by John Gribbin
The British expedition to make observations on the far side of the world, at Sumatra, set off on board HMS Seahorse on 8 January 1761, while a second team was sent to St Helena. The two observers on board the Seahorse were Jeremiah Dixon and Charles Mason, later to become famous as surveyors of the Mason–Dixon line in what became the United States. But only a couple of days after leaving port the ship was attacked by a French frigate and suffered severe damage, limping back to harbour with eleven dead and thirty-seven wounded. After repairs they set off again, but because of the delay had to make their observations from the Cape of Good Hope. Altogether, well over a hundred observers were involved in observations of the transit of 1761, but without measurements from the Pacific the data were not of the best quality. This made astronomers all the more determined to take full advantage of the 1769 transit, with national pride seen to be very much at stake. In 1766, promoting the possibility of an expedition to the Pacific, the Royal boasted that the British ‘were inferior to no nation on earth, ancient or modern’.fn6
Just over a year later, when plans for the expedition were well advanced, an unexpected bonus fell into their lap. The Cornish navigator Samuel Wallis returned to England with his ship the Dolphin, having circumnavigated the globe, and announced his discovery of the island now known as Tahiti. It was already vaguely known to mariners that there were Pacific islands, but Wallis was the one who stopped at Tahiti and made an accurate determination of its latitude and longitude. His return was just in time for Lieutenant James Cook, Captain of HMS Endeavour, to get the news (and some of Wallis’s crew members) before setting off on his own circumnavigation. Tahiti was clearly the best place known for the transit observations he intended to make, and his plans were revised accordingly.fn7
Cook was not only Captain of the Endeavour, but an experienced and competent astronomer who was one of the two officially nominated observers, the other being Charles Green; he even received a small stipend for his astronomical contribution on top of his naval pay. Green, who worked at the Royal Greenwich Observatory, was an expert at determining longitude using the lunar method, a key to determining the precise location of Tahiti. Cook’s ship reached Tahiti on 13 April, and the crew set about building a substantial observatory within an enclosure known as Fort Venus on a sandy spit of land they dubbed Point Venus (still known by that name). Some idea of the scale of the fortified enclosure can be gleaned from the fact that fifty-four tents were contained within its walls, housing not only the men but a blacksmith’s workshop and cookhouse as well as the observatory itself. As the day of the transit, 3 June 1769, approached, parties were also dispatched to the other side of the island and to a nearby island, Moorea, for additional observations.fn8 Although, as Cook noted in his journal, the day ‘prov’d as favourable to our purpose as we could wish, not a Clowd was to be seen the whole day and the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole of the passage of the Planet Venus over the Suns disk’, the different observers found that their measurements of the exact moments when Venus entered and left the Sun’s disc ‘Differ’d from one another … much more than could be expected.’
The Royal was disappointed by the results, and placed the blame for what they saw as an at least partial failure of the expedition on Green, who conveniently had died on the way home and could not answer back. But in 1771 the Savilian Professor of Astronomy, Thomas Hornsby, published an analysis of all the 1769 transit observations, which led him to conclude that ‘the mean distance from the Earth to the Sun [is] 93,726,900 English miles’, and in the same year Jérôme Lalande, who was also involved in the prediction of the return of Halley’s comet, used data from both the 1761 and 1769 transits (from sixty-two observing stations in 1761 and sixty-three, mostly different, observing stations in 1769) to come up with a distance of 153 million kilometres, plus or minus 1 million km. The modern value is 149,597,870 km (in round, imperial, numbers, 92,955,000 miles), and it seems likely that Hornsby got such a ‘good’ value as much by luck as by skill, with some errors cancelling each other out. But even Lalande’s estimate is impressive for the time. With the aid of Kepler’s laws, it meant that the distances of all the planets from the Sun could be calculated with reasonable accuracy for the first time.
The most important aspect of the observations of the two transits of Venus in the 1760s was not, however, the measurement of the distance to the Sun. It was the fact that the effort was made at all. This was the first time there was an organised international effort to tackle a major scientific problem, at great expense. It has been likened in scope with the European collaboration on the Large Hadron Collider at CERN in modern times. But the scientists at CERN do not have to battle with enemy frigates, storms at sea, scurvy and tropical diseases. They can reasonably expect that, unlike Green, they will get home after their experiments.
Cook may have been disappointed by the limited success of the transit observations, but his voyage was far from over. At Tahiti, he opened sealed orders from the Admiralty, which revealed why the Navy and the Government had been willing to subsidise this scientific expedition, contributing not only the ship but also £4,000 towards the costs. He was to head south in search of the fabled Terra Australis Incognita (unknown land of the south), and if he failed to find it to turn west and investigate New Zealand and New Holland (Australia), recently discovered by the Dutch. The rest, as they say, is history. Perhaps those territories would have fallen into British hands anyway, but it was the timing of the transit, and Halley’s promotion of the idea of an expedition to the Pacific region to make observations, that made it happen when it did.
