05/31/2012 09:23 am ET Updated Jul 31, 2012

The Roaringest Eclipse

On one summer day 243 years ago, the heavens became much larger than anyone had dared to imagine. And that same summer day had made them dare and imagine quite a bit.

The day was June 3, 1769, when some of the age's top explorers and scientists had scattered themselves across the Earth to witness a rare celestial event. Like other wild, breakthrough moments in human history -- think of the radio and the roaring 1920s or the fledgling Internet and the go-go 1990s -- great minds of the time dared to think big and venture beyond the bounds of the merely acceptable to bring their visionary ideas to life.

On June 3, 1769, the planet Venus spent some six hours crawling across the face of the sun, opening up a unique window that briefly allowed science to triangulate nearly every distance in the solar system. (There have only been four other Venus transits in the centuries since. The next transit is on June 5-6 of this year; the next one after that is in 2117.)

At stake for these scientist-explorers, including the latterly famous adventurers and surveyors Captain Cook and Mason & Dixon, was a matter of both the brain and the gut. The heady problem involved the architecture of the universe. How big is the cosmos? No one before had had the gumption or opportunity to peer over God's shoulder (as people in this very theistic time saw it) and measure out His greatest celestial achievement -- in the words of Genesis 1:1, "the heavens and the earth."

The matter of the gut involved needless shipwrecks and deaths at sea. Ships' navigators of the time, like their predecessors in the decades and centuries previous, couldn't do their jobs well enough. The lack of reliable navigation methods at sea made countless widows every year and helped keep European superpowers -- who would soon enough be discovering their world-conquering mojo -- mostly insular, non-global and sparring amongst themselves.

Yet it was during the same tumultuous decade of the 1760s that astronomers cracked the navigation puzzle wide open. Led by England's Astronomer Royal Nevil Maskelyne, a team of prodigies and whiz-kids invented a phenomenal 18th century GPS system, using the moon as the world's first global positioning satellite. These tech pioneers produced a reference book called The Nautical Almanac, first published in 1767, that cost 8 shillings ($50 today) and could suddenly translate the moon's position in the sky into longitude at sea.

It was a moment of world-transforming, peaceful revolution in an age that would soon come to know very bloody revolutions too.

Now what the system needed was a field test. This the 1769 Venus transit also provided. Both the scientific and military establishment in England summoned an obscure naval lieutenant named James Cook to helm a Venus transit expedition to the newly discovered South Pacific island of Tahiti. Cook's maiden voyage would also put the new Nautical Almanac -- cornerstone of the emerging proto-GPS system -- to its greatest test.

Cook's first voyage, aboard the HMS Endeavour, is only the most famous of these rip-roaring adventures across the Earth to and time the Venus transit from each far-flung locale. (The key to good Venus transit data was geographical diversity. So good arctic and good polar observations were essential to producing the best results.)

Observing the Venus transit on June 3, 1769, in other words, translated into braving a worldwide obstacle course of polar ice storms, brutal epidemics, uncharted seas, roughneck bandits, superstitious villagers, arctic wastelands and more than a little political intrigue.

Below is just a few tidbits from the overstuffed files of adventures, odysseys and extreme science centered around one day -- just four Venus transits ago.

"Confounded with the sky," en route to Mexico City, Mexico: Jean Chappe, the leader of a French-Spanish Venus transit expedition to the Baja peninsula, faced down natural disasters that would make many a Gore-Tex-coddled sportsman today pack up and go home. Crossing the Atlantic to Mexico, Chappe wrote in his journal a dare to the heavens to bring it on. "Nature has beauties in her horrors," he journaled, "Nay, it is there perhaps that she is most admirable and sublime. The calmness of a fine day is in some measure less interesting than the moments of distress."

Chappe would come to know many. His first distress came before he'd even landed at Veracruz. Riding a harbor pilot's boat to shore, Chappe saw the sky darken and foreboding rain pelt everything in sight. His expedition was now at the mercy of a Gulf of Mexico hurricane. Chappe had only offloaded half of his crew from the tiny Spanish brigantine that had brought them to the Mexican port city. But the stinging rain and bitter winds forced him to a storm shelter -- watching helplessly as some of the finest scientific instruments anywhere in New Spain tossed and jostled with every breaker and surge unleashed on the little nutshell of a boat.

Eight grueling days that followed on a mule train en route to Mexico City turned out to be the luxurious part of the journey. As the team prepared to forge west toward the Pacific, their guide advised them that bandits in western Mexico were notoriously cutthroat. It was best, the man said, to kill the bandito before he might offer you the same service.

"High mountains, dreadful precipices, dry deserts offered to us every day some new dangers," Chappe's assistant named "Pauly" later recalled:

"We came near dying a thousand times. We were besides crushed by an excessive heat which hardly left us strength enough to drag ourselves around ... Having reached to some valley or on the slope of some hills, we tried to take, on the bare ground, under the canopy of heaven, some rest so necessary on account of our cruel fatigue. We would be hardly half asleep, [and] we would be woke up all at once by some violent wind followed by a heavy rain, and some some sweeping torrents would cause us to fly with all we had, afraid at every step of falling into some of those precipices which surrounded us."

A "great expedition" above the Arctic Circle at the (crazy!) King of Denmark's invitation: The Hapsburg monarchy based in Vienna mounted their own Venus transit expedition to one of the northernmost towns in Europe, a remote outpost today called Vardø, Norway. The leader of this team was a Jesuit friar named, of all things, Hell.

Maximilian Hell was an astronomer and man of the cloth who had no idea what he was getting into. His invitation to provide an essential counterpoint to the tropical Venus transit expeditions came courtesy of the King of Denmark. King Christian VII, Hell learned only too late, also suffered from a degenerative dementia that turned his court into a den of thieving quacks and would-be shrinks -- vipers, each in their own way.

Hell and his assistant Joannes Sajnovics [pron. "SHINE-oh-vitch"] raced through Swedish and Norwegian mountain passes and dangerous snowscapes to reach the port city of Trondheim, where they might ship out through the Norwegian Sea and into the treacherous arctic. But they arrived in Trondheim on July 30, late enough in the season that the months-long passage to Vardø would face some of the most reliably horrendous ice storms, whiteouts and blizzards known to mariners.

Hell and Sajnovics were undeterred. Only one Venus transit remained in the entire century. The next one after that was 1874.

Nature, providing "beauties in her horrors" for Chappe, did not disappoint for Hell and Sajnovics either.

August alone proved more trying a month than the Austrian explorers could have anticipated. "We we had to sail on a famously dangerous eight mile portion of the sea, known for the many shipwrecks, situated far away from the shores," Sajnovics later recalled in a letter. "Our ship was lifted as high as on a mountain by the ever-growing waves and then it seemed to be falling back to the bottom of the sea, and then it was being thrown around to the left and right, threatening to fall apart among creaking sounds. The rainy fog was making the situation even more dangerous, because we could not see the mountains nor the rocks in the sea."

On June 5, 2012, Venus will pass in front of the sun in the same rare transit that sent men like Chappe, Hell and Cook racing to the ends of the earth and facing off against some of the gravest perils of a candlelit age. And for good cause: The transit provided inroads to solutions to fundamental problems of the 18th century.

A dot crossing a disk -- a circle within a circle -- reminds how sometimes small things can produced outsized rewards. All worth bearing in mind if you get the chance on June 5 to witness, as one Venus transit admirer two centuries ago put it, "the mighty dimensions of the sun shining in his strength."

More on observing this year's Venus transit can be found here. (And remember never to look directly at the sun. Safe observing tips here.) The above is adapted from The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus (Da Capo Press, June 2012)