In 1716 Edmund Halley (He of the comet, although he did not actually discover it – instead he determined that several previous recorded appearances of a comet were in fact the same one reappearing at intervals) published a paper showing how a transit of Venus could be used to calculate with remarkable precision the distance from the Earth to the Sun. A transit occurs when the planet passes directly between Earth and the Sun, and hence is observable as a black dot moving across the solar "surface" as viewed from Earth. Venusian transits are rare. We experienced two in our lifetimes but will never live to see another one; the solar system treats us to two separated by eight years (2004 and 2012) but then does not repeat the phenomenon for more than a century. The next one is in 2117.
Halley did not live to make anything of his idea, dying in 1742 and therefore missing out on the upcoming 1761 / 1769 pair of transits. Other astronomers took up the task, though. A worldwide effort led by Russian Mikhail Lomonosov attempted to coordinate hundreds of observations and measurements from every corner of the globe. Combining all of that data, even with the slow, cumbersome technology available in the 18th Century, would be a gold mine for astronomers. Some historians have suggested that this was the first truly international, coordinated scientific effort. Regardless, a great deal of data was collected and Halley's theory proved correct with time.
Many of the scientists who took part in the Venus effort were or would become famous. A pair of Englishmen, the famous Charles Mason and his assistant Jeremiah Dixon, would later become household names in America when they settled a border dispute between Pennsylvania and Maryland with a surveyed line that still bears their name. One who was not famous, though, was a Frenchman grandly named Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil. He failed miserably in his attempts to make observations in a series of misfortunes that borders on absurd.
In 1760 he set out to observe from the French possession of Pondicherry on the eastern shore of the Indian subcontinent. Reaching Ile-de-France (Mauritius) he found that further progress was made impossible by the ongoing naval war between France and Britain (in fairness, Mason and Dixon were fired upon numerous times by French ships too). He finally secured passage on a private merchant ship that had secured permission to travel to India by both navies. Unfortunately it went off course in a storm and floated aimlessly for five weeks, and when they finally reached Pondicherry they found that the French had lost it to Britain and Gentil was not allowed to disembark. The ship instead was forced to return to Mauritius, and on the appointed day, June 6, Gentil was unable to observe the transit from the deck of a ship pitching and rolling around in the Indian Ocean.
Surely that disappointed him, but he had another shot in the not too distant future. He remained in the Indian Ocean and took up projects like mapping the African coast, surveying Madagascar, and so on. He sent back word to France to let his family know that he would not be returning until after the second transit. After lengthy consideration he decided to observe the 1769 event from Manila. The Spanish colonial government, however, put him back on his ship when he arrived for some reason lost to history but probably having to do with the petty rivalries that defined the three great European powers in that era. Exasperated, he returned to Pondicherry (won back by France in a 1763 peace treaty) and set up his equipment to make his long-awaited observation. When it arrived – June 4, 1769 – Pondicherry, which had been chosen specifically for its unusually San Diego-like weather, was completely overcast. It was the first and only overcast day in more than six months Gentil spent at Pondicherry. He didn't see a thing.
Defeated, he returned to France on a ship on which dysentery broke out, killing many of the passengers and horribly sickening Gentil himself. When he finally arrived, haggard, half-dead, and spiritually deflated, he found that none of his letters to France had reached their destination. He had been removed from the Academy and declared legally dead; his wife re-married and avaricious relatives "enthusiastically plundered" his estate.
Only the direct intervention of Louis XVI, who found beyond pitiable the story of this man who had tried so hard to achieve something for the sake of the Academy and had been roundly kicked in the ass in return, restored him to something of a normal life. He was restored in his position at the Academy, remarried, and lived an additional two decades.
So if you had a bad week or you're having a bad day, it could be worse. You could be Guillaume de Gentil. Or on a ship when dysentery breaks out. Or both.