Out of the Shadow of a Giant

by John Gribbin

  Astronomers knew that there would be a solar eclipse visible from England on 22 April 1715, and Halley was determined that the best use should be made of this opportunity to observe such an event in detail. In 1714, he published a map showing the predicted path of the shadow of the Moon across England and Wales.fn3 The map was partly to reassure a populace still superstitious about such things that the eclipse was a natural event and nothing to worry about. The chart carried a reassuring explanation:

  The like Eclipse not having not for many Ages been seen in the Southern Parts of Great Britain, I thought it not improper to give the Publick an Account thereof, that the suddain darkness wherein the Starrs will be visible about the Sun, may give no surprize to the People, who would, if unadvertized, be apt to look upon it as Ominous, and to Interpret it as portending evill to our Sovereign Lord King George …

  But the main purpose of publishing the chart was scientific;

  … The Curious are desired to Observe it, and especially the Duration of Total Darkness, with all the care they can; for thereby the Situation and dimensions of the Shadow will be nicely determined; and by means thereof we may be enabled to Predict the like Appearances for ye future, to a greater degree of certainty than can be pretended to at present, for want of such Observations.

  Halley wanted to record the exact track of the eclipse across the country, and in addition to this general plea he wrote to several reliable people who were in the path of the eclipse, or better yet on the edge of the predicted path of the shadow, to ask them to make careful observations, which were then sent to the Royal and collated by Halley. This was the first systematic, large-scale observation of a solar eclipse.

  Halley himself observed the event from Crane Court in London, in the company of several Fellows and their distinguished guests, including visitors from overseas. One of the guests was Isaac Newton’s niece, Kitty Barton, who looked after Newton’s household. She was a noted beauty, and made a deep impression on one of the French visitors, Rémond de Montmort, who later wrote from Paris thanking Newton for presents ‘given by Mr Newton and chosen by Mrs Barton whose wit and taste are equal to her beauty’. She was, indeed, known in smart circles in London as ‘pretty, witty, Kitty’. But Halley’s eyes were on the heavens, not on her.

  The observations were, for once, blessed by good weather. In 1715, Halley published his account of the observations of the eclipse made by his scattered reporters, giving particular emphasis to the observations from near the edge of totality. He calculated that the southern edge of totality passed between Newhaven and Brighton on the Sussex coast, and between Herne and Reculver on the north Kent coast, while the equivalent northern line ran from Haverfordwest to Flamborough Head. The key to this aspect of Halley’s legacy was provided by the observations from places where the eclipse was just total, at the very edge of the shadow, as we shall discuss shortly.

  A year later Halley made another contribution to his scientific legacy, when he published a paper discussing the possibility of determining the distance to the Sun by observing a transit of Venus. He had been interested in the idea for decades, and had read a paper on the subject to the Royal in 1691, but the 1716 paper in the Philosophical Transactions was his definitive last word on the subject. The idea itself was not new, but Halley’s careful prescription of what should be done was. It included a detailed explanation of how and where the necessary observations should be made, and a plea that astronomers should take advantage of the opportunity to make such observations in 1761 and 1769, the dates of the next two such transits, by which time Halley himself would be dead. We shall describe what happened then in due course.

  Halley’s most famous legacy, though, concerns the comet that now bears his name. As we have discussed, he was calculating cometary orbits back in the 1690s and published most of this analysis in 1705; the final version appeared in 1726 but differs only slightly from the earlier version. Halley calculated that the same comet had been seen in 1531, 1607 and 1682, following an elongated elliptical orbit around the Sun, seventy-six years long, in accordance with the inverse square law of gravity. He predicted that the comet would be seen again round about the end of December 1758, and urged astronomers to watch out for it, ‘whereof if according to what we have said it should return again about the year 1758, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.’ As we shall see, his wish for due credit was granted, perhaps even more fully than he could have anticipated. But first we should describe Halley’s later life as Astronomer Royal and the grand old man of British science.

