A transit of Venus was a really big deal in astronomical circles during the last few centuries - timing the planet's apparent passage across the surface of the sun was the only way to get a better estimate of the Earth's distance from its very own star.
Transits visible from the Earth are rare. They occur in an odd pattern: eight years apart, then 121.5 years, then eight years, then 105.5 years. (The next one happens in 2012, then we wait 12 decades.)
Usually we read about successful measurements. Here's the other side of the coin.
Guillaume Le Gentil (Guillaume Joseph Hyacinthe Jean Baptist Le Gentil de la Galaisier to his friends) was born in 1725 in Coutances, a city in the northwest part of France near the English Channel. Le Gentil trained for the church, but became fascinated by astronomy. He did well, and discovered many Messier objects (galaxies, star clusters, etc., that looked like blobs in contemporary telescopes).
As the 1761 transit of Venus approached, he was one of many astronomers who scattered all over the world to view the event. Le Gentil headed for India, but conflict with the British and other problems delayed him. He ended up watching the transit from the rolling deck of a ship, making accurate measurements impossible.
Then he made the fateful choice to stick around exploring the region until the next transit in 1769. When the big day arrived, clouds obscured his view. Rats.
Dejected, he headed home, only to run to more delays - storms, dysentery, you name it. When he finally got back to France, he discovered he had been declared dead. His wife was remarried, and his relatives had divided up his property.
(There is a happier ending. He remarried, regained his property via lawsuits (and some help from the king) and got his job back. He died at 67.)