The Chance of a Lifetime: The Transit of Venus and the American Revolution

Early this evening (Tuesday, June 5, 2012), the Transit of Venus will be observable to amateur and professional astronomers across the globe. The next opportunity to view this major astronomical event will be December 2117. In other words, this is your last shot to see it. Even if you’re not much of a stargazer, you might like to understand the political significance of the Transit of Venus.

The first public reading of the Declaration of Independence took place in the yard of what is now called Independence Hall in Philadelphia. The crowd that had gathered outside the Pennsylvania Statehouse on July 8, 1776, listened intently as Colonel John Nixon ascended a wooden platform and began reading, “When in the Course of human events…” That wooden platform–much like the Enlightenment ideas in the Declaration of Independence– had been built for the observation of the Transit of Venus in 1769.

What follows is an excerpt from Chapters 4 and 5 of my book, American Independence. I focus here on the significance of the Transit in eighteenth century science, the history of the American observations, and on the Transit’s impact on early American political thought. If you want to read more, you can read any chapter of the book for free here, or you can buy a paperback copy here.

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At a meeting of the American Philosophical Society on February 7, 1769, fourteen members were selected as a committee for the observation of Venus’s transit across the face of the sun. With only four months until the event, the committee began preparations right away. The Society commissioned carpenters to build three observation decks: one in the State House yard in Philadelphia, one on the Rittenhouse farm in Norriton (now Norristown), and one on Cape Henlopen in Delaware. As carpenters hammered away at the Philadelphia observatory, located in the open square south of the Pennsylvania State House (now Independence Hall), the observation team gathered regularly to make calculations, to adjust instruments, and to clear any possible obstructions to their view.

Transit of Venus and Scientific Culture

The Transit of Venus was an eighteenth century “space race.” The transit generated worldwide scientific interest not because it was expected to reveal new facts about the planet Venus, but because it was the key for measuring the dimensions of the solar system. A careful observation of Venus’s transit across the disc of the sun was the best method yet devised for measuring the solar parallax, an astronomical figure that would permit scientists to recheck the distance from the earth to the sun (the “absolute unit” in astronomy), which was used to calculate the size of the solar system. The planet Venus crosses the sun’s face no more than twice per century because of the difference between the orbits of the earth and Venus. Moreover, each transit is visible only from certain places on earth, and the transit of 1761 had not been visible from North America. The Transit of Venus was the most important astronomical problem of its day, and by conducting exemplary observations of the phenomenon in 1769, the Americans hoped to put themselves on “the map” of European scientific culture.

Johannes Kepler had been the first to calculate the times of the Transit of Venus, but a mathematical error led him to the conclusion that the event would not be visible in 1639. An English minister, Jeremiah Horrocks, corrected Kepler’s error, and along with his friend, William Crabtree, made the only observation of the transit in 1639. Although the clouds had parted just before Horrocks’s observation, the accuracy of his results had been compromised because he was conducting church services when the planet first made contact with the sun.

Astronomers were ready for the next transit in 1761, which was a fiercely competitive affair, especially between Britain and France, who were then embroiled in the Seven Years’ War. Cloudy weather in Europe hampered observations at all but a few stations in the northern and southern extremities of the continent, and well-funded expeditions to distant observation sites met with mixed success. Charles Mason and Jeremiah Dixon tried to abort their voyage to Sumatra after an attack by a French warship killed eleven of their crew. Their Royal Society patrons replied that abandoning their mission would “bring an indelible Scandal upon their Character, and probably end in their utter Ruin.” After being forced to alter their course again, they ended up observing the transit from the Cape of Good Hope.

Around the world, astronomers awaited impatiently the next occurrence of the phenomenon in 1769 because it would allow them to check the correctness of the figures gathered from 1761 through repetition of the experiment. Again, expeditions embarked for far flung locales specifically to view the transit in places like Norway, Hudson Bay, Baja California, and Tahiti, where then-Lieutenant James Cook and his crew timed the event as the main impetus for their first voyage of discovery.

There were in 1769 two known methods of measuring the solar parallax. The first required only a single location but demanded perfect precision, instrumentation, and weather. Astronomers preferred a second method, proposed by the late Edmond Halley, who had referred to the Transit of Venus as “that sight which is by far the noblest astronomy affords.” In this method, astronomers measured the difference of absolute time of the transit at a variety of locales and triangulated the results, along with precise longitudes and latitudes, to generate a figure acceptable to the world scientific community. The latter method did not demand advanced instrumentation, which relieved Thomas Pryor, a member of the Philadelphia observation team, because he used his own 18-inch reflecting telescope with a “magnifying power he does not certainly know, but supposes it to be at least an hundred times.” Thus the Philadelphia team, armed with two 18-inch telescopes, one 12-inch telescope, and “a good time-piece,” promised themselves “the pleasure of making accurate Observations, if the weather should prove favourable.” And this last point was imperative. A stray cloud at the moment of contact could totally occlude the observation. The Philosophical Society team gambled, therefore, four months of effort on the weather conditions of just over eighteen minutes. They determined that the project was worth the risk, since the next transit would occur in 1874. For this band of amateur astronomers, the observation of the Transit of Venus was literally the chance of a lifetime.

