Peering into the skies might open the vastness of the universe and spark leaps of imagination, but astronomy, like writing, is a sedentary pursuit. It might even be considered a bit dull. A late 18th century job description for an assistant astronomer at Britain’s Royal Observatory suggested that candidates be “obedient drudges”: those with sloth-like dispositions, no doubt, were given special consideration.
But the 18th century stargazers whom Andrea Wulf chronicles in “Chasing Venus” proved themselves a different sort. Their exploits would put Indiana Jones to shame. They braved monsoons, frozen climes, tropical heat, hostile locals, war on the high seas, political upheaval, disease, and mental derangement, all in an attempt to observe one of the rarest of astronomical events: the transit of the planet of Venus between the sun and the earth. Transits occur only every 100 years or so, and always in pairs. (Mark your calendar and keep a telescope handy: Venus closes out its most recent cycle on June 6.)
Venus has tantalized and perplexed scientists since the very beginnings of modern astronomy. In 1716, the British astronomer Edmond Halley (of the eponymous comet) called on his colleagues across Europe to take up the challenge and track the planet. He would not live to see the transits that are the subject of Wulf’s book — in 1761 and 1769 — but he laid the groundwork for those who came after.
As Wulf explains in her appealing mix of science and travel, Venus held a key to unlocking the dimensions of the solar system. Scientists understood the distance between the planets and the sun relative to that between the Earth and the sun — how much nearer or farther was each. The problem? They didn’t know exactly how far the Earth was from the sun. They reckoned that if accurate data could be taken from the transit, they could derive, by way of trigonometry, that figure. Halley suggested that multiple observers fan out across the globe; their data would then be gathered and collated. Thus, writes Wulf, “could they achieve what had hitherto been almost unimaginable: a precise mathematical understanding of the dimensions of the solar system, the holy grail of astronomy.”
Wulf calls this Enlightenment-era project a “pivotal moment in a new era — one in which man tried to understand nature through the application of reason.” Like the figures on her pages, she possesses deep faith in the power of science, perhaps even unreasonably so. Wulf, the author of two fine books on botany in the Age of Reason, is interested in the humming knowledge networks that linked scientific societies of different nations, and how cooperation might triumph over conflict and national competition. Wulf writes with enthusiasm (even if she sometimes flirts with overstatement) and marvelous concision over the 200 pages of her text. Better yet, she explains complex scientific phenomena in clear, layperson’s terms: Here is a book both astrophysicists and poets can understand.
The obstacles confronting the platoon of observers were formidable. Britain and France were at war, but this did not deter fellow astronomers from linking up with each other. Indeed, a Frenchman, Joseph-Nicolas Delisle, took the lead. With contacts in Amsterdam, Basel, Florence, Vienna, and St. Petersburg, Delisle was a whirlwind planner and a hub of scientific back and forth. A skilled surveyor, his “mappemonde,” which highlighted the best spots around the globe to glimpse the transit, became an essential document for astronomers.
The theory of the transit was fine and good, but setting up the viewing stations proved a challenge. Getting to far-flung locations was dangerous work. For the 1761 transit, the British sent a man to St. Helena island, a tiny isolated speck in the south Atlantic. A colleague of Delisle’s, Jean-Baptiste Chappe d’Auteroche trekked 4,000 miles from Paris to the depths of Siberia, only to be attacked by villagers who thought his fancy scientific instruments had magical powers: They blamed him for bringing on devastating floods. Two British fellows named Mason and Dixon (surveyors of the famous line) were nearly smashed to bits by a French warship as they attempted to get to Sumatra. They nearly quit in fear and frustration.
But surely the most star-crossed (literally) of the Venus observers was the extravagantly named Frenchman Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière. His name notwithstanding, Le Gentil’s odyssey would be anything but nice. His was a story of tragic near misses. For the 1761 transit, Le Gentil was to journey to Pondicherry, then a French possession in India. War got in the way — the British laid siege to the town, and Le Gentil instead went to Mauritius, where he was waylaid by dysentery. On June 6, the day of transit, he was on a rolling ship, and he could not get an accurate fix on the planet. Eight years later, he made it back to Pondicherry for the 1769 transits, but weather marred the viewing. Poor Le Gentil had come so far “only to be the spectator of a fatal cloud.”
All together, several hundred scientists participated in 1761 and 1769 transits. Their efforts were not in vain. The various expeditions produced not only data about Venus but better maps of the world. The second transit saw the beginnings of James Cook’s heroic career as explorer and adventurer. The astronomers of the 1760s had no MIT super-computers or high-tech telescopes, but their calculations were remarkably accurate. There was some bickering about the results — the 1761 data was inconclusive — but a British estimate after the 1769 transit calculated the distance between the earth and the sun to be 93,726,900 miles, only 800,000 miles less than today’s figure. Venus, ever elusive, had yielded her secrets.