Travelers' Tales: 1761

Observers who sought out the 1761 event had mixed results. By 1761 the Seven Years' War was in full swing, and battles between England and France upset the travel plans of some astronomers. Plans drawn up by the Royal Society of London included an expedition to St. Helena lead by the future Astronomer Royal Nevil Maskelyne and one to Sumatra by Charles Mason and Jeremiah Dixon (the pair later surveyed what became known as the Mason-Dixon Line in the US). But due to the war Mason and Dixon modified their journey while en route and decided to advance no farther than Cape Town, South Africa, where they enjoyed excellent conditions for the transit. At St. Helena, Maskelyne caught glimpses of Venus through clouds but failed to obtain useful data.

The French had similar troubles. For example, there was the epic journey of Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil de la Galaisière He arrived in Mauritius a year in advance of the transit but could not continue because the British were besieging his final destination — the coastal fort of Pondicherry, India. While waiting for the blockade to be lifted Le Gentil fell ill, recovered, and finally joined a French warship bound for India to relieve the French colony. Despite being blown off course by a monsoon they reached the Indian coast, but passing ships notified them that Pondicherry had fallen. Le Gentil's vessel turned back toward Mauritius. June 6th was a beautiful day in the Indian Ocean, and Le Gentil saw the entire transit, but from the deck of his pitching ship he could make no scientifically useful observations.

Another hindrance to the 1761 observations was the unexpected discovery of the" black-drop effect." The key objective of most expeditions was to time the transit's internal (second and third) contacts — the precise moment when the limbs of Venus and the Sun barely touch. But to everyone's surprise, at these very instants the edge of the planet appeared extended or was rendered indeterminate by a nasty but suitably named phenomenon described as a black ligament or drop.

All in all, the 1761 observations were a sharp disappointment. The black-drop effect caused significant variations in the recorded times of contacts, even among observers at the same site, and seriously undermined attempts to refine the Earth-Sun distance.

Travelers' Tales: 1769

If nothing else, 1761 provided a dress rehearsal for the next (and last) transit of the 18th century: June 3–4, 1769. By then peace reigned across Europe, and Britain enjoyed a far reach over the surface of the Earth. With colonial possessions so vast that the Sun never set on their empire, the British organized two expeditions to, literally, the opposite ends of the planet.

Meanwhile, the French remained active; new expeditions were mounted while one simply continued on. Le Gentil decided to remain south of the equator so as not to be deprived of a chance to observe the last transit of his lifetime. He hoped to observe the transit from Manila, but after arriving in the Philippines he was ordered to Pondicherry — once more a French possession — where he experienced the most devastating experience of any traveling astronomer: he was clouded out.