Out of the Shadow of a Giant

by John Gribbin

  that Mr Edmund Hally a Student at Queen’s College in Oxford, with a friend of his might have their passage on the first ship bound for St Helen’s whether they are desirous to go & remayn for some times to make observations of the planets & starrs, for rectifying and finishing the celestial globe, being a place (he conceives) very fit and proper for that design; and that they may be received and entertained there, and have such assistance and countenance from the Compas [Company’s] officers as they may stand in need of. On consideration whereof had, It is ordered that Mr Hally with his friend doe take their passage for St Helena on the Unity with their necessary provisions free of charge; and that a lre [letter] be written to the Governor & Council of the said Island to accommodate them with convenient lodging during their stay there, and afford them such assistance and countenance as may be for their encouragement, to proceed in so useful an undertaking.fn3

  Halley’s plans must have been well advanced, because that letter from the King is dated 4 October 1676, and the Unity sailed at the beginning of November, just after Halley’s twentieth birthday. His friend, a Mr Clerke, sailed with him to assist with the observations, and Halley was provided with the best available astronomical instruments (in some ways, better than the ones Flamsteed had in Greenwich at the time), thanks to his father. His father also gave him an allowance of £300 a year, exactly three times the salary of the Astronomer Royal. The ship was owned by a family who lived near the Halleys in Winchester Street, which no doubt smoothed the arrangements.

  The voyage itself played an important part in Halley’s education. There is no hint of him ever having been to sea before, but as a passenger on a voyage that took three months to reach its destination (and on a similar ship for the voyage home), he had ample time to study the workings of such a vessel, and would have been particularly interested in the techniques used for navigation, which largely depended on astronomical observations. Indeed, it would be surprising if the captain had not taken advantage of having an experienced astronomer on board to consult him about these observations. Halley would also have had an excellent opportunity to study the techniques of sailing itself – the way the ship was run, and the relationship between the officers and the ordinary sailors, as well as the mechanics of sail-setting and steering in different wind and weather conditions. We can only speculate, but he must have put this opportunity to good use, because, as we shall see, a decade later when he went to sea again he proved to be an accomplished seaman and navigator.

  Halley and Clerke arrived at St Helena at the end of the southern hemisphere summer, and had to set up an observing site in rugged country high up on the slopes of the island’s largest mountain, Diana Peak, before they could begin work (the site overlooks the location of the later site of Napoleon’s tomb). When they began their astronomical work, they were plagued by bad weather, not just clouds, fog and rain but also by ‘mighty winds’, as Halley wrote in a letter to Sir Jonas Moore. Much later, in a paper published in the Philosophical Transactions in 1691, Halley recalled:

  In the Night time, on the tops of the Hills about 800 yards above the Sea, there was so strange a condensation or rather precipitation of the Vapours, that it was a great Impediment to my Celestial Observations; for in the clear Sky the Dew would fall so fast as to cover, each half quarter of an hour, my Glass with little drops, so that I was necessitated to wipe them so often, and my Paper on which I wrote my Observations would immediately be so wet with the Dew, that it would not bear Ink.

  They were also hampered by the uncooperative attitude of the governor of the island, Gregory Field; this does not seem to have been anything personal, since Field was so unpleasant and uncooperative to everyone that he was sacked by the Company in February 1678, too late to do any good for Halley. In spite of the difficulties, Halley and his assistant were able to measure the positions of 341 stars relative to the positions of those stars in Tycho’s catalogue which are visible from St Helena. Although Halley realised that Tycho’s catalogue would need revision as better observing instruments were developed, by locking his catalogue in to Tycho’s he ensured that, when Tycho’s positions were improved, his positions would still be accurate relative to the improved northern catalogue. His was the first major survey of this kind using telescopic sights, in either hemisphere, and remained the best guide to the southern skies well into the following century.