  By the time he became Astronomer Royal, Halley had been granted the only academic distinction that he ever seemed to care about. In 1710 he was awarded the degree of Doctor of Civil Laws by the University of Oxford. Up until that time, he had preferred the title ‘Captain’ to ‘Professor’, but from then on, he was happy to style himself ‘Doctor’ Edmond Halley. He was particularly pleased with the honour because it was made in recognition of his public service, not least his public service as a sea captain. The recommendation from the Chancellor of the University to the University Convocation read:

  Mr Edmund Hally having been Master of Arts near thirty years and often employed by her Majesty and her Predecessors in the Service at Sea in the remotest parts of the World to the great Satisfaction of the Lords of ye Admiralty and others of the first Quality in the nation; And now being Your Professor of Geometry and well known to be a Person of great knowledge not only in that Science but in most of ye other parts of Learning, I have thought fitt in consideration of his great merit and the Service he hath done to the public both at home and abroad to recommend it to You that you would confer the Degree of Dr of Civil Laws on him without fees …

  The mention of fees being waived is a significant indication of the esteem in which Halley was held; when Handel visited Oxford in 1733 to put on concerts, he declined the offer of an honorary degree because he was asked to pay a fee. The mention of services to ‘others of the first Quality’ also seems significant, hinting once again at services that are not a matter of public record. So it would be as Dr Halley that Edmond succeeded John Flamsteed as Astronomer Royal when Flamsteed died on 31 December 1719, somewhat to the relief of all those who had been frustrated by his general bloody-mindedness and unwillingness to make his observations widely available.

  Flamsteed’s death was not unexpected. He was seventy-three, and not in good health. Nor was Halley’s appointment as the second Astronomer Royal unexpected. Indeed, it seems to have been anticipated, since on 11 December, nearly three weeks before Flamsteed died, Thomas Hearne wrote to Richard Mead, one of the Visitors, ‘I must heartily congratulate you upon the success you have had on behalf of Dr Halley. This great Man had been neglected too long.’ On 10 January Hearne recorded that ‘Mr Flamsteed is dead and Dr Halley hath got his place at Greenwich’. Halley himself was now sixty-three, and moved quickly to install himself at the Observatory, ejecting Mrs Flamsteed, who took with her most of the instruments, claiming with some justification that they were Flamsteed’s property. So the first task of the new Astronomer Royal was to re-equip the Observatory, which he did, with the aid of a Treasury grant of £500, with much better instruments than those used by his predecessor.

  It might have been expected that Halley himself, in view of his advancing years, would make little use of the instruments. But on the contrary, in his sixty-fourth year he at last had an opportunity to begin a programme of observing the Moon over its entire eighteen-year Saros cycle. Maybe he was optimistic; maybe he expected his own successor to complete the task. But as it happens, he lived long enough to finish the job, his major observational achievement as Astronomer Royal. But he also lived long enough to see, and be involved in, the technology that would make this method of determining longitude obsolete.

  In 1714, the British government had offered a prize of £10,000 for anyone who could devise a method to determine longitude at sea so accurately that it would
produce an error of less than sixty nautical miles on a voyage to the West Indies and back, rising to £15,000 if the error was less than forty nautical miles and £20,000 if the error was less than thirty nautical miles. It was widely expected that an astronomical technique would eventually solve the problem. But at the end of the 1720s (the exact date is not certain), a young carpenter and self-taught clockmaker called John Harrisonfn4 visited Halley at Greenwich to show him a clock mechanism that he had devised, which he thought could be developed into a practical timepiece that would win the prize. Halley encouraged Harrison and introduced him to a leading clockmaker and Fellow of the Royal Society, George Graham. Graham not only gave encouragement but provided Harrison with £200, which Harrison and his brother James used to construct a wooden chronometer using his ideas (how Hooke would have loved it!). It was tested by the Royal Navy on ships travelling to Lisbon and back, producing spectacular results, and Halley was a member of the Royal Society board that officially endorsed Harrison’s project. There followed a long saga of development, with Harrison repeatedly frustrated by the doubts of the authorities, until more than twenty years later he produced his final, successful design (and even longer before he received part of the prize, but not all of it). James Cook carried one of Harrison’s chronometers on his second voyage, in 1772.

