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. It is a plausible idea, but to date no accounts of a round blemish, seen at the same time as a Venus transit, have been discovered.
For that matter, even the first transit of the telescopic era was missed. Johannes Kepler, a German astronomer and mathematician, predicted such an event for December 6, We now know that the transit's end was visible at sunrise from Italy, Austria, Germany, and Denmark. But it seems nobody in central Europe was looking, likely because of the turmoil caused by the ongoing Thirty Years' War.
Kepler died a year before that transit, and Pierre Gassendi, the only astronomer known to have kept watch, did so from Paris, which was outside the region of visibility.
The next transit, eight years later, also nearly went unobserved, because Kepler had failed to forecast it. The possibility of a Venus transit was picked up only a month before it occurred, thanks to the calculations of a brilliant young English astronomer, Jeremiah Horrocks. Horrocks and his friend William Crabtree were the only ones to observe a transit of Venus in the 17th century.
In contrast to the indifference of 17th-century astronomers, interest in transits soared in the s after English astronomer Edmond Halley proposed using them to determine the distance from the Earth to the Sun. It was Edmond Halley who first realized how to calculate the Earth—Sun distance by using measurements obtained during a transit of Venus.
His theory inspired astronomers in many nations to mount expeditions to observe the transits of and Copyright The Royal Society. As a young man, Halley journeyed to St. Helena, a bleak island in the South Atlantic, where Napoleon Bonaparte was exiled nearly years later. There he mapped the southern stars and in November observed a transit — not of Venus but of Mercury.
Transit of Venus: Measuring the heavens in the 18th century
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. If each observer timed the transit from beginning to end, the shift in the planet's position — its parallax — could be calculated and used to determine the Earth-Sun distance, a separation called one astronomical unit a. Although Mercury's transits are relatively frequent — on average, 13 of them occur each century — the planet's parallax is too small for the method to have great utility.
But Halley realized that the rarer transits of Venus were a different matter, and he proposed that they could be used to measure the distance from the Earth to the Sun to an accuracy of 1 part in As with the comet whose return he successfully predicted, Halley knew that he would not live to see the upcoming transits of and — the first since the time of Horrocks and Crabtree. Halley died in , but his method inspired other astronomers. The quest to use a transit of Venus to calculate the Earth-Sun distance became one of the great scientific obsessions of the 18th century.
Observers who sought out the event had mixed results. By 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. 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.
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. Australian watchmaker F. He observed through a 3. Russell Sydney, Courtesy Institute for Astronomy University of Vienna.
Another hindrance to the observations was the unexpected discovery of the" black-drop effect. 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 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.
If nothing else, provided a dress rehearsal for the next and last transit of the 18th century: June 3—4, 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. And the Distance Is. Using measurements of the transits of Venus of and , Johann Franz Encke — of Germany derived a value for the Earth—Sun distance that would stand for a generation. The transits had come and gone, but the observations of and were subject to scrutiny and calculation far into the 19th century.
In Johann Franz Encke, the director of Berlin Observatory, reviewed all the transit measurements and determined a parallax of 8. This was a great improvement over previous determinations but still well off today's value of ,, km. And what of the traveling astronomers who made these measurements?
In their race to be the first to lay claim to new territory, the British government and the Royal Navy came up with a secret plan: Send a naval officer on a supposedly scientific voyage, then direct him to undertake a voyage of conquest for the fabled Southern Continent. The man chosen for the job was one James Cook, a Navy captain who also had training in cartography and other sciences. Europeans already knew the Pacific had its share of islands, and some of them held the potential for enormous wealth.
Magellan was followed by a dozen other Europeans—especially Dutch and Spanish captains—over the next two centuries, some of them sighting the western shores of Australia, others identifying New Zealand. But the vastness of the Pacific Ocean, combined with the unreliability of maps, meant no one was sure whether the Southern Continent existed or had been discovered. Holland Rose. But to do so without drawing undue attention to their goals, the Admiralty needed another reason to send ships to the Pacific.
The Endeavour Flies Again
The Royal Society presented the perfect opportunity for just such a ruse. Founded in , the scientific group was at first little more than a collection of gentlemen with the inclination and resources to undertake scientific projects. As historian Andrew S.
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It was one of the most massive international undertakings to date. Captaining the Endeavour , Cook departed from Plymouth years ago on August 26, , in order to arrive in Tahiti on time for the transit, which would happen on June 3, His path carried him across the Atlantic and around the difficult-to-traverse Cape Horn in South America toward the south Pacific. Unfortunately for the scientists, the actual observations of the transit at points around the world were mostly useless. Telescopes of the period caused blurring around the planet that skewed the recorded timing of Venus passing across the sun.
But for Cook, the adventure was just beginning. Cook went on to follow those instructions over the next year, spending a total of 1, days at sea on this mission. He also traveled along the east coast of Australia, again becoming the first European to do so. Even as he took control of their land, Cook seemed to recognize the indigenous groups as actual humans. Their canoes are large, well built and ornamented with carved work. But the toll of that decision would be heavy. Cook estimated the native population on Tahiti to be , in By the time the French took control of the territory and held a census in , they found only 7, people of native descent.
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