The history of observing the transits of Venus
The early forecaste and transits in 1631 and 1639. Realiable forecasting transit of the transit of Venus is requires the orbits of inside planets. Astronomy was revolutionized when Johannes Kepler set up the so called writes Rudolph's tables named in honour of Rudoph II, Kepler's patron.. Kepler had predicted that a transit of Venus would follow on
6 December 1631. Kepler died before the first transit of Venus in 1631.
The nightly, 1631 transit of Venus was not visible in Europe. However in October 1639, Jeremiah Horrocks discovered that there would be another transit in December 1639. Horrocks was the first man to witness the transit of Venus.
In 1677 Edmond Halley (famous for so called Halley's comet) 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 remote places on Earth measure the exact time it takes Venus to cross the Sun. Knowing the difference in the times of the passage and the distance between the observers, Halley thought astronomers could calculate how far Venus is from Earth. Then, using Kepler's laws, astronoms could determine the AU.
Halley's method was improved on by the Joseph-Nicolas Delisle, french astronomer. 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.
All over Europe and North America, astronomers prepared for the 1761 transit of Venus, vying to be the first peple to determine the true Earth-Sun distance. They would need to observe it from distant places on Earth, covering several sites in each area, in case bad weather (or bad 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.