American Scientist

It is time to bring to a close this story of the 18th-century transits of Venus and the often amazing expeditions to the ends of the earth that they engendered. The purpose in measuring and timing the passage of Venus across the face of the sun on the very rare occasions it is seen to do this was to establish the scale of the solar system (and eventually the scale of the universe itself). Observers had to be sent to very distant parts of the earth because the longer the baseline between them, the more accurate would be the result, and in the ill-explored world of the 1760s this would cost more than one of them his life. But before we turn to the ultimate results of these undertakings we must look at one more of the expeditions, the most famous of them all, the British expedition to the South Pacific for the 1769 transit.

Early analysis of the 1761 transit observations was not entirely satisfactory, and it was expected that the 1769 transit (the last for more than a century) would offer better results. By 1765 Thomas Hornsby, Savilian Professor of Astronomy at Oxford, was urging the European powers to prepare their expeditions: "Posterity must reflect with infinite regret their negligence or remissness; because the loss cannot be repaired by the united efforts of industry, genius, or power." Calculation showed that the South Pacific, as yet hardly explored by Europeans, would be a desirable station, and in case science should not prove attraction enough, Hornsby noted that it would be a "worthy object of attention to a commercial nation to make a settlement in the great Pacific Ocean." Thus it was that the British expedition to the Pacific would have far more hopes behind it than merely establishing the scale of the solar system. Commerce, politics and empire were not to be denied. The Royal Society of London's estimate that £4,000 would be needed to mount the expedition met with little argument, and an appeal to the 30-year-old King George III was launched. "The Memorialists, attentive to the true end for which they were founded by Your Majesty's Royal Predecessor . . . conceived it to be their duty to lay their sentiments before Your Majesty with all humility, and submit the same to Your Majesty's Royal Consideration." Royal Consideration quickly arrived at acquiescence.

The Society had among its fellows just the man to command such an expedition: Alexander Dalrymple, a former professional sailor with much experience in eastern seas and an adept geographer and navigator. But where to find a ship? Clearly the Royal Navy must be the answer, as it had been for Mason and Dixon years before. And then a major snag. The Admiralty, it seemed, had never forgotten the last time it had allowed an astronomer, Edmond Halley, to command one of its ships on a scientific expedition (see Marginalia, January–February 1986). The result had been mutiny and the near loss of the ship. The First Lord of the Admiralty, Sir Edward Hawke, rather extravagantly announced he would sooner suffer his right hand to be cut off than sign another such commission. So Dalrymple was out. The Admiralty would find its own man. They picked a junior officer, then doing marine survey work on the St. Lawrence River in Quebec. His name was James Cook, the ship he was to command, the Endeavour.

Precious Hardware

The next question was just where in the South Pacific the expedition should go. Such reports of islands in the vast ocean as existed were not entirely reliable as to latitude and longitude, and one would not, as did Le Gentil in 1761, want to find oneself at sea when the crucial moment arrived. But even as the Endeavour was being fitted out there arrived back from the Pacific the good ship Dolphin. And what news! It had found an island that was a virtual paradise on earth, an island "such as dreams and enchantments are made of . . . ." An island where not only the surroundings were paradisiacal, but where the local culture was also utterly different from that of Europe. In particular, the sailors had discovered, no doubt within minutes of arrival, that the women were extraordinarily free with their sexual favors. The gift of anything metallic would hasten proceedings even further. The captain of the Dolphin had feared his ship would sink at her moorings as her crew enthusiastically ripped the nails from her decks. Her navigators had taken particular care in determining the island's latitude and longitude. Its name was Tahiti. The Endeavour would sail for Tahiti. Considerable quantities of nails would be among her cargo.

So on August 26, 1768 the Endeavour sailed from Plymouth, bearing southwest for Rio, then 'round the horrors of Cape Horn and across some 7,400 kilometers of the Pacific to Tahiti, arriving with almost two months in hand before the transit. Joseph Banks (26, later Sir Joseph, and eventually one of the Royal Society's most colorful presidents), who had joined the expedition as scientific leader and botanist, found previous reports to be accurate.

Soon after my arrival at the tent 3 hansome girls came off in a canoe to see us . . . and with very little perswasion agreed to send away their carriage and sleep in [the] tent, a proof of confidence which I have not before met with upon so short an acquaintance.

But cultural differences went well beyond sexual mores. Ownership seemed a very fuzzy concept, and casually stolen goods became a sore point. Particularly when an important astronomical instrument disappeared and had to be hunted down at gunpoint. The English crew set a poor example, as Banks noted in his journal after a near-perfect observation of the transit:

We also heard the melancholy news that a large part of our stock of Nails had been purloind by some of the ships company during the time of the Observation . . . . This loss is of a very serious nature as these nails if circulated by the people among the Indians will much lessen the value of Iron . . . .

The transit observations concluded, Cook, as per instructions, set off southwestward in search of the great southern continent postulated by philosophers of the day as the counterpart to the great land masses of the northern hemisphere. Instead, he discovered New Zealand and spent six months charting its coasts. Setting off westward once more, he ran into the east coast of Australia and worked northward, charting 3,000 kilometers of coast as he went. That took them out into the channel between the coast and nearly 2,000 kilometers of the Great Barrier Reef. Despite the crew's desperately careful sailing, the beautiful but treacherous reef claimed the Endeavour, and although they eventually got her off they had to beach her for many weeks on the desolate Queensland coast to make repairs.

