. Illegally excavated in Germany in 1999, and retrieved after an undercover police operation, it seemed at first too good to be true. Many experts feared that the dinner-plate-sized object was a fake, but extensive tests have shown that it is genuine. It is perhaps the oldest known visual representation of the cosmos, revealing for the first time that Bronze Age Europeans—like the supposedly more sophisticated inhabitants of ancient Egypt and Mesopotamia—paid close attention not only to the sun and moon, but also the stars. The tight group of seven stars represent the Pleiades as they appeared at that epoch, and the Disc may have been used to harmonize the solar and lunar calendars, a process hitherto thought to have been a Babylonian discovery a thousand years after the disc was made.

  Accurate calendars would, of course, have been useful for many purposes, such as choosing when best to plant crops, but it is hard to believe that this was the Nebra Sky Disc’s only purpose. Creation myths from around the world offer wildly varied accounts of the origins of the sun, moon, and stars and the significance of their behavior. The extraordinary imaginative energy they display plainly arose from deep concerns about our place in the universe and the meaning of life and death. Such concerns must surely also have influenced the designer of the disc. It has been suggested that the curved piece
of gold at the bottom of the disc represents a boat—perhaps one that safely carries the sun across the ocean after it has set. It might equally refer to the passage of the soul to the afterlife. We cannot help sensing that this extraordinary object, like the many prehistoric structures that are aligned with the heavens, embodies profound, if mysterious, spiritual beliefs.

  Fig 3: Diagram illustrating the equivalence of Polaris altitude and latitude.

  Until very recent times the heavens shaped the patterns of everyday life. The farmer judged when to sow his crops by looking at the night sky, and the sun and stars told him the time of day, long before the first mechanical clocks were invented. One of the western portals of the great thirteenth-century cathedral of Amiens in northern France is decorated with the signs of the zodiac, each one accompanied by a depiction of the activities appropriate to the month with which it was associated—such as sowing, reaping, cutting hay, and treading grapes. Similar motifs appear on many other medieval buildings. But people did not rely on the heavens only to plan their communal activities; they also thought that the sun, moon, and stars could foretell what lay in store for them as individuals or nations. This belief was—and is—so widespread as to qualify as a cultural universal.

  The skies were, of course, especially important for sailors. The moon enabled them to predict the height of the tide; “full” and “new” bring the “spring” tides, which have the widest range and produce the strongest currents, while the “half” moon signals the “neaps,” with the narrowest and weakest. By day the sun, rising in the east and setting in the west, told mariners roughly which way they were steering, as did Polaris—much more simply—by night. For navigators in the Northern Hemisphere, the height of Polaris was the crucial measure of latitude—the only one, in fact, until astronomers were able to produce accurate tables of the sun’s varying declination at the end of the fifteenth century.