The Day the World Discovered the Sun

by Mark Anderson

  The 1760s Venus transit voyages were also celebrated and immortalized in their day—recognized for a time as the scientific culmination of one of the Enlightenment’s greatest decades. Charles Green’s brother-in-law William Wales returned from his Hudson’s Bay 1769 Venus transit voyage to find a commission as Captain Cook’s new astronomer on the famous mariner’s second voyage. In 1775, upon completing Cook’s Pacific adventures, Wales then accepted an appointment as Master of Navigation Mathematics at Christ’s Hospital School in London. Wales was a popular instructor who shared tremendous stories—and posed equally tremendous mathematical problems—concerning some of the most legendary odysseys of the day. One of Wales’s star pupils was a sensitive boy who spent countless hours marveling over the “wonder and mystery of the universe”—and Wales’s wonderful tales of Pacific and polar adventures.

  The pupil was Samuel Taylor Coleridge, whose Rime of the Ancient Mariner distilled all he had learned from his Venus transit voyaging teacher into skeins of gossamer verse.18

  Back in America, a theologian and Methodist minister set out to make a definitive edition of the Bible that would bring his age’s greatest achievements in all fields of learning to the greatest story ever told. Adam Clarke, in his 1811 edition of the Bible, wrote a six-page footnote to Genesis 1:1, “In the beginning, God created the heavens and the Earth.”

  As Clarke noted, “The word heavens must therefore comprehend the whole solar system as it is very likely the whole of this was created in these six days.” The minister then unspooled in his copious biblical footnote one and a half pages of astronomical tables (!) giving distances to sun and planets, as well as pertinent facts about the known satellites of all the planets. “The columns containing the mean distance of the planets from the sun,” Clarke continued, “are such as result from the best observations of the two last transits of Venus, which gave the solar parallax to be equal to 8 [and] three-fifths of a degree.”19

  In 1830, American educator Hervey Wilbur wrote a popular book on astronomy. Even generations after the fact, the worldwide efforts to measure the 1761 and 1769 transits still commanded a tone of reverence. “The vast importance of correct observations of a transit of Venus is thus clearly seen,” Wilbur wrote, “as it enables man to throw his measuring line through millions of miles in space and gauge the mighty dimensions of the sun shining in his strength.”20

  On June 5–6, 2012, the world will witness the final Venus transit of the twenty-first century. (Venus transits almost always come in pairs, separated by eight years. The century’s other transit came on June 8, 2004.)

  In North and Central America, Venus will begin to cross into the solar disk when the sun is low in the afternoon or early evening sky. The sun will have set already when the transit ends, some six-and-a-half hours later. Europe, eastern Africa, the Middle East, western Australia, and South Asia, will see the latter parts of the 2012 Venus transit as the sun rises on the morning of June 6. Japan, Indonesia, eastern China, Russia, and eastern Australia as well as Alaska, Hawaii, northwestern Canada, and islands in the Pacific Ocean will see the transit in its entirety.

  These days, of course, the distance to the sun is well known—approx. 92,956,000 miles—and the value is more easily tracked and fine-tuned by radar measurements and other direct methods than by rare Venus transits. This is not to say, however, that Venus transits have ceased to be useful to astronomy.

  The 2012 Venus transit may in fact help guide one field of cutting-edge science. One of the hottest research areas in astronomy is the discovery and study of “exoplanets,” planets orbiting stars other than our own. At the time of this writing, 725 exoplanets have been discovered. New ones are being added to the tally practically every week. (The website http://exoplanet.eu keeps the latest tabs and statistics.)

  A key motivating question in exoplanet research involves estimating the number of Earth-like planets in the Milky Way capable of harboring life as we know it. (We can only study exoplanets in our galaxy and nearby satellite star clusters. Other galaxies are too far away for present-generation telescopes to be able to perform such detailed observations of individual stars.) The first step toward an answer will be finding a bona fide “sister” Earth somewhere else in the galaxy. But once a planet closely resembling ours is found, astronomers and astrobiologists will want to know what the atmosphere around the sister Earth is like.