  The rest of Halley’s life can be summarised briefly. He had served as Secretary of the Royal Society since 1713, but resigned in 1721 (he did not have to resign his Oxford professorship on becoming Astronomer Royal, although this seems to have become something of a sinecure). But he maintained contact with the Royal, attending meetings and the informal gatherings in coffee houses and taverns after meetings, and serving as Vice-President in 1731. In 1729, Queen Caroline, the consort of George II (whom Halley had met on his visit to Hanover) visited the Royal Observatory, and was intrigued to learn that Halley had served as a Royal Navy captain. We don’t know how much, if at all, Halley pressed his case, but she learned that as he had served more than three years in this capacity he was entitled to a pension of half-pay. Shortly after the visit, the King personally ensured that this pension was paid, and Captain Halley received it for the rest of his life.

  In January 1736 Halley’s wife, Mary, died (there are no details of the circumstances), and he seems to have suffered a slight stroke soon afterwards. This affected his right hand, but did not stop him observing, with the aid of an assistant. His son, another Edmond, died in February 1741, leaving no recorded children; Halley himself declined in strength through 1741, his eighty-fifth year, and died on 14 January 1742 (25 January New Style), quietly, sitting in a favourite chair after a glass of wine, and was survived by his two married daughters, Margaret and Katherine.

  He was also survived by a body of work which still included unfinished business, a legacy for the next generation of astronomers and beyond. The first of these projects to bear fruit was the famous prediction of the return of ‘his’ comet.

  As Halley and Newton had appreciated, predicting the exact time of the return of the comet depended on working out the perturbing influences of Jupiter and Saturn. The Frenchman Alexis-Claude Clairaut was the first person to devise a way to tackle the problem using a development of Newton’s equations, but even with the equations the problem still had to be solved numerically. This was a horrendous task involving lengthy and tedious arithmetical calculations, but it was carried out by the French astronomer Jérôme Lalande,fn5 with very considerable assistance from the splendidly named Mme Nicole-Reine Étable de la Brière Lepaute, a friend of Clairaut, who seems to have been an expert at astronomical calculations. Clairaut announced the result of these calculations – which took the pair more than six months to complete – to the French Academy of Sciences on the 14 November 1758, predicting that the closest approach of the comet to the Sun (perihelion) would occur in April 1759.

  The comet was first seen (on this visit) by a German astronomer, Georg Palitzsch, on Christmas Day 1758, but news of his observations did not spread quickly, and it was ‘discovered’ independently by the French astronomer Charles Messier, working at the Paris Observatory, on 21 January 1759. Perihelion actually occurred on 13 March that year, so the official date for the return is now given as 1759, not out of any disrespect to Palitzsch, but because perihelion is an accurate and unambiguous date, whereas the exact timing of the first observation of a returning comet is, as this example shows, largely a matter of luck.

  But whoever saw it first, the success of the prediction was sensational news. Lalande wrote:

  The universe beholds this year the most satisfactory phenomenon ever presented to us by astronomers; an event which unique until this day changes our doubts to certainty and our hypotheses to demonstration.

  It was the fact that this was a successful prediction that made the observations so important. It was all very well Newton (with a little help from Hooke) explaining why the orbits of the planets were elliptical, but astronomers already knew that the orbits were elliptical. And even before that, it was known that the planets were regular and predictable in their movements. But comets were the archetypal unpredictable phenomenon, appearing entirely without warning, rousing superstitious awe in the eighteenth century to an even greater extent than eclipses. Accurately predicting the date on which a comet would appear in the sky was the greatest triumph of the scientific method up to that time, and it established the ‘Newtonian’ way of understanding how the world works. The predicted return of the comet confirmed the accuracy of the inverse square law (suspected by others and proved by Newton), that gravity is an attractive (centripetal) force as Hooke had pointed out to Newton, and that the laws that govern the Universe are, as Hooke had also pointed out, the same as the laws which apply here on Earth. This world-view, which ‘changes our doubts to certainty and our hypotheses to demonstration’, owed at least as much to Hooke as to Newton, and its successful application was down to Halley. All three of them were dead by 1759, but their scientific legacies survived, and the scientific truth lived on. Where next could this understanding of the laws that govern the Universe take astronomy? Halley had already pointed the way there, as well.