  But the catalogue was only one of the fruits of Halley’s stay on St Helena. He took with him a pendulum clock, in pieces to be assembled on site, to help with timing his astronomical observations. He found that in order for the clock to keep correct time (in effect, to make sure that it ticked off exactly twenty-four hours in each day, from noon to noon by the Sun) the pendulum of the clock had to be shorter than when the same clock was in England. We now know that this is because the Earth bulges at the equator as a result of its rotation, so the effective gravitational pull felt at low latitudes is less than that felt at high latitudes. Robert Hooke was the first person to appreciate this, and, as we explain later, he used Halley’s measurements as evidence in support of the idea of a terrestrial equatorial bulge.

  But the observations from St Helena which made the most long-standing impact on astronomy, and opened up a way to measure distances across the Solar System, were made on 28 October 1677, when the planet Mercury, as seen from Earth, passed across the face of the Sun. Such events, known as transits, are rare but predictable, and this particular transit of Mercury was one of the main reasons why Halley wanted to be on the island that year. Because observers in different parts of the world are viewing (in this case) Mercury from slightly different angles, they will see it cross the edge (limb) of the Sun at slightly different times. This is a simple example of parallax. By timing the exact moments when Mercury crossed the Sun’s limb both going in (ingress) and out (egress) and comparing the interval between these measurements with similar measurements made in Europe, it would in theory be possible to use simple geometry and the laws of planetary orbits discovered by Johannes Kepler to work out the distance of the Earth from the Sun, in terms of multiples of the Earth’s radius.fn4 The idea had been known for some time, but Halley was the first to put it into practice. In spite of the terrible observing conditions he was able to write to Moore that:

  I have notwithstanding had the opportunity of observing the ingress and egress of Mercury on the Sun,fn5 which compared with the like Observation made in England, will give a demonstration of the Sun’s Parallax, which hitherto was never proved [that is, measured] …

  Unfortunately England was covered by cloud at the critical time, and French astronomers at Avignon were only able to observe the egress, when Mercury moved out of the line of sight to the Sun, not the ingress. So it was only possible to get a rough estimate for the Sun–Earth distance, which later turned out to be only about a fifth of the true value. Halley realised, however, that a transit of Venus would give a much better opportunity to make this measurement, a realisation which would lead to one of his two great posthumous successes (see Chapter Eleven).

  Halley and Clerke must have left St Helena only a few weeks after observing the transit, probably at the end of February, because in his diary entry for 30 May 1678 Hooke writes ‘met Hally from St Helena with S[ir]. J. Moore & Colwell at toothes.’ Toothes was one of the coffee shops frequented by Hooke and his friends. But over the following weeks Halley was too busy to spend much time in coffee houses. He immediately set to work on preparing his star catalogue, in which he included an account of how he had become interested in astronomy, descriptions of other objects he had observed from St Helena, such as star clusters and the Magellanic Clouds, as well as discussions of why the predicted positions of Jupiter and Saturn were incorrect in the published tables, and an analysis of the orbital motion of the Moon. He gave the resulting treatise an unwieldy title which began A Catalogue of the Southern Stars, or a supplement to the Catalogue of Tycho … and ran on for a total of more than a hundred words. Hardly surprisingly it is usually kn
own simply as A Catalogue of the Southern Stars.

  Halley’s catalogue was published (in Latin) early in November 1678. Hooke described the work to a meeting of the Royal Society, and at the end of the month the man Flamsteed now referred to as ‘our southern Tycho’ was elected as a Fellow. This honour was soon followed, on 3 December, by another, at the express wish (after a little prodding) of the King.