Owen Biddle supervised the observation at Cape Henlopen. John Ewing and Hugh Williamson headed the Philadelphia team, and William Smith led the team observing the transit from Rittenhouse’s farm in Norriton. The day of June 3, 1769, proved clear at all three observatories, and the teams of amateur astronomers, equipped with telescopes and pocket watches, successfully observed the entry and exit of the tiny silhouette of Venus arcing across the sun’s photosphere. David Rittenhouse, an expert surveyor, had calculated the longitude and latitude of the observatories, and he had also predicted with fine detail the exact motion and duration of the event. Biddle, Ewing, and Smith composed reports of each respective observation, which were presented to the Philosophical Society, published in its Transactions, and circulated in Europe, where the results were compiled with others to produce a more accurate measurement of the distance between the earth and the sun.

The American Philosophical Society took pride in its “committee” of astronomers who had ably contributed to solving the greatest problem in the “most sublime” of sciences. In the eighteenth century, one of the most important changes in the history of humankind’s self-understanding was occurring. The vastness of the universe and the immensity of past time were just beginning to be grasped, and the paradigmatic impact of these discoveries changed the nature of human relations. The Transit of Venus was an extraordinary alignment of the solar system, visible from only a few distinct spots on the planet. Under optimal conditions, amateur experimental scientists, aided by the instruments of chronometry and reflective optics, could observe and time this singular event. One epochal convergence in one fleeting moment, duly observed, held the key to unlocking the truths of the universe. This was a defining experience of Enlightenment culture, and by analogy, it explains Thomas Paine’s construction of the decision for independence in Common Sense.

Paine later described America as “the only spot in the political world,” where the “universal” principles of revolution could be at first adequately observed. In 1776 “an assemblage of circumstances conspired” to facilitate the discovery and advancement of political principles that would have been occluded anywhere else. Other pretended revolutions, he said, had “extended only to a change of persons and measures, but not of principles” and, therefore, were unable to escape the gravity of “the common transactions of the moment.” The “independence of America” swelled in significance through the combined optical powers of perspective and observation. America “made a stand, not for herself only, but for the world,” said Paine, because she “looked beyond the advantages herself could receive.”

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Epic Time and the Norriton Observation of the Transit of Venus

As William Smith [Provost of the College of Philadelphia, later renamed the University of Pennsylvania] related the situation in his account of the observation of the Transit of Venus, “The great discouragement which the different Committees” of the American Philosophical Society endured at first “was the want of proper apparatus, especially good telescopes.” The Pennsylvania Assembly agreed to fund some of the necessary equipment, most of which was directed (for political reasons) toward the Philadelphia observatory in the State House Yard. Owen Biddle used the Library Company of Philadelphia’s reflecting telescope for the Cape Henlopen observation. William Smith received word that colonial proprietor Thomas Penn was sending him from London a Gregorian reflecting telescope with a micrometer to be given to the College of Philadelphia when the observation was finished. John Lukens used a refracting telescope that Rittenhouse had assembled using “glasses” sent with Smith’s telescope from England. Rittenhouse himself observed the transit using a refracting telescope with a magnifying power of about 144 times. Lukens, the surveyor-general of Pennsylvania, had obtained from the surveyor-general of New Jersey an astronomical quadrant, a device used to measure the altitude of celestial objects above the horizon. The Norriton team also utilized, according to Smith’s account, “An excellent clock; a transit telescope, nicely moving in the plane of the meridian; and a very accurate equal altitude instrument, supported, in the observatory, on a stone pedestal.” Each of these last three items, as well as Rittenhouse’s refracting telescope and the observatory itself, were “Mr. Rittenhouse’s private property, and made by himself.”

Rittenhouse had begun building his observatory in November 1768 but, “through various disappointments from workmen and weather,” did not complete it until mid-April 1769. It was a primitive but sturdy wooden structure with an earthen floor, large windows, and openable shutters on its roof. Rittenhouse had hoped to receive an equal altitude instrument from Philadelphia, but, he reported, “Finding I could not depend on having it, I fell to work and made one, of as simple a construction as I could contrive.” His poor health kept him from conducting evening observations, so he was “obliged to content myself” with taking equal altitudes of “the Sun only.” On May 20, Rittenhouse installed thin perpendicular wires on the lens of his equal altitude instrument rather than the conventional scalp-plucked “crosshairs.” That day he began a series of exacting daily observations with the equal altitude instrument and his “meridian or transit telescope,” which he continued uninterrupted for at least a month and a half. Rittenhouse also paid special attention to an astronomical clock that he had made for the occasion; between March and May 1769, Rittenhouse “altered” his clock several times, took it down once for cleaning, and returned it to the observatory to be “regulated anew.”

