A Much-Scrutinized Planet
The earliest recorded sightings of Venus appear in a tablet dating back to the 16th century BC. 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.
Since then, 45 transits have taken place. 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, 1631. 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.
Halley, Venus, and the Astronomical Unit
In contrast to the indifference of 17th-century astronomers, interest in transits soared in the 1700s after English astronomer Edmond Halley proposed using them to determine the distance from the Earth to the Sun.As a young man, Halley journeyed to St. Helena, a bleak island in the South Atlantic, where Napoleon Bonaparte was exiled nearly 140 years later. 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. 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.u.).
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 500.
As with the comet whose return he successfully predicted, Halley knew that he would not live to see the upcoming transits of 1761 and 1769 — the first since the time of Horrocks and Crabtree. Halley died in 1742, 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.
Travelers' Tales: 1761Observers 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.
And the Distance Is . . .
And what of the traveling astronomers who made these measurements? They returned home, reported their results, often wrote a book about their travels and travails, and, in some cases, went on to make further astronomical discoveries. As for Le Gentil, it took him another two years, filled with numerous misadventures, to return to France, whereupon he found that his heirs had declared him dead and were in the process of dividing his property! Events finally took a better turn when he married a wealthy heiress, had a daughter on whom he doted, and lived in seeming contentment at the Paris Observatory.
Preparing for 1874
Encke threw down the gauntlet to astronomers worldwide to surpass the accuracy of this number at the next transits of Venus in 1874 and 1882. But first they would need to learn how to reduce the measurement errors caused by the black drop and other effects.
In order to practice anticipating effects like the black drop, and to get a better handle on the timing variations introduced by different visual observers in advance of the 1874 transit, astronomers at several institutions built artificial transit machines. In France, for example, Charles Wolf set up a series of lamps and screens in the window of a library at the Luxembourg Garden and had observers make timings with a telescope at nearby Paris Observatory. American researchers set up another transit simulator on a building near the US Naval Observatory in Washington, DC.
A New Tool
But suppose the observer could, in a sense, be eliminated altogether? As the 1874 transit of Venus was approaching, photography was coming into its own as an instrument of scientific research ("Picturing the Heavens," April 2004 issue, page 36.) Photography seemed to afford an objective and impartial record, not subject to the vagaries introduced by the "personality of the eye." And most conveniently, a photograph could be studied and measured long after the event was over.
In the buildup to the 1874 transit, photography was widely regarded as affording the best chance of obtaining an absolute and unvarnished record of the various fleeting contact phenomena and thereby obtaining the best value of the astronomical unit. Many nations, including France, Britain, the United States, Russia, and Germany, dispatched astronomers and their cameras to measure and photograph the transit from around the globe.
Amateurs usually employed eyepiece projection onto a white screen for solar viewing — the safest method then available. The only practical full-aperture solar filter obtainable in 1874 was smoked glass, but few wanted to smoke a valuable optical flat, much less their telescope's objective lens. So for direct viewing, observers typically placed an unsilvered prism in the telescope's optical path to deflect most of the Sun's heat and light away from the eyepiece. But enough heat and light got through to necessitate the use of tinted-glass filters to make viewing safe and comfortable, and even then there were considerable risks. In Australia one observer had the unnerving experience of having his dark glasses split, "causing him to lose the Ingress entirely."
As to photographic observations, most involved recording Venus's silhouette against the Sun so that its position could be carefully measured later. This meant taking special care that the wet-process plates then in use were carefully transported across great distances without bending or warping. Many plates were exposed, though most seem to have been discarded after they had served their purpose, and surprisingly few have survived.
The Transit of 1874: Disappointment
Under the circumstances, it is hardly surprising that publication of a refined value of the astronomical unit was not immediately forthcoming after the 1874 transit. Moreover, as was already becoming obvious even before the 1882 event, there were better methods for determining the Earth-Sun distance. One was to measure the position of Mars relative to the background stars during one of the red planet's close approaches to Earth. With Mars, David Gill (the Astronomer Royal in Cape Town, South Africa) deduced a value of the astronomical unit only 0.2 percent off the modern value, better than any achieved from timings of a transit of Venus.
The Transit of 1882: Derision
The transit of December 6, 1882, thus came to occupy an unusual position in the history of astronomy. In part as a result of inertia, and in deference to the important role that transits of Venus had historically played in the quest for the grail of the astronomical unit, a number of government-sponsored expeditions were funded. That did not play well with the press. For example, the New York Times lambasted the US Naval Observatory for sending several expeditions to remote points of the globe when the transit could be observed from the continental United States. Indeed, the results of the 1882 transit were not really scientifically important. What was unique about the event was that it was the first of its kind within reach of the public.
The New York Times reported:
This is the first time within the memory of man that the unlearned common people have been permitted to observe a transit, and it is the first revelation of the fact that a transit can be seen through smoked glass.People watched as they will again on June 8, 2004 — not in the hope of adding some tidbit of understanding to science, nor of refining the value of the astronomical unit, but out of sheer delight at the novelty of witnessing a rare happening in the heavens. Thus it will not be so much in the spirit of the great adventurers of the 18th century and the 1874 transit expeditions that we will watch the coming transit, but in a spirit of kinship with the "common people" who watched in 1882 realizing that this fleeting event would never again be seen in their lifetimes.
They relished it for its rarity, perhaps none more so than William Harkness of the US Naval Observatory, who reflected on the eve of the 1882 transit, "What will be the state of science when the next transit season arrives God only knows. Not even our children's children will live to take part in the astronomy of that day."
The astronomy of that day — our day — features gigantic telescopes on high mountaintops, twin rovers wheeling across the Martian surface, a probe about to enter orbit around Saturn, and an armada of space telescopes peering into the heavens from orbit and observing cosmic radiation across the entire electromagnetic spectrum. We will have our transit of Venus on June 8th, then another on June 6, 2012. After that, there will be no more until 2117. What will be the state of science then God only knows.
This abridged article appeared in its entirety the February and May 2004 issues of Sky & Telescope. Archive subscribers may download the articles without restriction; nonsubscribers may buy them for US$2.95 per article. The regular price of a one-year archive subscription is $19.95; Sky & Telescope magazine subscribers save 50% and pay just $9.95. See our archive subscription page to sign up.