Accounts of Halley’s work usually end here, saying that this was his last contribution to both science and discovery, made two decades after his death. But there is more. One of Halley’s ‘experiments’ actually did not achieve fruition until the end of the 1970s, not two decades but two centuries after his death, and once again it involved the Sun.
In the 1970s, the American astronomer Jack Eddy, working at the High Altitude Observatory of the US National Center for Atmospheric Research, became interested in the possibility of a link between climate change on Earth and changes in the level of solar activity, measured by the number of dark spots seen on the surface of the Sun each year. Over a cycle roughly eleven years long, the number of sunspots first increases to a peak then decreases to a minimum, but the peak level reached differs from one cycle to another. It happens that during the second half of the seventeenth century, exactly at the time of severe European cold mentioned earlier (Chapter Six), the Sun had been very quiet, with hardly any sunspots seen between 1645 and 1715. This interval of low solar activity became known as the Maunder Minimum, after the astronomer who first noticed it from his studies of old records. This led Eddy and his colleagues to investigate the possibility that the size of the Sun mighty be changing – that it was either shrinking or expanding, very slowly – and that this was affecting its output of heat. One way to test this was by looking at old records of transits of Mercury, to see how long it took for the planet to cross the disc of the Sun. If the Sun was larger in the past, for example, then the transit would have taken longer. But the observations were not precise enough to settle the question. There was, though, one set of very precise old data that could be used to test the idea: Halley’s study of the eclipse of 1715.
The exact position of the edge of the shadow on the Earth cast by the Moon during a solar eclipse depends on the size of the Moon (which is well known and doesn’t change) and on the exact position of the Moon in its orbit during the eclipse, which determines its distance from us and can be calculated. It also depends on the exact size of the Sun. At the end of 1980, a team of researchers headed by David Dunham reported a comparison of Halley’s records of the 1715 eclipse with data from an eclipse seen in 1976 in Australia and one visible from North America in 1979. They concluded that ‘between 1715 and 1979, a decrease in the solar radius of 0.34 + 0.2 arc seconds was observed
’. This stimulated further investigations using a variety of historical and modern data, including transits and other eclipse observation, which suggested that there has been a long, slow decline in the radius of the Sun of 0.01 per cent (about 44 miles/70 km) since the early 1700s. If this is a real phenomenon (and it is still possible that such a small apparent change might simply be appearing because some of the old records are inaccurate) it is probably part of a long-term cycle in which the Sun ‘breathes’ in and out; it does not mean that the Sun is going to shrink away and disappear. Whether or not the change is real, the links between changes in solar activity and climate are still unproven and a matter of (sometimes heated) debate among the experts. But that is not our concern here. What matters in the present context is that not only was Halley far-sighted enough to provide valuable data for future generations to use, he was good enough at his job to make sure that those data really were useful, 238 years after his death. Perhaps Dunham and his colleagues should have included Halley as co-author of their paper. But that really does mark the end of the direct contributions of Halley and Hooke to science.
How can we sum up the relative achievements of Hooke, Halley and Newton, and their contribution to the scientific revolution? Ironically, in view of Newton’s religious beliefs, the best approach is to treat them as a trinity. Hooke had the greatest physical insight, and even if we set to one side his other scientific achievements (microscopy, geophysics and the rest he was the first person to realise that the same laws of physics apply in the Universe at large as here on Earth, and to appreciate in particular that the inverse square law of gravity is a universal force and that it acts centripetally; Newton was a mathematical genius (his other activities, alchemy and theology, are best set to one side) who codified the new physics by providing a set of equations to describe the behaviour of everything from balls rolling down slopes to planets orbiting the Sun; Halley (apart from his other achievements as an astronomical observer and geophysicist) was the first person to apply those equations to new problems, rather than ‘merely’ explaining past observations, and use them to make successful predictions, the ultimate (indeed, only) test of any scientific theory. None of them deserves to be remembered in the shadow of any of the others, but if push came to shove, we would certainly place Hooke ‘first among equals’.
CODA
HOW TO DO SCIENCE
In an undated document, published after his deathfn1 but probably drawn up early in his career as Curator of Experiments, Robert Hooke set out his prescription for an ideal way to carry out scientific investigations. This not only gives insight into the way he, and the Royal Society, worked, but is relevant to anyone planning to do science today, or to understand why science provides the best description we have of the way the world works.
The Reason of making Experiments is, for the Discovery of the Method of Nature, in its Progress and Operations.