  This is where present-day Venus transits come in handy. When Venus passes in front of the sun, the sun’s light also passes through the tiny ring of Venus’s atmosphere at the planet’s outer edge. Astronomers routinely examine the spectrum of colors of light coming through their telescopes for hints of the distant object’s composition and other characteristics. For instance, the atom hydrogen naturally gives off a red photon of light (with a wavelength of 656.3 nanometers) when its electron descends from its second excited orbital state to first excited orbital state. Tracking this “H-alpha” spectral line throughout stars and galaxies in every corner of the universe has long been a centerpiece of visual astronomy, crucially assisting the study of an object’s motion, temperature, composition, age, and other properties.

  Every atom and molecule has its own signature pattern of colors and spectral lines that it preferentially absorbs when light passes through it. So when Venus crosses the sun’s face during a transit, the sun’s light during that brief window contains tiny spectral signatures—absorption lines—emanating from Venus’s atmosphere. The Venus transit, in other words, provides a rare but precious test case of an Earth-like planet with an atmosphere of known composition (thanks to Venus probes such as NASA’s Venus Express and the former Soviet Union’s Vega-1) as it passes in front of a well-studied star, our own.

  In 2004, a team of nine French, Swiss, American, Spanish, and German astronomers observed the Venus transit, discovering not only the strong signature of carbon dioxide in the Venusian atmosphere but even the characteristic wind speeds at various altitudes above the planet’s surface.21 Not unlike the 1761 Venus transit, 2004 provided a proof of principle for the larger ideas that the twin transit eight years later might be able to test more fully.

  In 2009, NASA launched the Kepler spacecraft, a dedicated exoplanet-finding, space-based telescope.22 Kepler’s raison d’être is to perform a kind of exoplanetary stakeout, continually keeping its telescope trained on a field of 145,000 stars in the constellations Cygnus, Lyra, and Draco. It primarily looks for exoplanets transiting their host stars. Astronomers then perform follow-up studies of the candidate exoplanetary systems that Kepler finds. Using observations such as the 2004 Venus transit data as reference points, they can then examine exoplanets’ atmospheres—wherever such atmospheres might be found—in finer detail. Hints of life on the exoplanet could even turn up. The presence of O2, molecular oxygen, in the Earth’s atmosphere certainly gives indications of lifeforms on this planet.

  In December 2011, nine American and French astronomers wrote a letter to Astronomy & Astrophysics, a prominent journal in the field, urging their colleagues to perform careful measurements of the century’s other Venus transit. “The June 2012 transit will be a unique occasion,” these scientists note, to study “a planet that was long believed to be the Earth’s twin sister . . . [and] to discriminate between Earth-like and Venus-like atmospheres of exoplanets transiting their stars.” 23

  As the two eighteenth-century Venus transits enabled science to grasp its place in the universe, so the pair of twenty-first century transits help turn up clues to some of the most fundamental questions in science today: Are we alone? If not, what is life like on other worlds scattered throughout the celestial beyond?

  Important note for anyone attempting to view the 2012 Venus transit: Do not view the sun without a proper solar filter on your eyeglasses, telescope, or binoculars. Viewing the sun directly without such protection can cause permanent eye damagae, even blindness.

  ACKNOWLEDGMENTS

  This book represents a homecoming of sorts. I have devoted
more than a decade to researching and writing about other subjects, not the least of which was Shakespeare and Elizabethan history. It’s taken a while to circle back to my original field of study, physics and astronomy. So my thanks first go to the good professors—Joel Weisberg, Bill Titus, Bruce Thomas, Cindy Blaha, William Gerace, Art Swift, Mike Skrutskie, among many—who opened up a whole universe to this undergraduate and later graduate student. Their lessons continue to inspire.

