Information sheet n° 20c :
History of the observations of the passages (3):
The transit of Venus of 1769

The experience gained at the time of the observation of the passage of 1761 will be used to improve the methods of observations for the passage of 1769. Lalande organized the observations of the french astronomers. The study of the sites favourable for the observations was made by Pingré. Le Gentil remained in Madagascar, went initially to Manila, then in Pondichéry where a fatal cloud deprived it of the observation. "C'est là, le sort qui attend souvent les Astronomes. J'avois fait près de dix mille lieues; il sembloit que je m'avois parcouru un si grand espace de mers, en m'exilant de ma patrie que pour être spectateur d'un nuage fatal, qui vint se présenter devant le Soleil au moment précis de mon observation, pour m'enlever le fruit de mes peines & de mes fatigues"(note 1).. Chappe accompanied by the engineer geographer Pauly, the designer Noël and the clock maker Dubois as well as two Spanish astronomers Vicente de Doz and El Salvador de Medina went in Low California on the West coast of Mexico, close of the Cape Lucas in a Spanish mission bearing today the name of San José del Cabo. The observation of the passage by Chappe and its collaborators was a success, they stayed there to observe the eclipse of the Moon on June 18, 1769 in order to determine with precision the longitude of their site of observation.they succumbed to an epidemic of typhus which decimated the three quarters of the population. Only Pauly survived the epidemic. The third french mission was a travel by sea of which the goal was not only the observation of the Venus passage but to test the marine clocks invented by Berthoud. Pingré and the Comte de Fleurieu, sponsoring the voyage, observed the Venus transit from the Cape François in Saint-Domingue.

In England the observation of the passage of 1769 was actively prepared. Since 1763, James Ferguson described the future passage in the Philosophical Transactions and two years later, Thomas Hornsby published an important report on the advisability of observing the next passage. "In this uncertainty, the astronomers of the present age are peculiarly fortunate in being able so soon to have recourse to another transit of Venus in 1769, when, on account of that planet's north latitude, a difference in the total duration may conveniently be observed, greater than could possibly be obtained, or was even expected by Dr. Halley from the last transit". In November 1767, a special committee was created to prepare the observation of the passage of 1769. This committee decided to send three teams of observers. A first team of observers, composed of Dymond and Wales, went to Fort Churchill in Hudson Bay. A second team trained by the father Maximilen Hell, assisted by the Danish astronomer C. Horrebow and a young botanist Borgrewing, was to go to Vardö, a small island in the north of the Scandinavian peninsula and a last team was to go in the islands of the South Seas like had suggested it Thomas Hornsby. This last mission, was also used to explore the South Seas and was entrusted to a young unknown lieutenant, James Cook, the observation of the Venus passage having to be made in Tahiti, islands discovered two years earlier by Samuel Wallis. The observation in Tahiti was made by Charles Green and James Cook. To these three teams it is advisable to add Bayley and Dixon, Bayley observed the passage to the Cape North and Dixon observed it on the Norwegian island of Hammerfest. In addition to these observations, there were also those carried out (approximately 90) in the american british colonies under the impulse of Winthrop, author of the only american observation in 1761.

The imperial Academy of Russia under the impulse of the tzarina Catherine II also invited many foreign astronomers to come to observe the Venus passage. It was the case of the german Jesuit C. Mayer, of the swiss astronomers Mallet and Pictet and of the swedish J. Lexell, the famous mathematician Leonard Euler also made the voyage. Russia sent these observers on many sites distributed on its vast territory (Yakutsk, Orks and Orenburg in the south of the Ural, the Cola peninsula, St-Petersbourg).

Visibility of the passage of 1769, projection of Hammer

Third place for the number of observations carried out at the time of the first Venus passage, the English will pass in the first place with 69 observations on distinct sites. They were followed by France with 34 observations only, which marks the future decline of the scientific hegemony of France in Europe. Finally the passage of 1769 showed 151 professional observations, distributed on 77 sites. In spite of the numerous instruments used for the observation, (27 achromatic refractors -there were only three for the observation of the passage of 1761-), the observations did not make it possible to provide a final value to the solar parallax. Moreover it should be announced that this observational campaign had made many victims in the team of Chappe in Mexico, and also during the voyage of Cook.
The following table provides the various values allotted to the solar parallax following these observations.

Authors Values
William Smith 8,6045" (1770)
Thomas Hornsby 8,78" (1770)
Pingré et Lalande 9,2" et 8,88" (1770)
Pingré 8,80" (1772)
Lalande between 8,55" and 8,63" (1771)
Planmann 8,43" (1772)
Hell 8,70" (1773/1774)
Lexell 8.68" (1771) and 8,63" (1772)

One can conclude that the parallax lies between 8,43"and 8,80", which represents a clear improvement compared to the values obtained after the first passage which gave a parallax ranging between 8,28"and 10,60".

The reduction of the observations of the passages of 1761 and 1769 was taken again thereafter by J.F. Encke in 1824 and S. Newcomb in 1890.

Note 1:

"It is there, the fate which often awaits the Astronomers. I travelled nearly ten thousand miles; it seemed that I traversed a so great space of seas, by exiling me of my fatherland that to be spectator of a fatal cloud, which was presented in front of the Sun at the precise moment of my observation, to remove me fruit of my sorrows & my tirednesses"