Whosoever therefore doth rightly make Experiments, doth design to enquire into some of these Operations; and, in order thereunto, doth consider what Circumstances and Effects, in that Experiment, will be material and instructive in that Enquiry, whether for the confirming or destroying of any preconceived Notion, or for the Limitation and Bounding thereof, either to this or that Part of the Hypothesis, by allowing a greater Latitude and Extent to one part, and by diminishing or restraining another Part within narrower Bounds than were at first imagin’d, or hypothetically supposed.
The method therefore of making experiments by the Royal Society, I conceive, should be this.
First, to propound the Design and Aim of the Curator in his present Enquiry.
Secondly, to make the Experiment, or Experiments, leisurely and with Care and Exactness. Thirdly, to be diligent, accurate, and curious, in taking Notice of, and shewing to the Assembly of Spectators, such Circumstances and Effects therein occurring, as are material, or at least, as he conceives such, in order to his Theory.
Fourthly, After finishing the experiment, to discourse, argue, defend, and further explain, such Circumstances and Effects in the preceding Experiments, as may seem dubious or difficult: And to propound what new Difficulties and Queries do occur, that require other Trials and Experiments to be made, in order to their clearing and answering: And farther, to raise such Axioms and Propositions, as are thereby plainly demonstrated and proved.
Fifthly, To register the whole Process of the Proposal, Design, Experiment, Success, or Failure; the Objections and Objectors, the Explanation and Explainers, the Proposals and Propounders of new and farther Trials; the Theories and Axioms, and their Authors; and in a Word, the History of every Thing and Person, that is material and circumstantial in the whole Entertainment of the said Society: which shall be prepared and made ready, fairly written in a bound Book, to be read at the Beginning of the Sitting of the said Society: the next Day of their Meeting, then to be read over, and further discoursed, augmented or diminished, as the Matter shall require, and then to be sign’d by a certain Number of the Persons present, who have been present, and Witnesses of all the said Proceedings, who, by Subscribing their Names, will prove undoubted testimony to Posterity of the whole History.
Which, of course, is the practical application of the Society’s motto, Nullius in Verba.
PICTURE SECTION
View towards Rocken End (Isle of Wight) from Whale Chine with, in the foreground, gravel and blown sand above the Cretaceous, ferruginous sands. © Ian West
Some of Hooke’s equipment, including (bottom right) his microscope. © Royal Institution of Great Britain/Science Photo Library
The title page of Hooke’s great book. © The Royal Society
The eyes and head of a fly, from Micrographia. © Natural History Museum, London/Science Photo Library
Hooke’s drawing combining a star map with a detail of the Moon showing craters. © The Royal Society
A view of the dome of St Paul’s, designed by Wren using a technique devised by Hooke. © Historical Images Archive/Alamy
One of Hooke’s churches, St Benet Paul’s Wharf. © Michael Grant/Alamy
An engraving ofthe Monument, London from a book printed around 1850. © Stephen Dorey/Bygone Images/Alamy
Ramsbury Manor, Wiltshire, designed by Hooke. © Chronicle/Alamy
Coloured engraving of the Royal Observatory at Greenwich at the time of Flamsteed. © Science Photo Library
View of the Frost Fair on the Thames looking downstream towards London Bridge during the Great Frost of 1683–4. © Sheila Terry/Science Photo Library
Engraving of Halley’s diving bell of 1716. © Sheila Terry/Science Photo Library
Apparatus used by Hooke and Boyle to demonstrate that the sound of a ringing bell cannot travel through a vacuum. © Royal Astronomical Society/Science Photo Library
Johannes Hevelius at left, carrying out astronomical observations with his second wife Elisabetha Koopman Hevelius. © Royal Astronomical Society/Science Photo Library
Hevelius’s plan for a tower observatory, showing various telescopes, including his 150-foot refracting telescope. © World History Archive/Alamy
A ‘Pink’, similar to the ship commanded by Halley. © Lordprice Collection/Alamy
Halley’s wind chart (see text), published in 1686. © Royal Astronomical Society/Science Photo Library
Halley’s chart of magnetic compass readings, published in 1700. © Stephen A. Schwarzman Building/The Lionel Pincus and Princess Firyal Map Division/New York Public Library/Science Photo Library
Star map showing the path of Halley’s Comet across the sky in 1759. © Art and Picture Collection/New York Public Library/Science Photo Library
Eighteenth century diagram of the Solar System showing the orbits of the known planets, and the orbits of five comets. © Library of Congress, Geography and Map Division/Science Photo Library
Map prepared by Halley showing where the transit of Venus due in 1761 could be observed. © American Philosophical Society/Science Photo Library
Halley’s Comet, photo
graphed from Peru on 21 April 1910. © Harvard College Observatory/Science Photo Library
A map of England drawn by Halley showing the passage of the shadow of the Moon during the total solar eclipse of 22 April 1715. The dark circle gives the size of the Moon’s shadow at a given time during the eclipse; the diagonal stripe shows locations from which totality was observed. © Science Photo Library