Ordinary Meeting, 2004 May 26


Historical Transits of Venus

The speaker explained that he would describe the observations of past transits of Venus based on research undertaken using the collection of the Library of the Royal Astronomical Society. Some might look upon the forthcoming transit with a certain sense of apathy: what could be interesting about a small dark blob crossing the face of the Sun? Its rarity would excite some – for it was surely one of the rarest of all easily observable events – but studying the historical context of past transits also allowed observers to appreciate the importance of such events in past times. Though there was limited scientific interest in the transit of 2004, the same was not true of historical events, and extraordinary lengths had been taken by some to make their observations.

The speaker explained that the primary problem which made transit observation so difficult in the past was one of selecting an observing site. The average duration of a transit was around six hours, and to observe it in its entirety, the Sun had to be above the horizon throughout. This was only the case for a quarter of the world for any given transit – the UK by chance having the good fortune of lying in this quarter for the forthcoming transit on June 8. The speaker explained that it was of crucial scientific interest to past observers to measure the start and end times of the transit, and so viewing the whole event was vital, yet this often necessitated long voyages to distant places.

Historical research into transit expeditions often proved a frustrating business. The infamous black-drop effect – to be explained in more detail later – would give rise to substantial imprecision in the timings of first and last contact, and, as these measurements were scientifically vital, the expeditions would be seen largely as failures. The jury still seemed to be out as to the exact cause of this phenomenon which so troubled past observers. As a result, few of the observations were properly recorded, and many were discarded. To his excitement, however, Mr Hingley had found a substantial album in the RAS library recording detailed observations of the 19th century transits. The identity of its owner, and how it had found its way into the library, was unknown, though the speaker had strong reason to believe it to have belonged to a certain Father Stephen Perry. The subject of this album would be returned to later.

The speaker started with the first observed transit – that of 1639. One of the most important outstanding questions in 17th century astronomy was the scale of the Universe. For the preceding 2000 years, the Earth had been viewed as the centre of the Universe, and the planets and fixed stars as being attached to revolving spheres of various radii around it. The speaker used the famous illustration from Thomas Digges' A perfit description of the Caelestial Orbes (1576) to depict this view, advocated for so long by the Pythagoreans. In 1543, however, Copernicus had published his De Revolutionibus, proposing the heliocentric system, which by the 17th century was winning favour. The publication of Kepler's Laws of Planetary Motion had come in the early 1600s – explaining planetary orbits in terms of ellipses around the Sun – and shortly thereafter came Galileo's telescopic observation that the planets could be resolved to appear as whole new worlds with moons orbiting around them. The speaker showed a corresponding figure of the Copernican system.

It was in this context that the 17th century transits of Venus in 1631 and 1639 came. The former was predicted by Kepler, who also asserted that such events should follow a 120-year cycle. His calculations were not of sufficient accuracy to reveal that the timing of the 1631 event would be such that it would not be observable from Europe, but following his death in 1630 it was not even clear whether there was any attempt at observation. Kepler's calculations also predicted a near-miss transit in 1639. However, Englishman Jeremiah Horrocks (sometimes Horrox), observing that the old tables of planetary positions were substantially in error, realised the need for new measurements. Thus, with less than a month before the event, he came to realise that a transit would in fact occur. On November 24 (Julian calendar) of that year, at 3.15pm, he obtained the distinction of being the first person to have observed a transit of Venus. Only the first half-hour of the transit was visible, before the Sun set.

Mr Hingley noted that it was also Horrocks who realised the scientific value of the event. He used it to refine the orbit of Venus and its angular size. He was thus able to produce more accurate tables of planetary positions. He even, though his method was flawed, derived an estimate of the Earth-Sun distance of 59,000,000 miles from his observations – within a factor two of the presently accepted value. The speaker explained that whilst Kepler's Laws gave the ratios of the sizes of the orbits of the planets from their orbital periods, the overall scale of the solar system remained undetermined. Horrocks realised that by determining the size of any one of the planets' orbits, Kepler's Laws gave all the others. He would die in 1641, aged a mere 22, and his work was not to become widely known until 1662, when it reached the attention of Polish astronomer Johannes Hevelius, who published it at his own expense. However, the speaker remarked that Hevelius chose to add his own commentary and notably modified Horrocks' results as he didn't believe the huge scale they implied.