When Smith and Lukens arrived at Norriton on June 1, 1769, two days before the transit, they found “every preparation so forward, that we had little to do but to examine and adjust our respective telescopes to distinct vision. Mr. Rittenhouse had completed his observatory, fitted up the different instruments, and made a great number of observations, for fixing the latitude and longitude of the observatory, and ascertaining the going of his time-piece.” The team coordinated a methodical scheme for their observation. With the purpose that “each of us might the better exercise our own judgment, without being influenced, or thrown into any agitation or surprise by the others,” they agreed to “transact every thing by signals” and in silence. John Sellers and Archibald McClean leaned out one window of the observatory to take signals from Lukens. Two members of Rittenhouse’s family who “had been trained by him to services of this kind” stood in another window “to count the time, and take his signals.” Smith was inside the observatory and “within the hearing of the beats of the clock,” so he was “to count and set down my own time.”

At two o’clock, everyone moved to their stations, but “a great concourse of many of the principal inhabitants of the county” made them apprehensive that “that our scheme of silence and order might be interrupted by the impatience and curiosity natural on such occasions.” Smith informed “the gentlemen, who had honoured us with their company, that the accuracy and success of our observations would depend on our not being disturbed with the least noise, till the contacts were over.” During the twelve minutes between Smith’s announcement and the first contact, “there could not have been a more solemn pause of expectation and silence, if each individual had stood ready to receive the sentence that was to give him life or death.” Smith further described the scene,

So regular and quiet was the whole, that, far from hearing a word spoken, I did not did not even hear the feet of the four counters, who had passed behind me from the windows to the clock; and I was surprised, when I rose up and turned to the clock, to find them all there before me, counting up their seconds to an even number; as I imagined, from the deep silence, that my associates had yet seen nothing of Venus.

As Smith sat in the cool shade of the observatory scanning the edge of the sun, the only sound he heard, besides the beating of his heart, was the ticking of a clock.

Rittenhouse’s telescope was set up farthest from the observatory, and laying on the ground for maximum stability, he peered through the long tube of his refracting telescope. At the moment of first contact, he gave a hand signal to his assistant, Thomas Barton, who in turn waved a handkerchief “to the counters at the window, who, walking softly to the clock, counting as they went along, noted down their times separately, agreeing to the same second.” Rittenhouse had signaled at the time “to the best of my judgment” when “the least impression made by Venus on the Sun’s limb could be seen through my telescope,” but his estimate of the time was earlier than that of his colleagues.

Even though the air was “beautifully serene and quiet” and the Sun’s edges “perfectly defined” and “free from any tremulous motion,” the observation was not so clear-cut as they had hoped. As Smith put it, “The idea I had formed of the contact was—that Venus would instantaneously make a well-defined black and small dent or impression on the Sun.” In fact, the actual “appearance was so different” that he “was held in suspense for five or six seconds”—an eternity in the calculation of the astronomical unit—“to examine whether it might not be some small skirt of a watery flying cloud.” In fact, what Smith, Rittenhouse, Lukens, and every other observer of the transit saw was what is now called the “black drop effect.” Smith described “the disturbance on the Sun’s limb” as “so undulatory, pointed, ill-defined, waterish, and occupying a larger portion of the limb than I expected.” The “internal contact,” or the exit of Venus from the sun’s disc was no better, as Smith said, “The thread of light, coming round from both sides of the Sun’s limb, did not close instantaneously, but with an uncertainty of several seconds, the points of the threads darting into each other, and parting again, in a quivering manner, several times before they finally adhered.” Smith claimed to wait “for this adherence with all the attention in my power” but still differed “a few seconds” from Lukens, “who took the same method of judging.” The Norriton team, it turned out, was among the first to attribute this bleeding “undulatory motion” to the presence of an atmosphere on Venus, “varying the refraction of the rays” of the sun.