With supplies running low, the Endeavour put in to Batavia (Jakarta) for refreshment and more permanent repairs. So far the crew's health had been fine; indeed, Cook, with his insistence on sauerkraut as a defense against scurvy, was famous for protecting the well-being of his crews, but he had no defense against the malaria and dysentery ("the bloody flux") of tropical Batavia. By the time the ship set off to cross the Indian Ocean, round southern Africa and sail the length of the Atlantic, nearly half the crew had died and most of the remainder were severely stricken. But finally, on July 13, 1771, more than two years after the transit, the survivors, weak and shaken, arrived home. Among those they left behind was Charles Green, the expedition's official astronomer. It was reported that he "had been ill some time … [and] in a fit of phrensy got up in the night and put his legs out of the portholes, which was the occasion of his death."

It says something of Cook the man that he would undertake two more expeditions to the Pacific despite these previous experiences. It was, of course, to cost him his life.

So another chapter in the history of the transits of Venus was closed. No one alive then would see another. It remained only to determine how well they had done in arriving at their goal of calibrating the astronomical unit, the distance between the earth and the sun.

The Black Drop

Three problems permeated the analysis: first, the curious and unexpected phenomenon called "the black drop"; second, uncertainties in the distances between observers; and third, the problem of how to combine redundant observations.

The black-drop problem surprised observers. They were trying to determine the exact moment when the edge of Venus's disk was just tangent to the edge of the sun's disk as Venus began or ended its transit, but what they saw was an elongated black ligament joining the two edges and persisting even when Venus's disk was clearly within that of the sun. This so surprised and unsettled the observers that even when two of them were standing alongside each other their reported times could be half-a-minute apart, when they were expecting agreement to within a few seconds. As Cook himself reported,

This day prov'd as favourable to our purpose as we could wish, not a Clowd was to be seen the whole day and the Air was perfectly clear . . . [yet] we very distinctly saw a . . . dusky shade round the body of the Planet which very much disturbed the times of the Contacts . . . . We differ'd from one another in observeing the times of the contacts much more than could be expected.

Today we understand this as being the result of sunlight refracting through the very dense atmosphere of Venus, but it certainly degraded the timing of the transits.

The accuracy of the final results also depended directly on knowing the length of the baseline between observers—in effect knowing accurately the latitude and longitude of each observer. But since methods for determining longitude in the 1760s were inadequate, to say the least, these baselines were not well determined, and the accuracy of the final results was correspondingly diminished.

The third problem reminds us that although this series of articles has described those few expeditions that went to remote parts of the earth to observe the transits in 1761 and 1769 (and their observations carried the most weight), there were additionally many other observers who saw the transits from home, if home happened to be in the right hemisphere at the right time. The initial analysts of the data faced the problem of getting the best single answer from multiple locations and observations, when in principle only two observations were needed. Methods for combining redundant observations were only in their infancy and would not come to fruition until the work of Legendre, Gauss and Laplace in the early 19th century led to the method of least-squares.

Thus contemporary analyses of the 1760s data yielded a variety of answers. Typical was the analysis of Lalande in 1771, who found values of the earth's mean distance from the sun (the astronomical unit) in the range of 152 to 154 million kilometers (Mkm). But more than a century later in 1891, when locations had been much better determined and mathematical methods improved, Simon Newcomb, dean of late-19th-century American astronomy, from the same data determined a value of 149.7 ± 0.9 Mkm, and when he combined the 1761 and 1769 transits with those of 1874 and 1882, he found an overall transit value of 149.59 ± 0.31 Mkm.

Before we compare this to the latest determination, let it be said we now know that of the methods developed after the last transit of Venus up to the mid-20th century (which included trigonometric parallaxes of asteroids, gravitational perturbations by the sun and improvements in the constant of stellar aberration), none would surpass in accuracy (although often in claimed precision) the results of the transits of Venus.

Modern astronomy has turned back to Venus to calibrate the astronomical unit, but now in quite a different way. Today giant radiotelescopes are used as radar guns, pumping out a tremendously powerful radio signal directed at Venus, and minutes later, switched to receiver mode, detecting the faint echo returning from the planet, the round-trip time being measured by atomic clocks. This interval, together with the speed of electromagnetic waves, yields the distance of Venus at that moment—and thus, through Kepler's third law (see Part I of this series), the value of the astronomical unit. The current value stands at 149,597,870.691 ± 0.030 kilometers. This astonishing result, if taken at its claimed precision, almost defies comprehension. It is the equivalent of measuring the distance between a point in Los Angeles and one in New York with an uncertainty of only 0.7 millimeter!

So when the next transit of Venus finally comes along in about five years (June 8, 2004), we are not likely to expect new exactitude in determining the astronomical unit, but we might give thought to the words of William Harkness, a key American figure in the 19th-century transits, writing just after those transits:

There will be no other [transit of Venus] till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last [18th century] transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity which has led to our present advanced knowledge was just beginning. What will be the state of science when the next transit season arrives God only knows.

© J. Donald Fernie