Sextant: A Voyage Guided by the Stars and the Men Who Mapped the World’s Oceans. David Barrie

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 first 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.

      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 1: Principles of the Meridian Altitude

      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 upwards, the gap between the disc and the horizon widening all the time. It was the sun and I was watching the earth turn.

       Fig 2: Diagram illustrating the sun’s varying declination. (G.P. is the geographical position: see Glossary.)

      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 upwards, 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 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^fn1 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 USA 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, on 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 behaviour 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) observed from the centre 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