Moving ahead to the 18th century transits, the speaker noted that the repeating periodicity of 105 years, 8 years, 120 years and 8 years between transits had by this time been well established. It was Edmund Halley who in 1678 proposed that by timing transits of Mercury, or even better Venus, and comparing results from widely spaced geographic locations, the distance of the planet could be triangulated. Sadly, whilst Halley was to observe a transit of Mercury, Venus would not transit in his lifetime. Once again, Mr Hingley emphasised the importance of historical context here. Terrestrial surveying was becoming widespread at this time, and it would be expected that any seafarer should be familiar with the technique. Halley's inspiration in proposing that it could be extended to the solar system was thus clear.

The next such transits would be on 1761 June 6 and 1769 June 3. Whilst a variety of circumstances would preclude useful observation of the former, valuable data would be gathered from the second. It was rarely remembered that the official justification for Captain Cook's First Voyage of 1768-1771 aboard the Endeavour was to obtain scientific measurements of the transit of Venus from Tahiti. The voyage was proposed by the Royal Society, keen to ensure British involvement in this scientific opportunity, and one result of this was that the young Joseph Banks secured a place amongst the crew. He was later to be appointed President of the Royal Society and knighted. The purpose of the trip was in truth twofold: the navy themselves had great territorial interest in mapping the South Pacific, searching for the proposed Great Southern Continent, supposedly required to counter-balance the northern landmasses. They jumped at the opportunity, as under the cloak of a scientific expedition, such exploration would be less likely to run into trouble from rival sea-faring nations sailing the same trading routes. To this day, the achievements of Cook's voyage in charting New Zealand and discovering Eastern Australia were remembered best. Perhaps one reason for this was the infamous black-drop effect, or 'oil-drop effect' as Cook called it – the blurring of the edge of the planet with that of the Sun at second and third contact. Charles Green, the official astronomer on the expedition, found himself unable to obtain the level of precision he sought in his timings, and thus the transit observations were broadly considered a failure.

The speaker also recalled the story of another of the 18th century observers, Frenchman Le Gentil. Hoping to travel to Pondichery in India to view the transit, his plans were foiled when he arrived only to discover it to have been taken by British forces, with whom France was at war at the time. With only two weeks to go before the transit, Le Gentil was forced to set sail to Mauritius, and whilst he would be treated to a fine view of the transit from the deck of his ship, the all important timing of the event was impossible at sea. He decided to spend the following eight years exploring the Indian Ocean, and then observe the 1769 event. This he did, and he would eventually find himself back in Pondichery on the day of the second transit, 1769 June 3. However, a change of wind the preceding evening would bring thick cloud and render observation impossible. As a final blow of misfortune, when the dejected Le Gentil returned to France in 1771, he would find his heirs dividing up his estate, presuming in his long absence that he was dead.

It was generally surprising how few observations appeared to have been made of the 18th century events, however some relics of observing programmes had been left behind. These included the Kew Observatory in Richmond, built by King George III, himself a keen amateur astronomer, in order that he might make his own observations. The Observatory continued to function until 1840, whereupon it became primarily a meteorological observatory. The speaker showed the clock used by the King, as well as his telescope, both now in the collection of the Armagh Observatory. Mr Hingley went on to show the records of the King's observations, as well as those made from the Greenwich Observatory, remarking that whilst observers at the latter were said to have differed in their timings by several seconds, it was noted that none differed from the King's by more than a second.

Moving forward to the 19th century transits in 1874 and 1882, the speaker explained that the prime mover in both the British and the international observing programmes on these occasions was to be Sir George Airy, the man who held the post of Astronomer Royal for nearly half of that century. In studying his papers, the speaker had come to the conclusion that Airy worked with certain terms of reference held firmly in mind. He appeared to value measurement very highly, as was shown by his efforts in 1854 towards improving the precision with which the Gravitational Constant was known. But he seemed less concerned about making discoveries – perhaps this was why his response appeared to have been lacklustre when presented in 1845 with Adams' prediction of the location of the new planet Neptune. Thus, he saw the transits of Venus as a tremendous opportunity to refine the scale of the solar system. His excitement would go so far that he would propose a trip to Antarctica for the 1874 transit, but though the importance of this event was appreciated, the expedition was deemed too costly.