  On the other hand, science is only one small part of the 1760s Venus transit story. To historians Simon Schaffer, Per Pippin Aspaas, William Sheehan, John Westfall, and Stephen Wepster as well as Cliff Thornton and Wendy Wales of the Captain Cook Society, I owe a debt of gratitude for their kind assistance and feedback during the preparation of this manuscript. A central piece of the history—the foundation of chapters 7, 9, and 12—lay buried in a manuscript for which I only had the Hungarian translation. Translating the entire document affordably and promptly into English was an odyssey all its own. (This book, not unlike the voyagers whose story it tells, followed an unforgiving timetable laid down by a forthcoming Venus transit—in this case the June 2012 transit, by which time we needed to have our book on bookstore shelves.) I am grateful for the assistance that Zsuzsa Racz, Amanda Solymosi, Roy Wright Tekastiaks, Merilee Karr, Dave Goldman, Paul Olchvary, and Stephen desJardins each provided in the search. And Ilona Dénes, the translator whom I did hire, consistently provided superb work that was prompt, careful, and professional. My special thanks to her.

  This book also drew heavily on the resources generously provided by a number of research libraries and librarians. Thanks to Adam Perkins at the Cambridge University Library and to the many helpful staff at the Smith College libraries (Neilsen, Hillyer, and Young Libraries), Forbes Library (Northampton, Mass.), the Amherst College Frost Library, the University of Massachusetts Du Bois Library, Harvard University’s Widener and Houghton Libraries and Yale’s Sterling Library. In the earliest stages of publicizing and promoting this book, at the time of this writing, I have also benefited from the kind assistance of Meg Thatcher, Aliza Ansell, Gerit Quealy, Max Germer, Jennifer Margulis, Kris Bordessa, Lauren Ware, Brett Paesel, and Mona Gable.

  This book simply would not have existed were it not for the patient, generous, and resourceful guidance of my literary agent, Jennifer Weltz, at the Jean V. Naggar Literary Agency. Her good grace and genial humor have been a wellspring of motivation and inspiration throughout this project. I am all the more grateful that her colleagues at JVNLA—Jessica Regel, Tara Hart, Laura Biagi, Elizabeth Evans, and Alice Tasman—have contributed their tremendous talents to this project as well. My editor, Robert Pigeon, has been a steadfast and true supporter of this project from his first exposure to the idea to its final production and beyond. I extend my thanks as well to the dedicated editorial, production, promotion, and marketing teams, at Da Capo and the Perseus Books Group—including Kevin Hannover, Timm Bryson, Sean Maher, Karstin Painter, and Lara Simpson Hrabota.

  This book greatly benefited from the input of readers who graciously gave of their time to critique and provide crucial feedback on early drafts of the manuscript. For their helpful critiques and insights, I remain indebted to Burl Gilyard, Joe Eskola, Megan Eskola, Sabrina Feldman, Wendy Wales, Cliff Thornton, Danielle Dart, and Kirsten Jamsen.

  My friends and family—especially my father and two children—have endured more deadlines and provided more spark and light in my life than I could have hoped to ask for. To them I am forever grateful. And, finally, I owe every good word to my wife, Penny, whose love and support throughout this often supremely challenging project has never flagged. Thank you.

  TECHNICAL APPENDIX

  This book requires no specialized mathematical or astronomical training to explore the human drama of the 1761 and 1769 Venus transit voyages. However, as a supplement to the discussion, the present appendix answers readers’ curiosity about the specific methods the astronomers mentioned in this book used to calculate the solar distance. In so many words: How, specifically, can one use the Venus transit to find the distance to the sun? And why wasn’t there an easier way?

  The answer to the second question sets the stage for the first.

  Every astronomical object in the sky appears as if projected onto a flat screen. Nothing immediately apparent about any star or planet might give the casual observer hints about its distance.

  The primary reason ancient astronomers distinguished between stars and planets was that stars remained fixed in their positions with respect to one another, as observed night after night. Planets (from the Greek for “wanderer”), however, moved across the familiar stellar tapestry over the course of weeks and months.

  From the time of the ancient Mayan, Chinese, Greek, and other early civilizations, astronomers, typically also serving as astrologers, devised elaborate theories to explain planets’ wanderings—and what those wanderings might portend for kings or great events of the day.

