A Once and Future Transit

By Carol Van Strum

On Tuesday, June 5, will occur one of the rarest of astronomical events: a transit of the planet Venus across the face of the Sun. For people living today, June 5 is the last Venus transit in our lifetime; the next will not happen until December 11, 2117 and December 8, 2125. Weather permitting, people in North America should be able to see at least part of Venus’s passage across the solar disk on June 5. This is an event not to miss!

Two excellent, complementary books together provide a roadmap to the history and significance of this rare occasion:

Transit of Venus: 1631 to the Present, by Nick Lomb, 2011. Powerhouse Museum, paperback, $24.95

The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus, by Mark Anderson, 2012. Da Capo Press, hardcover, $26.00

A transit occurs when a planet passes between Earth and the Sun across the Earth’s orbital plane (ecliptic). Because Venus and Mercury are the only planets with orbits closer to the Sun than Earth’s, they are the only planets that ever pass between us and the Sun. Mercury’s orbit being smaller and closer to the Sun, the innermost planet transits more frequently than Venus, but is harder to see. Venus transits are much rarer and usually occur in pairs eight years apart, separated by roughly 122 years. The June 5 transit is the second of this century’s pair (the first occurred on June 8, 2004).

Venus transits occur in eight-year pairs because for every eight orbits of Earth, Venus takes thirteen circuits around the Sun before crossing Earth’s orbit at the right moment again. (Just to confuse things, since Venus actually reaches the eight-year rendezvous 2.45 days earlier than Earth, occasionally Venus will arrive too early for conjunction and make only one transit in a century.) Calculating planetary orbits and the transit times of Venus was one of the great achievements of 17th Century science.

Historically, until now people have witnessed a total of only seven transits of Venus — in 1631, 1639,1761,1769, 1874, 1882, and 2004 — after Galileo’s invention of the telescope in 1609 made seeing a transit possible. Each of those transits ushered in a staggering burst of mathematical and scientific discoveries, the earliest being developed well before the 1631 transit by Johannes Kepler (1571-1630), Imperial Mathematician and Astrologer to Emperor Rudolf II of Bohemia. Kepler’s laws of planetary motion, developed during 20 years of calculations for a set of tables on the positions and motions of stars and planets, became the cornerstone of astronomy for centuries, in particular his third law, which holds that the square of the time a planet takes to go around the Sun is proportional to the cube of its distance from the Sun.

“Kepler’s third law meant that, once the actual distance of one planet was known, the distances of the other planets could also be deduced,” writes Nick Lomb. The transits of Venus of the 18th and 19th centuries could provide the one key distance — from Earth to Sun — needed to determine the distances of all the planets. In the course of his calculations, Kepler also discovered that both Mercury and Venus would transit the Sun in 1631 and wrote an “Admonition” to astronomers about when the transits would occur and how to observe them. Venus would transit, he predicted, on December 6, 1631. Sadly, Kepler died a year before that date, and actual observation and reporting of a Venus transit did not occur until the 1639 transit, triggering two centuries of attempts to record, measure and time the transits accurately.

Nick Lomb, curator of the Sydney Observatory and leader of its 2004 transit of Venus team, chronicles each of the Venus transits from 1631 to the present with historical anecdotes, clear diagrams of planetary motion, and a wealth of illustrations, from Kepler’s 1608 horoscope cast for General Albrecht von Wallenstein to spectacular photographs of the 2004 transit of Venus. He provides charts for viewing the June 5 transit from various points on the globe, and directions for observing the Sun’s surface safely. His book is a useful and very readable handbook for viewing and understanding transits as well as a fascinating tour of scientific history.

As a hair-raising companion volume, The Day the World Discovered the Sun chronicles the first international scientific effort to span the globe: the race to observe and measure the second-ever known transits of Venus, in 1761 and 1769. After attempts to measure the 1761 transit failed, in the midst of multiple wars for world domination, governments and scientists collaborated in feverish, harrowing expeditions to remote locations where the 1769 transit could be viewed in entirety. By sail through uncharted Pacific waters, by sled into Arctic territory, braving typhus epidemics, bandits, and extremes of heat, cold and storms, teams from England, Austria, France, Russia, Sweden, Denmark and Spain raced to the points where the transit could be viewed.

The goal of these teams was to measure with precision the timing of the transit from different locations, in order to determine through triangulation the distance of the Earth from the Sun, the elusive number that had so long prevented any calculations of distances from other planets and the size of the solar system, as well as accurate measurement of the Earth’s diameter and circumference. It would take years for the teams to return to Europe, and more years for their measurements to be collected and analyzed, but a secondary agenda had more immediate results: the perfection of navigation tables to determine longitude, enabling world trade to flourish long before accurate clocks became affordable.

Mark Anderson tells in extraordinary detail, often in the words of the men themselves, the adventures of three of those teams: Captain Cook’s legendary voyage to Tahiti, a French/Spanish expedition to Baja, Mexico (then New Spain), and an Hungarian astronomer priest named Father Hell sent by a demented king of Denmark to the far Arctic town of Vardo, in Norway. Each team’s tale is a chronicle of nonstop disasters, shipwrecks, storms and setbacks, to say nothing of perils and terrors largely forgotten in our day of instant communications and rapid air travel. The remarkable thing is that men embarked on such harrowing travels eagerly, simply to watch a little ball move across the sun.

The unintended consequences of these adventures, as both authors point out, turned out to be as important as their original goal: the cure and preventative for scurvy, which had taken the lives of so many mariners; the development of the first ‘instant’ information transmitter, a visual telegraph system across Europe; the discovery of an atmosphere around Venus; the establishment of the Mason-Dixon line in the U.S. by two astronomers after their unsuccessful mission to measure the 1761 transit of Venus. Similarly, observations of the 2004 and upcoming June 5, 2012 transits may provide the key to discovering other Earth-like planets in our galaxy and their potential for supporting life.

Observing and trying to comprehend the heavens are perhaps the oldest and most characteristic aspects of being human. Together, Anderson’s and Lomb’s books are a grand introduction to one of the rarest events the heavens offer.

Both books also evoke a haunting, perhaps unintended notion: that this last Venus transit of our lifetime is a poignant transit for humanity, too: a meeting place of heroic past and unknown, scary future.

Note: Transits of Venus are only possible during early December and June when Venus’s orbital nodes pass across the Sun. Transits of Venus show a clear pattern of recurrence at intervals of 8, 121.5, 8 and 105.5 years. The table at this NASA link lists all transits of Venus during the 800 year period from 1601 through 2400. For a more complete and detaled list, see Six Millennium Catalog of Venus Transits: 2000 BCE to 4000 CE.