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.