  The first astronomer to predict and observe a transit of Venus was a young Englishman called Jeremiah Horrocks. He was born in 1618, near Liverpool, and attended Emmanuel College, Cambridge, from 1632 to 1635, but like many of his contemporaries did not bother graduating. Horrocks was a dedicated amateur astronomer (there were no professional astronomers in England then) who also worked in the family business of watch- and instrument-making. In 1629, Johannes Kepler had published a pamphlet in which he predicted, based on the astronomical tables he had produced, that there would be a transit of Mercury in 1631, and transits of Venus in 1631 and 1761, with a ‘near miss’ in 1639. Horrocks was too young to observe the transits in 1631, and nobody else seems to have bothered, but when he reworked Kepler’s calculations using the best available data he found that Kepler had made a tiny error and there would, in fact, be a transit of Venus in 1639, which he predicted would occur at about 3 pm on 24 November on the Julian calendar (4 December on the Gregorian calendar). He was able to observe the event, using the standard technique of projecting an image of the Sun through a telescope on to a white surface, and his friend and colleague William Crabtree made similar observations. Horrocks wrote an account of the event in Latin (Venus in sole visa), but his death from unknown causes early in 1641 at the age of twenty-two meant that it was not published in his lifetime. However, Johannes Hevelius was sent a copy, and published it in 1661, causing great interest at the Royal Society, among other places.

  Following Horrocks’ work, it was clear that transits of Venus always occur in pairs, following a pattern that repeats every 243 years. The two transits in a pair are separated by eight years, then there is a gap of 121.5 years followed by another pair of transits and a gap of 105.5 years before the whole pattern repeats. So the total length for the cycle is (8 + 121.5 + 8 + 105.5) = 243 years.

  In
1663 James Gregory, the Scottish mathematician who invented the Gregorian telescope design, pointed out that by observing and carefully timing a transit of Mercury from two widely separated points on the surface of the Earth it ought to be possible to calculate the distance to the Sun using geometry, essentially the same system of triangulation, involving parallax, that is used by surveyors to calculate distances to objects without ever visiting them. The essential point is that the widely separated observers see the planet pass in front of the Sun from slightly different angles, so that they record slightly different times for key moments of the transit, such as the time when the edge of the planet first seems to touch the edge of the disc of the Sun and the duration of the transit. With this information and the precise locations of the observing sites it is possible to calculate the angles involved (too small to measure directly) and work out the distances. Although Halley observed a transit of Mercury from St Helena in 1677, there was no concerted effort by astronomers to make observations from other locations, and there was insufficient data to put the idea into practice. But it sowed the seed for Halley’s publications of 1691 and 1716 urging for a proper investigation of the pair of transits of Venus due in 1761 and 1769, and providing a detailed plan both for the necessary observations and the technique for using the results to work out the distance to the Sun. Venus is a better choice for the job, because it approaches much closer to the Earth than does Mercury (or, indeed, any other planet), although before the job was carried out nobody knew exactly how close. But even in the case of Venus, a transit lasting typically for around six hours has to be timed to an accuracy of about 10 seconds for the parallax effect to be noticeable.

  By 1761, Europe was again at war. This one was known as the Seven Years’ War, although it actually lasted from 1754 to 1763; however, there had already been a Nine Years’ War, and this time most of the fighting was confined to the period from 1756 to 1763. Nevertheless, this did not stop several countries making plans to observe the transit – if anything, the war spurred competition among the belligerents (notably Britain and France) to outdo each other. Indeed, the British effort was a badly funded project hastily cobbled together when the Royal Society learned that the French were planning something. Halley had suggested that observations should be made from sites such as Hudson’s Bay, Norway and the Far East. This broad spread of observing sites was possible because the event occurred in northern summer, when the nights are short. This meant that observers at high northern latitudes could watch the beginning of the transit before sunset, and then pick up the end of the transit after the short northern night, while observers on the far side of the world simultaneously made observations close to midday. But the plans were partially frustrated by the war.