A French astronomer traveled to Siberia to observe the 1761 transit of Venus, assisting in a worldwide effort to determine the scale of the solar system for the first time
From the book The Day the World Discovered the Sun, by Mark Anderson. Reprinted by arrangement with Da Capo Lifelong, a member of the Perseus Books Group. Copyright © 2012.
June 6, 1761
The cloud bank to the east glowed red. Jean-Baptiste Chappe d’Auteroche had been living at his mountaintop observatory, avoiding contact with the superstitious townsfolk and instead looking to the skies. His lot, he’d surmised, was not to make inroads with the locals but rather, as a fellow French philosophe put it, to make “a communication of flying bridges, as it were, that reunite one continent with another and pursue all the tracks of the Sun.”
Three days before, Chappe had pointed his nineteen-foot telescope at a solar eclipse, recording in his logbook the exact moment when the eclipse ended. His pendulum clock—which he’d previously set to noon when the sun reached its highest altitude in the sky—read 6:11 am and 4 seconds. He’d already calculated that the same solar eclipse would be visible in St. Petersburg as well. So when he later returned through Russia’s capital city en route to Paris, Chappe could then compare notes with observers there. Since the eclipse ended at the same instant, whether seen from Tobolsk or St. Petersburg, the difference in time between these geographically separated measurements was exactly the difference in longitude between the locales. Chappe derived that Tobolsk was 65.8490 degrees east of the National Observatory in Paris. (Today, Chappe’s longitude would be written as 68.1862 degrees east of the prime meridian, the British Royal Observatory in Greenwich, England. By either standard, Chappe’s error was an impressive 0.0719 degrees or 4.3 arc minutes—translating to 3 miles at Tobolsk’s latitude.)
The night before the transit, all looked calm. “The sky was clear,” Chappe recalled. “The sun sunk below the horizon free from all vapors. The mild glimmering of the twilight and the perfect stillness of the universe completed my satisfaction and added to the serenity of my mind.”
By morning, however, the 4:30 sunrise had brought a dark veil. Clouds loitered. As the increasingly cloudy and sleepless night progressed, Chappe paced the observatory floor. His assistants, whom Chappe had woken earlier in the night, left their master alone—knowing they’d only be needed if clear skies returned. “I found myself relieved by their absence,” Chappe wrote.
Soon after dawn, Chappe heard a commotion outside. Tobolsk’s governor, the local archbishop, and some nobles had assembled at the new observatory to take in the heavenly spectacle. The first light of day shone upon the French visitor whose anxiety grew with each troubled glance at the clouded-over sky.
“The idea of returning to France, after a fruitless voyage, of having exposed myself in vain to a variety of dangers,” he recalled, “[with every] expectation of success, which I was now deprived of by a cloud…threw me into such a situation as can only be felt.”
Chappe had instructed his assistants to set up a tent outside the observatory with the secondary telescope. The arrangement provided all they’d need to view the transit—but still permit Chappe to perform his own delicate observations with the privacy he demanded.
As the dawn’s blush gave way to early morning light, an easterly wind peeled back the top layers obscuring the sun. And with the increasing transparency, the mood both inside the observatory and in the nearby tent lightened. “The clouds began to exhibit a whitish color, which grew brighter at every instant,” Chappe wrote. “A pleasing satisfaction diffused itself through all my frame and inspired me with a new kind of life.”
To everyone’s pleasant surprise, the residents of Tobolsk—so vocal in their opposition to the Frenchman’s entourage weeks before—had shut themselves up in their houses and churches, some fearing God’s imminent wrath. Today, the armed guards assigned to protect Chappe proved an unnecessary precaution. Chappe instead enlisted their help in moving his nineteen-foot telescope out onto the lawn.
Doomsday had been postponed. Instead, the morning of June 6 brought a clear patch through which Chappe could view unobstructed the first hints of a tiny black sphere piercing the sun’s sacrosanct disk. The assembled crowd in the nearby tent now had something to see.
Henceforth no excuses remained. Heartbeats quickened as Chappe cued his interpreter inside the observatory to shout out every second of every minute on the pendulum clock. A stream of numbers cut the hush with metronomic quickness. “Cinqante-cinq minutes et un…deux…trois…quatre…”
The seven o’clock hour approached as Chappe adjusted his telescope. Like a smooth, circular pebble descending into a thick fluid, Venus began crossing the solar limb. Eighteen minutes after Venus first excited the assembled crowd with its initial appearance, the first crucial moment of its solar transit approached—when the sun had enveloped the entirety of Venus’s shadow. No words the interpreter had ever said meant more to Chappe than the sequence of numbers he shouted through the observatory door. “Vingt-quatre…vingt-cinq…vingt-six…”
At 7:00 am and 28 seconds, Chappe recorded in his logbook the moment of internal contact between Venus and the sun. “I…felt an inward persuasion of the accuracy of my process,” Chappe recorded. “Pleasures of the like nature may sometimes be experienced. But at this instant, I truly enjoyed that of my observation and was delighted with the hopes of its being still useful to posterity when I had quitted this life.”
Many other observers of the 1761 transit reported difficulty recognizing the instant of interior contact, elongating the measure of a moment into a guessing game extended over tens of seconds. This surprise phenomenon, ruining many otherwise useful transit observations, results from an optical illusion that makes the sun’s limb briefly pucker inward with a plasticity that appears to connect it with Venus’s distorted disk. Chappe, on the other hand, reports no problems with what was later dubbed the “black drop effect.”
The aromatic musk of Russian tea—hints of honey and Spanish pepper—spiced the air of this increasingly beneficent morning. Chappe had refused dinner the night before, and the twelve or more hours since his last meal would have left the explorer with little more than adrenaline to fuel him. The city’s nobles, gathered around the nearby tent, provided a counterpoint. Spirits like bilberry wine and quas—a commonplace Russian drink made of fermented meal and malt—likely blurred these momentous few hours into pleasantry while delicacies like caviar and roasted quail tempted hungry men in Chappe’s party to join in. (“All these [Siberian game] birds,” Chappe grumbled, “have a disagreeable fishy taste.”)
The sun continued rising, and the clouds continued to clear. Around 10:00 am, Venus had reached the halfway point in its trans-solar journey. At the exact median, Chappe tended to the second smaller telescope to make a different kind of measurement. As an independent check against the transit’s time records, he also measured the separation between the nearest edge of Venus and the sun’s enveloping arc. His ten-foot telescope, in less demand after drink and disinterest had peeled away some of the observatory’s guests, made its 3,000-mile journey to yield one crucial number of angular distance. Inside the smaller telescope’s eyepiece a translucent set of hash marks provided the ruler that yielded 6 arc minutes and 2 arc seconds of angular separation between planet and star at the transit’s halfway point. And to check his check, Chappe also measured out the entire diameter of the sun: just over one-half of a degree—31 arc minutes and 37 arc seconds.
Now measured as if for a new outfit, a star ascended. It gave day to the Earth, as it always does. But on this day its closest watchers had reached into the beyond for their first grasp at a universe of knowable depth.