  After the Polish astronomer Nicolaus Copernicus (1473–1543) committed the ultimate heresy of replacing the earth with the sun as the center of the solar system, the German mathematician Johannes Kepler (1571–1630) made quantitative sense of Copernicus’s framework. From a lifetime of studying detailed planetary observations by Danish astronomer Tycho Brahe (1546–1601) and others, Kepler ultimately derived three basic laws of planetary motion, which remain in use to this day.

  Kepler’s third law states that the square of the time a planet takes to complete one orbit of the sun is proportional to the cube of that planet’s distance from the sun. In a simple equation:

  P2 = a3 (1)

  Where P is the planet’s orbital period (the length of time, measured in earth years, the planet takes to complete one orbit) and a is the planet’s average distance from the sun, measured in fractions of the earth-sun distance (astronomical units or AU). From well before Kepler’s day, detailed charts of Venus’s motions established that the planet completes one orbital period, one Venusian “year,” every 0.615 Earth years.

  Multiply 0.615 by itself and take the cube-root of the result to find Venus’s distance from the sun as 0.72 AU.1 But what, in practical distance units such as miles, is an AU?

  Here is where astronomy remained stuck for more than a century.

  Clever attempts to leverage precision measurements of Mars’s and Mercury’s orbits brought some astronomers in the late seventeenth and early eighteenth centuries close to answering the question.2 But planets move slowly across the sky, and tracking their motions with respect to background stars was necessarily imprecise. Discovering the sun’s distance required greater precision.

  It’s useful now to introduce an important term: the “solar parallax,” not the solar distance, is actually the quantity astronomers sought. Solar parallax is an angular measurement, representing one-half of the angular size of the earth as seen from the sun. To use the analogy of a circle, the solar parallax is like the angular “radius” of the earth, as subtended from a distance of 1 AU. Fortunately, converting between solar parallax and solar distance is relatively easy. The distance to the sun is just the radius of the earth divided by the solar parallax. In real numbers, 92,956,000 miles = 3,963.2 miles ÷ 8.79414 arc seconds. (To do this on a calculator, an extra factor of 206,265 is needed to convert arc seconds to “radians,” the natural unit of angular measurement.)

  In 1716, soon to be Astronomer Royal Edmund Halley published an astronomical call to arms, revealing that for a brief window in June 1761 and again in June 1769, the planet Venus would be moving across a kind of interplanetary yardstick—the face of the sun.

  Astronomers at different locations across earth could then time the duration of the Venus transit and compare answers (along with exact measurements of the observers’ latitude) to triangulate the sun’s distance.

  Halley’s method did not call for the observers to know their longitudes. And given how difficult longitude was to determine at the time, whether at sea or on land, Hall
ey’s method seemed to be a quick and easy route to the solar parallax, such a crucial number in science.

  However, one of Halley’s protégés, the French astronomer Joseph-Nicolas Delisle, examined the English method in closer detail and found it wanting. In 1761, for instance, the difference between the shortest and longest transit times would be just 13 minutes, making crucial the accuracy of each Venus transit duration measurement down to the second. It would also require the weather’s cooperation for five or more hours. And the location of the longest transit time would be in the Indian ocean, notorious for its changeable weather conditions.

  Delisle thus developed a supplementary method of discovering solar parallax from the Venus transit. Delisle’s technique required an observer to mark the exact local time for just one of the four points of contact between Venus and the sun. (Those four points are the planet’s outer and inner limbs touching the sun’s edge on entry and exit—external and internal points of ingress and egress, respectively.) Delisle’s method did only require observation of a single moment in the transit, thus making stations of multiple observers more likely to have at least one overlapping data point even in very uncooperative weather. Crucially, however, Delisle’s method also required knowing both latitude and longitude of the observing station.

  The story in the present book describes the compromise Halley-Delisle method that astronomers used in the 1761 and 1769 Venus transit voyages: measure latitude, longitude and as much of the Venus transit as possible. Astronomers after the fact would then use both Halley’s and Delisle’s methods to discover solar parallax, ideally enabling them to cross-check their results as well.3

  For the purposes of the present appendix, we’ll also consider two approaches to deriving solar parallax or distance. Neither is strictly the Halley or Delisle approach. Rather the first is much simpler—although far less accurate—than the other.