Читать книгу Sextant: A Voyage Guided by the Stars and the Men Who Mapped the World’s Oceans - David Barrie - Страница 11

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 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 harmonise 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 behaviour. 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.

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.

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

As this diagram makes clear, the height of Polaris above the northern horizon is equivalent to the observer’s latitude. Thus if Polaris is vertically overhead, you must be at the geographical North Pole, while if it appears right on the horizon you must be on the equator. In this diagram, its height above the horizon is 50 degrees and it follows that you must be 50 degrees north of the equator – in other words your latitude is 50 degrees North.^fn2

So vital was Polaris to the seafarer that the English simply called it ‘the Star’, and Shakespeare knew enough to press it into service in Sonnet 116:

Let me not to the marriage of true minds

Admit impediments. Love is not love

Which alters when it alteration finds,

Or bends with the remover to remove:

O, no! it is an ever-fixèd mark,

That looks on tempests and is never shaken;

It is the star to every wandering bark,

Whose worth’s unknown, although his height be taken … ^fn3

In medieval Latin, Polaris was Stella Maris – ‘star of the sea’ – a term that was applied also to the Virgin Mary, whose sky-blue cloak is emblazoned with a star in many early European paintings. The theologian Alexander Neckham (1157–1217) likened Mary to the Pole Star standing at ‘the fixed hinge of the turning sky’ by which the sailor at night directs his course. Polaris must have seemed a perfect symbol of the Mother of God, the immaculate spiritual guide and intercessor. The name ‘Stella Maris’ was sometimes also applied to the ship’s steering compass on which the thirty-two ‘points’ are still often marked in the form of a star.⁷ Even today ‘Stella Maris’ is a common name for fishing boats and ‘true north’ – solidly reliable, unlike its variable magnetic cousin – was marked on old charts with a star. (True north marks the direction of the geographical north pole, which is fixed,^fn4 whereas the magnetic north pole – like its southern counterpart – wanders and is at present several hundred miles distant from it.)

*

Before the end of the thirteenth century the Venetian explorer Marco Polo recorded that he had measured the altitude of Polaris on the coast of India, though how he did so is unclear. The earliest references to the measurement of the height of Polaris by Portuguese navigators date from the mid-fifteenth century, but it seems unlikely that they were using purpose-built instruments to make their observations.⁸ In theory they could have used the astronomer’s astrolabe, an elaborate device that permitted the observer to find the time of day, as well as solving other astronomical problems. It had by then achieved a high level of sophistication but it is doubtful that many ordinary sailors would have known how to operate one, and such a complicated and costly instrument would not have been required merely for the purpose of measuring the altitudes of heavenly bodies.

A radically simplified version, known as the mariner’s astrolabe, was however widely adopted for use at sea during the sixteenth century,⁹ by which time solar declination tables were available. It was a metal disc or hollow circle with a scale of degrees engraved on its circumference. While it was suspended from its ring, the navigator would adjust the ‘alidade’ (a revolving bar with peep-hole sights) so that it was aligned either with the sun or with Polaris and its height could then be read off from the scale. The motion of the ship, however, and the effects of the wind limited the usefulness of any instrument that had to be freely suspended and it would have been much easier to obtain accurate readings from it on dry land.

The Portuguese poet Luís Vaz de Camões (c.1524–80), who sailed out to India in 1553, witnessed the use of an astrolabe at first hand – though it is not clear which kind – after his ship anchored off the west African coast. In his great epic The Lusiads (first published in 1572) he describes the scene:

Like clouds, the mountains we spied

Began to reveal themselves;

The heavy anchors were readied;

Now arrived, we took in the sails.

And so that we would better know

Where we were in these remote parts,

Using that new instrument, the astrolabe,

An invention of subtle skill and wisdom,

We then landed on an open shore

Where the crew scattered, wishing

To see strange things in the land

Where no other people had trod.

But I, with the pilots, on the sandy beach

To find out where I was,

Remained to take the height of the sun

And measure the painted universe.

We reckoned that we had already passed

The great circle of the Tropic of Capricorn

Being between it and the frozen southern

Circle, the most secret part of the world … ¹⁰

The seaman’s quadrant, an even simpler device that was probably in use at a much earlier date than the mariner’s astrolabe, relied on a plumb-line so it too would have been of limited utility on the open sea. Later developments included the cross-staff (first mentioned in 1545)¹¹ and the more sophisticated back-staff (invented by the English navigator John Davis in the late sixteenth century). These were much more practical than the astrolabe and quadrant, but they were hardly precision instruments. In the latter part of the seventeenth century, as the scientific revolution fast gathered momentum, astronomers started to explore ways in which more accurate sights could be obtained from the moving deck of a ship. It was from this process that the sextant eventually emerged.

Fig 4: Back-staff (left) and cross-staff (right).

Fig 5: Diagram of a sextant, showing the Double Reflection Principle.

The sextant employs the principle of double reflection to enable the user simultaneously to observe the horizon and a chosen heavenly body, and to measure the angular distance between them with great accuracy. Its key virtue is that it marries the two in a single, steady image that is completely unaffected by the movement of the observer (or the deck on which he or she is standing) so long as the instrument is kept pointing in the right direction. The sextant is also versatile. Unlike the astrolabe or quadrant it can be used for measuring angles in any plane – for example, between two heavenly bodies, or between two objects on the surface of the earth.