  Halley had made an overt attempt to butter the King up by inventing a ‘new’ southern constellation, which he called Robur Carolinium (‘Charles’ Oak’) in recognition of the oak tree in which Charles had hidden after the Battle of Worcester. The name did not stick, and no such constellation is now recognised, but no doubt Charles was pleased. There had clearly also been some behind-the-scenes machinations concerning Halley’s academic status, because on 12 November the Provost of Queen’s College had written to the Secretary of State, Sir Joseph Williamson, picking up on earlier communications, now lost. ‘I have spoken with Mr Vice-Chancellor about procuring a degree for Edmond Halley,’ he wrote. ‘If you will procure the King’s letter, it will be both effectual and not unpleasing to the University. He is now of almost 6 years standing and less than a master’s degree cannot be conferred on him.’ The reason the university needed a letter from the King was that there were (and are) strict rules about the amount of time undergraduates had to reside at the university, which clearly Halley had not kept; nor had he taken any examinations. But the King was happy to oblige. This was not entirely unprecedented; other people had been awarded degrees at the King’s behest. But on this occasion it is worth quoting his letter, dated 18 November, at length, because as a result of it Halley received the first degree ever awarded specifically for research:

  … having received a good account of the proficiency in Learning of our Trusty Wellbeloved Edmund Hally of Queens College in that our University especially as to the Mathematick and Astronomy, Whereof he has (as We are informed) gotten a good Testimony by the Observation he has made during his abode in the Island of St. Helena; We have thought fit for his Encouragement hereby to recommend him effectually to you for his Degree of master of Arts; Willing and Requiring you forthwith upon the receipt hereof (all Dispensations necessary being first granted) to admit him the sd. Edmund Hally to the said Degree of Master of Arts without any Condicion of performing any previous or subsequent Exercises for the same any Statute or Statutes of that Our University to the contrary in any Wise notwithstanding …

  The earliest opportunity the University had to confer the degree was at a Congregation held on 3 December, by which time Halley was already a Fellow of the Royal Society. He had arrived with a bang on the scientific scene, displaying a multiplicity of skills which has been neatly summed up by Alan Cook:

  In his first and early major campaign Halley showed an outstanding ability to plan and persuade, to organise and to carry through his project. He showed theoretical insight into fundamental matters of astrometry, and acquired more practical experience as an observing astronomer with up-to-date instruments than anyone else of his day. He had accomplished the three things he set out to do, to observe in the south, to produce the first catalogue after Tycho, and to use modern instruments in a major systematic campaign.

  This was clearly a young man – only just twenty-two in November 1678 – who was going places. And the next place he went, this time on semi-official Royal Society business, was Danzig; that business also involved Hooke.

  Johannes Höwelke was an astronomer of the old school, born in 1611, who, as was the custom at the time, particularly on the continent, had Latinised his surname and was known to all as Hevelius. He came from a wealthy family of brewers, and was able to indulge his passion for astronomy by building an observatory in Danzig (spread across the roofs of several houses), which was the best in Europe until the construction of the Greenwich and Paris Observatories. In 1662, Hevelius’ first wife died, and the following year he married a girl of sixteen, Elizabetha. The daughter of a merchant, she soon proved to have a good business head herself, more or less running the brewing business and helping her husband with his astronomical observations. His work was known and admired across Europe, and he was elected as a Fellow of the Royal Society in 1665. But doubts about his work began to emerge in the 1670s.

  As an astronomer of the old school, Hevelius made all his observations using open sights. In 1673, he published a book (part one of an opus called Machina coelestis) in which some of these observations were reported. After the publication of this book, Flamsteed and Hooke were among the astronomers who criticised the old-fashioned technique used by Hevelius. Flamsteed wrote in the Philosophical Transactions that:

  We have heard that the celebrated Johannes Hevelius has indeed undertaken the restitution of the fixed stars, yet seeing he is reputed to use sights without glasses, it is doubtful if we shall obtain from him much more correct places than Tycho left us.

  Hooke, always an innovator and never afraid to speak his mind, pointed out the deficiencies of open sights and the advantages of telescopic sights in his second Cutlerian Lecture in 1674. The lengthy title of the published lecture begins ‘Animadversions on the first part of the Machina Coelestis’, which leaves the reader in no doubt where Hooke is coming from. Hooke goes into great detail in describing and discussing both the instruments used by Hevelius and his own instruments, and referring to previous criticism of the telescopic sights made by Hevelius in correspondence with Hooke. His exasperation is clear:

  [I] am sorry I have been forced to say so much in vindication of Telescopical Sights; and that in the doing thereof, I have been necessitated to take notice of the imperfections, that are the inseparable concomitants of Instruments made with Common [open] Sights. Nor should I have published these my thoughts, had I not found them so highly decryed by a person of so great Authority, fearing that hereby other Observators might have been deterr’d from making any use of them, and so the further progress of Astronomy might have been hindred.

