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. Helena, Maskelyne caught glimpses of Venus through clouds but failed to obtain useful data. 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.This Venus Tablet, as it is known, was found among the ruins of Nineveh (near present-day Mosul, Iraq) and now resides in the British Museum. Might Venus have been observed on one of these occasions and unwittingly set down as a sunspot?The planet's silhouette is large enough to appear as a small, naked-eye spot during a transit, and there are many records, especially by astronomers in ancient China, of "blemishes" on the Sun's face.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.Halley died in 1742, but his method inspired other astronomers.
For that matter, even the first transit of the telescopic era was missed.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).There he mapped the southern stars and in November 1677 observed a transit — not of Venus but of Mercury.At once he realized that if two observers were widely separated in latitude, they would see a transiting planet move along different chords as it traversed the Sun.
Johannes Kepler, a German astronomer and mathematician, predicted such an event for December 6, 1631.