The many transits to track the transit of Venus

How many men have died pursuing their science? How many have travelled to the little known reaches of the Earth in pursuit of their science? Both questions can be explored in the stories of the transit of Venus during the 18^th and 19^th centuries. From remote Kerguelen Island to Nordkapp, Tahiti to plague-ridden Baja California, the wilds of Siberia  to war-torn Pondicherry — all these places were visited in pursuit of a measurement and a question, just how far is the Sun from the Earth?

It was Johannes Kepler who determined that this measurement could be obtained by observing a transit of Venus across the face of the Sun.  As with many other sciences the technology for accurate observations would take a little time to catch up with the science.

In 1761 more than 60 observing stations were established from the Gulf of Bothnia to Calcutta, from St Helena in the South Atlantic to Bencoolen in Sumatra. In some cases just finding a destination was the battle. Guillaume Joseph Hyacinthe Jean Baptiste Le Gentil de la Galaisière (glorious name; we’ll call him Le Gentil for short) was one such scientist. He reached Ile de France (Mauritius) with few problems, but then the journey on to Pondicherry was beset with problems. War had again broken out between England and France, so there was no ship free to carry him on to India, and he contracted dysentery. When he did get a ship, bad weather delayed him and still aboard ship the pitching deck was no place for accurate observations. Was he deterred? No—he would repeat his efforts for an observation of the transit in 1769, and again despite all his efforts be defeated by the weather. As he wrote: “This is the fate that often awaits astronomers.”

Jean Chappe d’Auteroche made a long trip across Europe for the transit of 1761. A land journey would, so we in the 21^st century think, pose few problems surely. But his destination was in Siberia and it took him 2½ months to reach St Petersburg — just half way to his destination. Arriving in Tobolsk, his plethora of equipment led the local people to believe him a magician, particularly when the local river flooded well past its usual spring levels upon his arrival. The governor set a guard around him and unlike Le Gentil, Chappe’s viewing of the transit was unimpeded by the weather. He, like Le Gentil, would observe the 1769 event as well, this time from Baja California. All except one of his party on that expedition would die for their science, struck down by a local plague, after they had successfully made their observations.

The results of the 1761 observations were inconclusive, partly because the spread in latitude achieved by the scientists who could observe the whole event was insufficient. For the 1769 transit scientists were determined to do better. This year the optimum viewing sites would be in the Pacific Ocean region, still barely explored at this time. The Royal Society was keen to send a team of observers, the Royal Navy would assist with a vessel. James Cook was selected to captain the ship and was promoted from Master’s Mate to Lieutenant in the process. He was also the official observer, assisted by Charles Green from the Royal Observatory. Cook had proved his credentials in Newfoundland and by his observation of an eclipse of the Sun. He combined many fine attributes as a navigator and surveyor and one other outstanding quality, luck. This luck was observed again and again over the course of his career, until that fateful last voyage. The Royal Navy was prepared to send Cook out to the little known Pacific Ocean and to find a suitable site for making the observations, but fortune smiled and in May 1768 Captain Samuel Wallis of the Dolphin returned to England and reported the discovery of King George Island—or Tahiti as it should be known. An island paradise right in the central south Pacific. Cook and his ship Endeavour with crew and scientists, including Joseph Banks, departed England during August 1768. Tahiti was reached the following April with ample time to establish a viewing site. The Tahitians were friendly, perhaps overly so, and light fingered too. A vital piece of equipment was misappropriated but retrieved, repaired and in place for the transit. The day dawned, in Cook’s words “as favourable to our purpose as we could wish. Not a cloud was to be seen the whole day and the air was perfectly clear.” He had taken precautions and posted observers on several other sites around the island. All observers recorded the transit. Cook and Green the chief observers were disappointed with their results and the differences in their timings, yet time would prove their accuracy.

 With the transit observations over, Cook could open his secret instructions and would chart New Zealand and discover the east coast of Australia.

The next transit of Venus would occur in 1874 and Australia would be a prime site for the observations. In New South Wales, Victoria and South Australia, at this time all still autonomous colonies, preparations were made. In addition, the United States sent a team of observers to Tasmania, as well as to the remote Indian Ocean island of Kerguelen. Henry Russell, Director of the Sydney Observatory was taking no chances and established three sites in addition to Sydney. They had few problems. In Victoria Robert Ellery, Government Astronomer posted an additional three sites in addition to Melbourne Observatory. All four sites were affected by cloud to some extent.

In Adelaide, Government Astronomer Charles Todd experienced poor weather but his assistant, Ringwood, made a startling observation of a bright halo around Venus. The Americans in Tasmania experienced poor weather conditions.

The 1882 transit was viewed again by observers from many countries, even though interest in the astronomical event was waning as other methods of determining the Sun’s distance from the Earth were developed. Australia was again favourably positioned and observation posts established. Weather again played its part and clouded some of the stations. Charles Todd set up his main observation post in Wentworth and was reasonably successful: Adelaide was overcast and clouded out.  New South Wales was badly affected by cloud and no view was obtained from any of its viewing sites. Victorian observers this time were generally favoured however.

The fortunes of all of the observers of the four transits viewed during the 18^th and 19^th centuries fluctuated widely in their success rate. These dedicated men of science however pursued their goals with an astonishing single-mindedness. Distance was no issue—after all they were intent on calculating an even greater distance. And as many of their results were analysed and re-analysed in the years after the event their results were amazingly accurate, given the technology of their time.

To hear more about the transit of Venus, its history and the astronomy tune into the livestream of this booked out event at riaus.org.au/livestreaming or book your seat to view the live stream at the State Library of South Australia.

By Valerie Sitters 

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