Day 7: Stayed in my bunk ’til 0745 and then sat in the sun reading Slocum while watching clouds building up – the barometer is falling and the weather is starting to break. The wind veered to W and increased to F 5 so we took down the main and ran on under genoa at 6–7 knots. Colin was still not comfortable though, so we went right down to the pocket handkerchief No. 2 stays’l which cut our speed to 4 knots.
After lunch – the usual sandwiches though the bread is getting mouldy round the edges – I went to sleep or tried to for two hours. Much rolling and rattling. It’s amazing how much the weather affects one’s mood out here. All the same, we’ve made good progress so far and today we’ve covered 144 miles, noon to noon. Our course is now 105° magnetic. Helped Colin work out our noon position: 42° 34' N, 46° 16' W.
Celestial navigation would be easy if the sun and all the other heavenly bodies stood motionless in the sky – as Polaris does, almost.fn1 It would then be possible to fix your position by sextant sights without the need to know the time or even the date. The cosmos, however, is not that obliging. Not only does the earth rotate completely once every day, but its axis of rotation – currently inclined at roughly 23.5 degrees to the plane of its orbit around the sun – also changes gradually over a cycle of about 25,800 years.fn2 To complicate matters further, the earth’s orbit around the sun is elliptical rather than circular, with the result that the interval between one passage of the sun over the observer’s meridian and the next is not quite constant. So not only do the heavens appear to be in motion, but that motion itself is also changeful. This is most obviously revealed by the variations in the path of the sun across the sky – which is measured by its declination to the north or south of the equator – the phenomenon that gives rise to the seasons. The behaviour of the planets and the moon is yet more complex.
The ancient Greeks and Romans, who leaned heavily on earlier Babylonian learning, had a well-developed understanding of the paths that the various heavenly bodies described, as did the Arab astronomers who followed them. They clung, however, to the view – associated with the astronomer Ptolemy (c.90–168 CE) – that the earth was at the centre of the universe, and this theory prevailed until the time of Copernicus (1473–1543).1 Though Ptolemaic orthodoxy may have been misguided, it did not prevent astronomers producing accurate solar declination tables as far back as the end of the fifteenth century. These enabled mariners for the first time to adjust their observations of the sun to allow for the seasonal changes in its meridian altitude. Now they could determine their latitude at noon as the sun crossed their meridian, as well as after dark (from the height of Polaris), subject to the limitations of the instruments then at their disposal. Moreover, they could continue to find their latitude when south of the equator – when Polaris had disappeared below the northern horizon. This breakthrough helped the Portuguese to open up an enormously valuable trade route into the Indian Ocean round the Cape of Good Hope. Early in the sixteenth century the Portuguese also devised a rule for determining latitude by reference to the stars of the Southern Cross – which lie some distance from the south celestial pole.2
While latitude could be determined quite easily, the earth’s motions meant that the measurement of longitude was a much more difficult challenge. Early in the sixteenth century the astronomer Gemma Frisius (1508–55) realized that a promising approach to solving the longitude problem would be to find a way of measuring time accurately – whether on land or sea. An observer equipped with an accurate enough clock set to the time at a reference meridian could in principle compare the time of an event (such as sunrise or sunset) with the predicted time of the same event at a reference meridian – such as Greenwich.fn3 The observer’s longitude could then be derived by converting the time difference in hours and minutes into a spatial displacement measured in degrees and minutes east or west – one hour being equal to 15 degrees of longitude (360 divided by 24).
It was not until the early seventeenth century that Copernican theory was firmly established on the basis of the observations of Tycho Brahe (1546–1601), Galileo Galilei (1564–1642) and Johannes Kepler (1571–1630). Galileo’s momentous discovery of the moons of Jupiter and, soon afterwards, of the changing phases of the planet Venus not only provided overwhelming evidence that the earth was not the centre of the universe but also opened the way to a proper understanding of planetary motion.3 The invention of the first pendulum clock in the 1650s by Christiaan Huygens (1629–95) also marked a great advance. It was now possible for astronomers to measure time with sufficient precision to predict with great accuracy the positions of all the major heavenly bodies day by day – though, as we shall see, there was one troublesome exception: the moon. The establishment of the two great Royal Observatories in Paris (1667) and Greenwich (1675) contributed notably to this process. These technical developments, coupled with major theoretical advances – of which the publication in 1687 of Newton’s laws of motion was the most significant – were crucial steps on the path to the eventual solution of the longitude problem.
By the end of the seventeenth century, the laborious observations of astronomers had yielded the first accurate ephemeris tables.4 An observer on dry land supplied with a pendulum clock could now regulate it by reference to the predicted events and thereby establish his or her longitude. French scientists were the first to apply the new technology to the making of accurate terrestrial maps and the results were sometimes surprising. In 1693 a new map of the coast of France based on an elaborate survey supervised by the astronomers Jean Picard (1620–82) and Philippe de La Hire (1640–1718) revealed that the kingdom had shrunk. The port of Brest, for example, had moved 50 miles to the east of its position on the best existing map. King Louis XIV is reputed to have complained that he had lost more territory to his astronomers than to his enemies.5
The French undertook much basic research – including heroic efforts to determine the precise shape of the planet, a knowledge of which was essential if maps were accurately to reflect reality. Scientists were sent all over the world in an attempt to decide whether Newton’s prediction that the earth bulged slightly around the equator was correct. If it did, the geographical length of a degree of latitude would increase as one moved away from the equator towards the poles. While one such expedition went to Finland and another to South Africa, a third, led by Louis Godin, set out in 1735 for the Andes to try to measure a degree of latitude at the equator. Godin and his team endured extraordinary hardships, first struggling through tropical jungles and then working at heights of over 16,000 feet on the freezing mountaintops, as they carefully measured baselines and extended a network of triangles along the mountain chain over a distance of some 200 miles.fn4 Their efforts, combined with the work of the other expeditions, confirmed Newton’s prediction.6
The British were initially slow to learn from the French map-makers, but a growing awareness of the great military and commercial advantages conferred by good maps and charts prompted action. Murdoch Mackenzie Senior (1712–97) led the way with his pioneering marine survey of the Orkney Islands in the 1740s, based on a rigid system of triangulation using precisely measured baselines, the first of which was laid out on the frozen surface of a lake.7 Mackenzie’s were the first accurate British charts, and he also invented the system of symbolic abbreviations that survives to this day. His Treatise on Maritim Surveying (published in 1774) was to set the pattern for every hydrographic survey conducted over the next hundred years, and in it he listed the quadrant and sextant as essential items of the marine surveyor’s equipment. Of the sextant he had this to say:
This instrument may be used with great Advantage in Maritim surveys, on most Occasions; being more portable,