  Hooke is reasonableness itself in summing up the situation. He refers to Hevelius as an ‘excellent Person’ who in his astronomical work ‘seems not to have spared either for labour and vigilancy, or for any cost and charges that might check his purpose, for which he hath highly merited the esteem of all such as are lovers of that Science’. But, says Hooke:

  if he had prosecuted that way of improving Astronomical instruments, which I long since communicated to him, I am of opinion he would have done himself and the learned World a much greater piece of service … by publishing a Catalogue ten times more accurate.

  Hevelius was unconvinced, and the matter rumbled on. Perhaps ‘rumbled’ is too mild a term. Hevelius developed a great antipathy towards Hooke, as shown by a letter he wrote to Flamsteed on 14 June 1676:

  To be honest, I repeatedly have serious doubts as to whether, with his telescopic lenses and polemoscope mirrors, or with any other devices whatsoever, that man (I repeat, that man)fn6 will be able to produce results more definitive or consistently more complete than ours.

  Without modifying his views on Hooke, early in 1677, Hevelius wrote in more friendly terms to Flamsteed, saying ‘let each allow the other to help on Astronomy at his own risk and by his own methods’.fn7 He also wrote to Oldenburg with details of his observations. But the matter could not be left entirely for each astronomer to work in his own way and with his own instruments. That was all very well in Tycho’s day, but by the late 1670s astronomy was becoming an international endeavour that needed recognised standards. Apart from the work done by Hevelius, Flamsteed was carrying out a detailed survey of the northern skies using telescopic sights, and Halley had already carried out the first survey of the southern skies. All of these observations (and those from Paris and other places) had to be matched up with one another and combined to chart the heavens. It was crucial to know how far Hevelius’ published results could be trusted. Sure that he would be vindicated, Hevelius invited the Royal Society to send an astronomer, armed with telescopic instrume
nts, to make observations from Danzig and compare these with observations made by the same observer using the open sights instruments at the Danzig observatory. There was an obvious choice. Halley was already an experienced observer, had no ties, his own portable instruments, and plenty of money to finance his own travel without costing the Royal a penny. He was trusted by Flamsteed, and had sent Hevelius an early copy of the Catalogue of the Southern Stars, with a letter in which he said that he would be happy to recalculate his star positions relative to the northern survey then being carried out by Hevelius, instead of relying on Tycho’s catalogue. In April 1679, armed with a scarcely necessary letter of introduction from the Royal, Halley set sail for Danzig on what was technically a private visit, although Hevelius seems to have regarded him as an official envoy of the Society. He arrived on 26 May, and the two astronomers made observations together, sometimes with others as well, every clear night for the next two months.

  It turned out that the measurements made by Hevelius were more accurate than Halley had anticipated, largely because Hevelius used two observers to make the measurement, one lining up a sight on one star while the other moved a second sight to another star to measure the separation of the two stars, from the angle between the two sights. This was better than having a single observer use one sight that was moved from the first star (after noting its position) to the second star. Impressed, Halley wrote to Flamsteed on 7 June with a description of the instruments and the observatory, and then discussed the measurements of star positions made by different observers. ‘I assure you I was surpriz’d to see so near an agreement in them, and had I not seen, I scarcely could have credited the Relation of any; Verily I have seen the same distance repeated several times.’ The measurements also agreed with those obtained using Halley’s own instruments, with telescopic sights. But this was hardly a fair comparison, because the portable instruments could not be expected to be as accurate as permanent, full-size instruments like those Flamsteed was now using in Greenwich. As Halley must have been aware, but did not point out, this implied that Flamsteed’s instruments were more accurate than those of Hevelius.