An impression of the importance attached to an accurate measurement of the AU can be gleaned from the fact that 176 observers attempted to observe the 1761 transit from 117 stations (see Transits of Venus, Richard A. Proctor, Worthington and Co., New York, 1875, p. 51). The Astronomer Royal, Nathaniel Bliss (1700 – 1764), strolled into his garden and observed from Greenwich. Abbé Jean-Baptiste Chappe d’Auteroche (1728 – 1769) left France in November 1760 and reached Tobolsk (Siberia) via St Petersburg, on April 10, 1761. An English expedition set out for Sumatra but only reached the Cape of Good Hope; the one to St Helena actually arrived. Mikhail Lomonosov (1711 – 1765) the Russian polymath not only observed the transit from his St Petersburg home but also noticed a luminous ring around Venus just as it entered the solar disc, thus discovering the planet’s atmosphere. The French Academician, Guillaume Gentil de la Galaisière set out for Pondicherry, in India, on March 26, 1760. Unfortunately war broke out between England and France. His frigate turned back, and he made useless observations of the transit on board ship somewhere in the Indian Ocean. Gentil then stayed in the East and was told to observe the 1769 transit from Pondicherry, only to be clouded out. When he finally got back to Paris he found he had been presumed dead and his heirs were dividing up his estate.

The results from the 1761 observations gave the solar distance as anywhere between 155 million and 125 million kilometres. It was concluded that too much reliance had been placed on the Delisle method.

The 3rd June 1769 was the date of the next transit and Ferguson suggested that, following the Halley method, the northern parts of Lapland and the Pacific Solomon Isles were “the most proper places”. The King of Denmark sent a German astronomer north to Wardhuus, Lapland; unfortunately the Solomon Isles were under Spanish control and they would not let a French astronomer land. Mexico, California and Kamchatka were also favoured. The Royal Society, at great expense, sent Captain James Cook and the astronomer Charles Green to Tahiti in the coal-barque Endeavour. They set sail from Deptford on July 30^th 1768 and arrived on April 10^th 1769. Even though many now used similar telescopes, the timing errors were still very noticeable. Due however to the huge number of measurements that were made, the best results gave the solar parallax to an accuracy of about ± 2%.

By the mid-nineteenth century alternative methods such as accurate observations of the Moon and Mars were beginning to yield better estimates of the AU. Even so, observers of the December 1874 transit tried to overcome the black-drop effect by building artificial transit machines to practice on. Astronomers then tried to improve their timing skills by observing these machines, hoping that they could reduce their reaction time and improve their hand-eye coordination. Wet-plate photography was blossoming and offered the chance of impartial and recordable sequence timing. Pierre Jules Janssen (1824 – 1907) even devised a quick-fire revolving camera that turned out to be a forerunner of the cinematographic camera. France, Britain, Russia, Germany and the USA all organised expeditions.

Lord Lindsay of Balcarres sailed on his private yacht with David Gill (later Her Majesty’s Astronomer at the Cape of Good Hope) to Mauritius. They took with them fifty borrowed chronometers in order to establish the time of observations. Others went to Hawaii, New Zealand, Tasmania, Kerguelen’s island and Crozet’s island in the southern hemisphere and Egypt, Nagasaki, and Vladivostok in the northern.

To aid the British effort the House of Commons voted £10,500 to defray the cost of instruments and observations, and a further grant of £5000 was made to cover photographic apparatus and photographers.

Mr Richard Proctor, the famous populariser of astronomy, argued with Sir George Airy, the Astronomer Royal, about suitable sites, suggesting that a station on the Antarctic continent (Possession Island, near Victoria Land) should be used for both the 1874 and 1882 transits, and also that a site in northern India would be useful. The multitude of southern sites was supposedly to mitigate the strong chance of inclement weather. The Royal Navy was given the task of ferrying the astronomers to and fro. The Hydographer (Admiral Richards) was distinctly unimpressed. Even though the astronomers might be enthusiastic he still thought them to be “deficient on many subjects which it is necessary to take into account …. We are told to sail to the Antarctic Continent and to visit a variety of small rocky islets interspersed over the Southern Ocean …. many of which are actual myths, while on those which do exist it is certain that there is no anchorage for a ship, and that even landing would be generally impossible.”

The end-results of all this planning, travelling, expense and observing effort were dismal. High-quality optics, and improved observational techniques had reduced the black-drop problem somewhat, but even the new photographic approach resulted in distorted images. It turned out that the errors had only been reduced by a factor of two over the 120 years since the previous pair of transits. Many were the accounts of continuous cloud, poor timekeeping and excessive image turbulence. On Kerguelen (48.5° S; 70° E, an isolated island in the Southern Ocean, south of a line between South Africa and Australia) the British expedition was instructed by Airy to stay an extra 12 weeks in order to make over a hundred observations of the Moon. These provided an accurate value of the longitude of the site. However, the astronomers nearly ran out of food and were forced to go onto half rations.

Interest in the December 1882 transits started to seriously wane. Even so the French Academy of Sciences organised expeditions to Haiti, Mexico, Martinique, Florida, Patagonia, Chile, Rio-Negro, Cape Horn, Argentina and Montevideo. David Gill, however, recognised the fact that transit observations would never yield good results. He wrote “the transit of Venus in 1882 was therefore awaited at the Cape without special interest.” One famous French astronomer, Urbain J. J. Leverrier (1811 – 1877) would have nothing to do with either the 1874 or 1882 transits. He concentrated on three gravitational approaches. The orbits of Venus and Mars are perturbed by the mass of the Earth; the orbit of Earth is perturbed by the mass of the Moon and the speed of the Moon around its orbit depends on the gravitational influence of the Sun. All three effects depend on the astronomical unit and their careful measurement yields a more accurate value of the AU than the transit approach.

The US Naval Observatory was criticised in the press for sending extravagant expeditions to distant parts. Even worse, funding for the analysis of the observations was mostly used for other purposes and interest in analysing the observations faded away.

In the century that followed, observers turned to Mars, the Moon, nearby asteroids and radar for accurate measurements of the astronomical unit. They sensibly tried to replace ‘special occasion’ transit observations, which can be rendered useless by all kinds of weather, travel and observing accidents, by more reliable large-observatory techniques that permitted many repetitions of significant observations to be made in relative comfort.

Today the astronomical unit is quoted to a few tens of metres. In 2004 the Venus transit is treated by most as an oddity of mere historic interest. Professional astronomers are indifferent. Astronomy has changed almost beyond recognition since 1882. The next transit cycle starts in 2117. Our children’s children’s children might be tucked up in bed on Mars by then.

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