Chasing Venus: Observing the Transits of Venus 1631-2004
What is a Transit of Venus?
Discovering the Transits: The 1631 and 1639 Transits
Measuring the Universe: The 1761 and 1769 Transits
New Possibilities: The 1874 and 1882 Transits
The Transits of 2004 and 2012
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Measuring the Universe: The 1761 and 1769 Transits

Hunting the Astronomical Unit

In 1677 Edmond Halley (of Halley's comet fame) began to think about using transits to find the Earth-Sun distance (the astronomical unit or AU). He finally developed a workable method: astronomers at two widely separated places on Earth measure the exact time it takes for Venus to cross the Sun. Knowing the difference in the times of passage and the distance between the observers, Halley thought astronomers could calculate how far Venus is from Earth. Then, using Kepler's laws, they could determine the AU.

The French astronomer Joseph-Nicolas Delisle improved on Halley's method. If the two observers know their exact positions on Earth, he argued, they only need to record the moment when the edge of Venus lines up with the edge of the Sun. Even if clouds prevent them from seeing the entire transit, they will still have valid measurements.

Mobilizing for the 1761 and 1769 Transits

All over Europe and North America, astronomers prepared for the 1761 transit of Venus, vying to be the first to determine the true Earth-Sun distance. They would need to observe it from widely separated places on Earth, covering several sites in each area, in case bad weather (or rotten luck) affected some observations.

The best place to see the entire 1761 transit was India and the East Indies. The most distant site that would complement those locations was Siberia. Accordingly, French and British academies of science sent astronomers to both regions and other distant points around the world.

Those expeditions helped astronomers better prepare for the 1769 transit when Russia, Sweden, and Denmark joined in observations. The 1769 transit was observable from America, so many astronomers watched it in places like Boston, Philadelphia, and Providence. Click here to see a map representing past expeditions.

To the Ends of the Earth

The 1761 and 1769 transit-of-Venus expeditions suffered great trials and achieved moderate success.

Due to the expense of mounting major expeditions, a great deal of additional scientific activity was included. A number of expeditions, especially the well-known voyage of Captain Cook, made many natural history and anthropological observations and brought back specimens and artifacts. The voyage signaled a new age of scientific exploration.

More than 120 transit observations were recorded in 1761, but most were of poor quality, due to optical problems and inexperienced observers. In 1769 more than 150 observations were recorded - with no better results. Nevertheless, the range of values for the Earth-Sun distance was narrowing to between 94 and 96 million miles. Astronomers would have to wait another century for the next transit of Venus and their next attempt at finding the precise answer.

The Perils of Chasing Venus

Traveling across the world could be grueling in the 1700s. Most of the transit expeditions suffered hardships, but two French astronomers were particularly unfortunate.

Guillaume Le Gentil de la Galaisière traveled to India to observe the 1761 transit from the French colony of Pondicherry. Before he arrived, the British captured the colony, forcing Le Gentil to return to the French island of Mauritius. He did not get there in time to observe the transit. Rather than return home, he studied the Indian Ocean cultures, planning to watch the 1769 transit from Manila in the Philippines. But he was ordered back to Pondicherry, again under French control. While Manila had beautiful weather during the transit, it rained in Pondicherry. Le Gentil missed both transits. When he returned home in 1771, he learned that he had been declared dead. After many expensive legal battles, Le Gentil managed to prove he was actually alive.

For the 1769 transit, Jean Chappe d'Auteroche led a party to the southern tip of Baja California. They observed the transit under ideal conditions, but a few days later an epidemic struck the area, killing three-fourths of the local population and everyone in the expedition except for Chappe and Pauly, his engineer. Chappe lingered for a short while but died before leaving Baja. Pauly managed to get back to France with all of the observation records.

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