Mr Hingley returned to the subject of the anonymous album mentioned at the start of the talk, found in the Library of the RAS. Upon the front were embossed the words 'Ad majorem Dei gloriam' (translated 'to the greater glory of God'), the motto of the Society of Jesus. This would suggest its owner to have been a Jesuit, and combined with the content of the album, seemed to identify him to be Father Stephen Perry. This figure was known to have accompanied the British transit expeditions of both 1874, aboard HMS Volage to Kerguelen Island, a desolate place in the Indian Ocean of comparable size to Wales, and of 1882, to Madagascar. The primary scientific achievements of Perry's life appeared to be in the field of geomagnetism, and he made full use of the transit expeditions to collect magnetic samples. He would go on to accompany several other voyages, including finally an expedition to observe the total solar eclipse of 1889, on which he would lose his life at sea through sickness, five days after the eclipse. The speaker felt one of the most interesting aspects of studying transit records was the great many names of astronomers that cropped up. It was a unique opportunity to learn about figures other than the most famous, such as the Herschels, whose names seem so ubiquitous in other 19th century astronomical records. The speaker also identified the era of these two transits as an especially interesting one in Naval history, coming partway through the transition from wind-powered sailing to steam power. Perry's voyages would have been on ships which combined both, as it was not yet possible to maintain steam-power for long periods.

A notable feature of Perry's album was the number of photographs of the inhabitants of his observing locations which appeared amongst its pages. It was clear that Perry saw this as an important part of the trip, and the speaker recalled a comment, said by David Dewhirst to have been made by Arthur Eddington, that meetings of the RAS shortly after eclipses invariably included so many images of the local population around the observing sites that they seemed like a meeting of a missionary organisation. Also featuring heavily amongst its pages were images of the mobile observatories set up in preparation for the transit. The speaker noted that these observatories were typically built from wooden panels which had been carefully designed so that they could be collapsed in such a way as to double as a packing crate for transporting the telescope. In Perry's photographs, stencilled lettering could be seen on the walls of some observatories, revealing the observing station to which it had been sent.

Mr Hingley pointed out that the 19th century transit observers were, in the very truest sense, great explorers. Stories came back of ships rolling through 45° as they were buffeted by typhoons in the South China Sea, and in one instance of a telescope being lost overboard. Upon arrival at their destination, observers had to determine the local sidereal time and the longitude of their observatories before their timings could be useful. It was also necessary to carefully align the polar axes to account for the latitude of the observatory. In an attempt to minimise the black-drop effect, Airy devised a Transit Simulator, which was viewed through a telescope and used to practice making timings of ingress and egress. In the event, however, it was concluded that timing the model was very much easier than timing the real transit, and despite Airy's efforts to ensure the most accurate timing possible, observers disagreed as to the exact timings by several seconds.

The speaker concluded his talk with a series of images from Perry's album of the various observing stations he visited in the 1874 expedition, including the German and US observatories. It appeared that the British had missed out on the best observing locations, since they had been preparing to sail on the HMS Encounter, but had had to find a replacement ship at short notice when the rudder of the Encounter was found to have been damaged during the Ashanti War. This somewhat surprised the speaker, whose understanding was that this had been a land war. After some delay, they set sail aboard the Volage. Yet, despite the amount of effort put into observing the 19th century events, surviving photographic plates remained so very rare. The observing sites, the speaker commented, were increasingly becoming recognised as such, with plaques being erected. Monuments now stood at the (believed) site of Horrocks' 1639 observations, and of Captain Cook's observatory on Tahiti. The latter was known to this day as Point Venus.

To close, the speaker listed those great achievements which he felt had only come about when they did as a result of transit expeditions: first and foremost those of Captain Cook's First Voyage – the discovery of Australia and the mapping of New Zealand. The invention of the Jansen Revolver, a camera capable of taking a series of exposures at speed, designed by Jules Janssen for use during the 1874 transit, was surely another. In many ways this could be thought of as a precursor to the modern video camera. Perhaps the greatest achievements of all were in learning about the inhabitants of the islands visited, sampling the local cuisine, and returning home with stuffed specimens of the local wildlife.

Following the applause for Mr Hingley's superbly researched talk, The President expressed his gratitude to the speaker before inviting questions. Mrs Hazel McGee enquired as to whether the 19th century observers had used filters or projection in their solar observation. The speaker said this was difficult to answer, as it was usually not recorded. Writers had evidently considered it too obvious to note such mundane detail. However, he presumed that filters had been used. Dr William Sheehan, in the audience, added that it was documented that as an exception, Horrocks had used solar projection.

The President wished members clear skies for June 8, and took the opportunity to remind members of the IAU Colloquium 196, to be held June 7-11 in Preston, which would be centred around the transit, and include an opportunity to observe it from the Alston Observatory. For observers wanting to see the event naked eye, information about how to do so safely would be included in the June Journal. Mr Boles then welcomed Mr Martin Mobberley to present his Sky Notes.




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