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Venus of Longitude

The Day the World Discovered the Sun by Mark AndersonOn Tuesday, June 5, 2012, people across the world get to witness a rare, celestial shadow play. The planet Venus will, for about seven hours, cross the disk of the sun, a "transit" that happens less than twice per century. (The next Venus transit after Tuesday will be on December 11, 2117.)

And rare as it is, the Venus transit has an outsized impact on the world. The transit launched the legendary careers of Captain Cook and Mason and Dixon — surveyors who wouldn't have staked out the definitive line between American North and South without first proving themselves on a Venus transit voyage. The transit, with the help of a little clever triangulation, enabled science to measure out the distances to the sun and planets with uncanny precision. (So, the transit also lies at the heart of the world's first international, big science project — forefather to the likes of NASA's Apollo program and the Human Genome Project.)

The Venus transit also stands at the crossroads of perhaps the single greatest technological challenge in human history: practical navigation at sea.The Venus transit also stands at the crossroads of perhaps the single greatest technological challenge in human history: practical navigation at sea.

It on was this last point that I found a skewed account in recent popular histories — and ultimately tried to right the record a bit with my new book.

Dava Sobel's bestseller Longitude is a tremendous work of historical nonfiction. I greatly admire Sobel for rightly resuscitating the reputation of the brilliant clockmaker John Harrison — winner of the lion's share of Britain's famous Longitude Prize. But in rescuing Harrison, Sobel also caricatured Harrison's competition and their motivations and accomplishments — and in the process missed a high-stakes human drama at least as compelling as the Harrison story.

In the age of ubiquitous GPS smartphones and car-dashboard units, it's easy to forget how revolutionary navigational technology can be. Throughout the 18th century and before, untold scores of people died every year because ships' navigators couldn't do their job. Naval and civilian vessels wrecked with astonishing frequency because they couldn't find where they were.

For instance, the British warship HMS Ramillies slammed into the Devon coastline, stern first, on February 14, 1760, because her captain thought he was easing into a sheltering bay elsewhere in the English Channel. But instead Ramillies had wandered into the Channel's most deadly stretch of coast. The next morning, bloated bodies bobbed in the surf as the villagers onshore snatched stray shards of lumber and hardtack for their pantries.

Ramillies' wreck, killing 700 men, was just one of the age's countless sacrifices at the altar of longitude.

Finding longitude at sea was difficult for the same reason there's no "east pole." The Earth spins along its north and south axis, and so the stars above the north and south poles remain fixed. Get a fix on Polaris, the north star, and finding your latitude is just an arithmetic problem. However, the celestial grid toward the east and west is constantly shifting.Get a fix on Polaris, the north star, and finding your latitude is just an arithmetic problem. However, the celestial grid toward the east and west is constantly shifting. So, finding longitude, instead, means somehow keeping track of Greenwich Time at sea.

Compare Greenwich Time with your local time, and you know your longitude. But keeping Greenwich Time on a rocking and swaying ship (making pendulum clocks useless) was so difficult as to utterly baffle such great minds as Edmund Halley, Isaac Newton, and Galileo Galilei.

Making a good, spring-wound watch that could keep time at sea might seem, with 20/20 hindsight, to be the best solution. And it was — once the technology became affordable in the early 19th century. (The pocket-watch story is the story of Sobel's Longitude.)

But throughout the 18th century — the age of a nearly unstoppable Europe conquering the planet, the establishment of truly global trade routes, and the foundation of the British Empire — affordable and practical longitude involved instead a proto-GPS system.

England's Nautical Almanac used the moon as its global positioning satellite. The moon became a universal timekeeper whose position in the sky gives Greenwich Time. The Nautical Almanac came of age during the 1760s, the same decade that Venus transit mania swept the salons and scientific societies of Europe and the New World.

The almanac's developers, and developers of parallel almanacs across Europe, were the same astronomers who chased the Venus transits of 1761 and 1769. Since Venus transit observations required both far-flung expeditions across the Earth and cutting-edge navigational tools, it's no surprise that these same longitude pioneers used the Venus transit voyages of the 1760s as the ultimate prototyping and testing grounds for their proto-GPS system.

Throughout the Enlightenment, in other words, astronomy remained the world's way-finding science, cracking open the field of navigation and heralding a new age of worldwide expedition, trade, and conquest. The road to today's global society and economy passes right through the moon — and through the Venus transit itself.

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Mark Anderson is a science and technology journalist and author. His new book, The Day the World Discovered the Sun: An Extraordinary Story of Scientific Adventure and the Race to Track the Transit of Venus, has just been published by Da Capo. You can also find Anderson and more about his new book on Tumbler.

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