1769 Transit of Venus

Date: 1769
Owner: Chez Charles-Antoine Jombert
Source Type: Images


Baja California, a remote area on the periphery of New Spain, was one of 151 observatories throughout the world (from Siberia to Tahiti) that turned their telescopes towards the sun on June 3, 1769, to watch Venus pass in front of it. A half century earlier, British astronomer Edmund Halley predicted that observing the transit of Venus (an event that happens only twice every 113 years) would facilitate the calculation of the distance from the earth to the sun, a length now known as one Astronomical Unit (AU). Although there was a Venus transit in 1761, many of the observations were flawed and 1769 (the last passage until 1874) was seen as the last opportunity for the generation of Enlightenment astronomers to observe this crucial event. The resources of Europe's largest empires were thus deployed to distant corners of the globe to stargaze from various latitudes and longitudes.

This chart notes the observations of seven disparate observatories, including Captain Cook's in Tahiti, one on the Arctic Ocean, and two in Baja California. The San Joseph observatory was the work of a joint scientific expedition from France and Spain led by the French astronomer Jean Baptiste Chappe D'Auteroche and two Spaniards, Vicente de Doz and Salvador de Medina. (Carlos III sent Doz and Medina to observe both the transit and any suspicious behavior by the French astronomers). The second observatory in California (at Santa Ana) was set up by Joaquin Velazquez de Leon, a Mexican scientist and royal officer who had taught himself the necessary astronomy to make accurate measurements of the transit and owned two adequate telescopes. From his makeshift observatory, Velazquez made the measurements recorded here, that were included in Chappe D'Auteroche's report and then integrated into the calculations used to measure the size of the earth and distance to the sun.

There were, however, great sacrifices made to conduct these observations. The French and Spanish expedition became infected by an epidemic of typhus and 11 of the 17 Frenchmen, including Chappe D'Auteroche, died in California a few months after the transit. 8 of the 11 Spaniards also died of this disease. The transit, though, underscored the necessity of doing astronomy in observatories all over the earth and, combined with the many natural advantages of observing in Latin America, set the stage for many future expeditions and permanent observatories for the scientific benefits of foreign powers.

References: Engstrand, Iris Wilson. Royal Officer in Baja California, 1768-1770: Joaquin Velazquez de Leon. Los Angeles: Dawson's Bookshop, 1976.

Nunis Jr., Doyce B., Ed. The 1769 Transit of Venus: The Baja California Observations of Jean-Baptiste Chappe D'Auteroche, Vicente de Doz, and Joaquin Velazquez Cardenas de Leon. Trans. By James Donahue, Maynard J. Geiger, and Iris Wilson Engstrand. Los Angeles: Natural History Museum of Los Angeles County, 1982.

CITATION: Observatoins de la duree du passage de Venus en 1769. In Chappe D'Auteroche, Jean Baptiste. Voyage en Californie pour l'observation du Paggage de Venus sur le disque du soleil, le 3 Juin 1769. Redige & publie par M. Cassini fils. Paris: Chez Charles-Antoine Jombert, 1772.