Fig 6: Hadley’s reflecting ‘quadrant’.

The sextant was the offspring of an earlier invention, the so-called reflecting ‘quadrant’. Sir Isaac Newton can take credit for designing the first device of this kind, plans for which were shown to the Royal Society in 1699.¹² Another Fellow of the Royal Society, John Hadley (1682–1744), came up with two designs, similar to Newton’s though apparently not derived from them, which he presented to that institution in May 1731.¹³ One of these was widely adopted following successful sea trials conducted the following year by the Oxford Professor of Astronomy, John Bradley, who was later to become Astronomer Royal.¹⁴ By one of those strange coincidences that seem common in the history of science, an American – Thomas Godfrey – independently came up with a similar design almost simultaneously.¹⁵

Confusingly, the reflecting quadrant is actually an octant – its arc is one-eighth of a circle (45 degrees) rather than one-quarter. It is capable of measuring angles up to 90 degrees, thanks to the double-reflection principle. The invention of ‘Hadley’s quadrant’ marked a revolution in the history of marine navigation. For the first time, it was possible to measure the altitudes of heavenly bodies from the moving deck of a ship at sea with great precision – in fact, with the help of a vernier scale, quadrants permitted readings to the nearest minute of arc (one-sixtieth of a degree).¹⁶ Its larger cousin, the sextant, which was later to become the instrument of choice for accurate offshore navigation, could measure angles up to 120 degrees. Both the quadrant and the sextant were far superior to any instruments previously available for measuring angles at sea, in terms of both precision and ease of use. The sextant’s original design was so perfect that it has to this day remained essentially unchanged, and with its introduction the solution of the greatest problem of celestial navigation – the accurate determination of longitude at sea – at last became a practical possibility.

*

Having learned how to do a mer alt, I could, in theory, have found my way home to England simply by following the right parallel of latitude: 49 degrees 30 minutes North brings you nicely into the English Channel – halfway between Ushant and Scilly.^fn5 Mariners relied entirely on ‘latitude sailing’ of this kind for hundreds of years before the longitude problem was solved, but it is subject to one potentially disastrous drawback: unless you know how far east or west you have travelled, and the coordinates of your destination, you cannot be sure when you are going to arrive. Latitude sailing also suffers the disadvantage that the shortest distance between two points on the surface of a sphere is a ‘Great Circle’ (a circle whose centre coincides with the centre of the earth), not a parallel of latitude.^fn6 The difference can be significant if the voyage is a long one.

You might suppose that it would be a simple matter for the sailor to work out his position in mid-ocean just by measuring the distance he has travelled on a particular course. This is known as ‘dead reckoning’ (DR), a term that could well have been chosen by someone with a black sense of humour, though its actual origins remain mysterious. Even today, when it is possible to measure speed and distance travelled through the water with great accuracy, DR is an imprecise science. Many factors affect a vessel’s rate of progress ‘over the ground’ (that is, relative to the seabed), some of which are very hard to assess. Ocean currents are one example. These can be strong, but they are fickle, seldom running steadily in one direction or with a constant strength. Steering an accurate course is also much trickier than the landsman might suppose: it is nothing like driving a car down a road. Leaving aside human error, and the tendency of sailing vessels to ‘sag’ (drifting sideways – or making ‘leeway’ – when heading to windward, or ‘close-hauled’), the magnetic compass itself is subject to large errors – which were not well understood until the nineteenth century. Unless correct allowances are made for all these effects, the navigator will soon be lost. DR is, in practice, highly unreliable and especially so over long distances because the errors tend to accumulate – as Álvaro de Mendaña’s experiences in the sixteenth century vividly showed.

There is an extraordinary passage in Moby Dick where Herman Melville contrasts the reliability of celestial navigation with the uncertainties of DR in order to dramatize Captain Ahab’s descent into madness. Consumed by hatred for the white whale that has cost him his leg, Ahab takes his last mer alt seated in the bows of one of the open whaleboats in which he hopes to hunt it down.

At length the desired observation was taken; and with his pencil upon his ivory leg, Ahab soon calculated what his latitude must be at that precise instant. Then falling into a moment’s revery, he again looked up towards the sun and murmured to himself: ‘Thou sea-mark! Thou high and mighty Pilot! Thou tellest me truly where I am – but canst thou cast the least hint where I shall be? Or canst thou tell where some other thing besides me is this moment living? Where is Moby Dick?’

Ahab gazes thoughtfully at the quadrant, handling its ‘numerous cabalistical contrivances’ one after another, and then mutters:

‘Foolish toy! babies’ plaything of haughty Admirals, and Commodores, and Captains: the world brags of thee, of thy cunning and might; but what after all canst thou do, but tell the poor, pitiful point, where thou happenest to be on this wide planet, and the hand that holds thee: no! not one jot more! Thou canst not tell where one drop of water or one grain of sand will be to-morrow noon: and yet with thy impotence thou insultest the sun! Science! Curse thee, thou vain toy … Curse thee, thou quadrant!’

To the astonishment of his crew, Ahab then dashes the instrument to the deck:

‘no longer will I guide my earthly way by thee: the level ship’s compass, and the level dead-reckoning, by log and by line: these shall conduct me, and show my place on the sea. Aye,’ lighting from the boat to the deck, ‘thus I trample on thee, thou paltry thing that feebly pointest on high; thus I split and destroy thee!’¹⁷