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Solving for Time

July 8, 1776, proved to be a warm, clear day. John Nixon had been appointed by the Sheriff of Philadelphia and the Pennsylvania Committee of Safety to read the Declaration of Independence to the public in the State House yard in Philadelphia. This was the first of dozens of similar ceremonies in every part of America. Beginning at eleven in the morning, the various committees in Philadelphia began to converge, along with thousands of residents from the city and surrounding towns, upon the State House grounds. Just before noon, Nixon, who had been reviewing and remarking his copy of the Declaration to pronounce it with clarity to the multitude assembled before him, ascended a set of wooden steps. From his perspective, he could see the crowd’s expanse filling the square like the stars would fill the Philadelphia sky that evening. He lifted the sheet of paper just below his chin, took a deep breath, and began. “When in the course of human events, it becomes necessary for one people…” For the first time, the gathered public heard their independence declared, and as Nixon continued, the hushed crowd listened with nervous expectation. No one in the State House yard that day knew how events would turn out, and no one could be sure that the Americans had made the right decision to declare independence at that particular moment in time. Some of the audience applauded politely, feigning support for a measure they thought premature. Others standing in the square that day, like Thomas Paine and John Adams, wondered in silence if the decision had come too late. Most of the auditors, however, were convinced that independence had arrived at just the right time. As John Nixon neared the end of the text, he raised his voice even more, “…in the name and by the authority of the good people of these colonies solemnly publish and declare, That these United Colonies are, and of right ought to be, FREE AND INDEPENDENT STATES.” When Nixon spoke the phrase introduced by Common Sense, the square erupted in cheers. Several members of the crowd that day looked up at the bright sunshine and then down at John Nixon standing atop the sturdy wooden platform that had been originally erected in 1769 as an observatory for the Transit of Venus. They were reminded of a single extraordinary moment when the universe seemed to converge on that very spot, when space and time opened like a door that had been sealed for centuries. A group of amateur astronomers had stood on the very same platform seven years earlier and measured the breadth of the solar system. And the key to it all was time.

As we shall see in the next chapter, the most significant point of dispute in the debate over independence was time. Very few Americans had any problem with the idea of independence, but when Common Sense transformed the perpetual future of the idea into the immediate present of the decision for independence, many of the colonists balked.  The decision for independence was, in Paine’s mind, a very scientific question that could be solved using mathematics. He isolated time as the crucial variable in the independence equation. A large number of Americans anticipated independence in about a half century. Paine zeroed in on “this single position” and argued that if it was “closely attended to” it would “unanswerably prove” his theorem. “The argument turns thus,” said Paine, “at the conclusion of the last war, we had experience, but wanted numbers; and forty or fifty years hence, we should have numbers, without experience.” Since “the proper point of time, must be some particular point between the two extremes, in which a sufficiency of the former remains, and a proper increase of the latter is obtained,” Paine concluded, “that point of time is the present time.”

Paine’s lifelong passion, as we have seen, was science, but he was also an avid student of mathematics. While Paine served as an aide-de-camp to General Nathanael Greene in late 1776, the general noted that even in the midst of war, “Common Sense,” as Paine was called by many Americans, was “perpetually wrangling about Mathematical Problems” with two of Greene’s senior officers. Paine had become infatuated with mathematics when he was studying for the entrance exam to qualify as an excise officer. Though he used only a limited set of mathematical skills in the excise, he demonstrated a sustained passion for complex mathematical problems that helped fuel his interest in natural philosophy. Direct mathematical arguments would not become a critical component of Paine’s political writing until his later economic essays, but during his time as an exciseman we can witness the emergence of his ability to grasp the complex causality of systems. Paine made a reasoned argument in The Case for the Officers of the Excise (1772) that underpaid and exploited excise officers became more prone to corruption, which ultimately managed to clog Britain’s revenue stream.

In late 1775 and early 1776, when Paine was writing Common Sense and his Appendix, he saw the mathematical logic of independence, but he also remembered his lecturers back in London who had translated Newtonian mechanics for an audience largely ignorant of mathematics. Men like James Ferguson and Benjamin Martin communicated and convinced, not by the means of mathematical proof, but by mechanical demonstration. In order to persuade his American audience to declare its independence immediately, Paine could not simply describe independence; he needed to help them experience independence. With this in mind, Paine strolled up Arch Street toward David Rittenhouse’s shop.

About Ben Ponder, Editor-at-Large

Ben Ponder, PhD, is Editor-at-Large at Media Rostra. Ben has received decorative pieces of paper conferring upon him an unnamed set of “rights and privileges accorded thereto” from the University of Arkansas, Regent College, and Northwestern University (where he was a Presidential Fellow). He studied (in alphabetical order) architecture, classics, communication, history, political science, rhetoric, and theology. He is the author of American Independence: From Common Sense to the Declaration (“Sizzling.” – TMZ) and the co-editor of Making the Case: Advocacy and Judgment in Public Argument (“Six-pack abs-olutely great!” – US Weekly). Ben is currently an executive in the educational software industry. He and his organic wife, Amy, live with their four free-range kids in a farmhouse Ben designed and built. His personal site on the Interweb is, and he can be reached on Twitter @ponderben.