Longitude

Walker & Company

CHAPTER ONE

Imaginary Lines

When I'm playful I use the meridians of longitude and parallels of latitude for a seine, and drag the Atlantic Ocean for whales.

--MARK TWAIN, Life on the Mississippi

 : :  Once on a Wednesday excursion when Iwas a little girl, my father bought me abeaded wire ball that I loved. At atouch, I could collapse the toy into a flatcoil between my palms, or pop it open to make a hollowsphere. Rounded out, it resembled a tiny Earth,because its hinged wires traced the same pattern ofintersecting circles that I had seen on the globe in myschoolroom--the thin black lines of latitude and longitude.The few colored beads slid along the wirepaths haphazardly, like ships on the high seas.

My father strode up Fifth Avenue to RockefellerCenter with me on his shoulders, and we stopped tostare at the statue of Atlas, carrying Heaven and Earthon his.

The bronze orb that Atlas held aloft, like the wiretoy in my hands, was a see-through world, defined byimaginary lines. The Equator. The Ecliptic. The Tropicof Cancer. The Tropic of Capricorn. The Arctic Circle.The prime meridian. Even then I could recognize,in the graph-paper grid imposed on the globe, a powerfulsymbol of all the real lands and waters on theplanet.

Today, the latitude and longitude lines governwith more authority than I could have imagined forty-oddyears ago, for they stay fixed as the world changesits configuration underneath them--with continentsadrift across a widening sea, and national boundariesrepeatedly redrawn by war or peace.

As a child, I learned the trick for remembering thedifference between latitude and longitude. The latitudelines, the parallels, really do stay parallel to eachother as they girdle the globe from the Equator to thepoles in a series of shrinking concentric rings. The meridiansof longitude go the other way: They loop fromthe North Pole to the South and back again in greatcircles of the same size, so they all converge at theends of the Earth.

Lines of latitude and longitude began crisscrossingour worldview in ancient times, at least three centuriesbefore the birth of Christ. By A.D. 150, the cartographerand astronomer Ptolemy had plotted them onthe twenty-seven maps of his first world atlas. Alsofor this landmark volume, Ptolemy listed all the placenames in an index, in alphabetical order, with the latitudeand longitude of each--as well as he could gaugethem from travelers' reports. Ptolemy himself hadonly an armchair appreciation of the wider world. Acommon misconception of his day held that anyoneliving below the Equator would melt into deformityfrom the horrible heat.

The Equator marked the zero-degree parallel oflatitude for Ptolemy. He did not choose it arbitrarilybut took it on higher authority from his predecessors,who had derived it from nature while observing themotions of the heavenly bodies. The sun, moon, andplanets pass almost directly overhead at the Equator.Likewise the Tropic of Cancer and the Tropic of Capricorn,two other famous parallels' assume their positionsat the sun's command. They mark the northernand southern boundaries of the sun's apparent motionover the course of the year.

Ptolemy was free, however, to lay his prime meridian,the zero-degree longitude line, wherever heliked. He chose to run it through the Fortunate Islands(now called the Canary & Madeira Islands) off thenorthwest coast of Africa. Later mapmakers movedthe prime meridian to the Azores and to the CapeVerde Islands, as well as to Rome, Copenhagen, Jerusalem,St. Petersburg, Pisa, Paris, and Philadelphia,among other places, before it settled down at last inLondon. As the world turns, any line drawn from poleto pole may serve as well as any other for a startingline of reference. The placement of the prime meridianis a purely political decision.

Here lies the real, hard-core difference betweenlatitude and longitude--beyond the superficial differencein line direction that any child can see: The zero-degreeparallel of latitude is fixed by the laws of nature,while the zero-degree meridian of longitudeshifts like the sands of time. This difference makesfinding latitude child's play, and turns the determinationof longitude, especially at sea, into an adult dilemma-onethat stumped the wisest minds of theworld for the better part of human history.

Any sailor worth his salt can gauge his latitude wellenough by the length of the day, or by the height ofthe sun or known guide stars above the horizon. ChristopherColumbus followed a straight path across theAtlantic when he "sailed the parallel" on his 1492journey, and the technique would doubtless have carriedhim to the Indies had not the Americas intervened.

The measurement of longitude meridians, in comparison,is tempered by time. To learn one's longitudeat sea, one needs to know what time it is aboard shipand also the time at the home port or another placeof known longitude--at that very same moment. Thetwo clock times enable the navigator to convert thehour difference into a geographical separation. Sincethe Earth takes twenty-four hours to complete onefull revolution of three hundred sixty degrees, onehour marks one twenty-fourth of a spin, or fifteen degrees.And so each hour's time difference between theship and the starting point marks a progress of fifteendegrees of longitude to the east or west. Every day atsea, when the navigator resets his ship's clock to localnoon when the sun reaches its highest point in the sky,and then consults the home-port clock, every hour'sdiscrepancy between them translates into another fifteendegrees of longitude.

Those same fifteen degrees of longitude also correspondto a distance traveled. At the Equator, wherethe girth of the Earth is greatest, fifteen degreesstretch fully one thousand miles. North or south ofthat line, however, the mileage value of each degreedecreases. One degree of longitude equals four minutesof time the world over, but in terms of distance,one degree shrinks from sixty-eight miles at the Equatorto virtually nothing at the poles.

Precise knowledge of the hour in two differentplaces at once--a longitude prerequisite so easily accessibletoday from any pair of cheap wristwatches--wasutterly unattainable up to and including the eraof pendulum clocks. On the deck of a rolling ship,such clocks would slow down, or speed up, or stoprunning altogether. Normal changes in temperatureencountered en route from a cold country of origin toa tropical trade zone thinned or thickened a clock'slubricating oil and made its metal parts expand or contractwith equally disastrous results. A rise or fall inbarometric pressure, or the subtle variations in theEarth's gravity from one latitude to another, couldalso cause a clock to gain or lose time.

For lack of a practical method of determining longitude,every great captain in the Age of Explorationbecame lost at sea despite the best available chartsand compasses. From Vasco da Gama to Vasco Nunezde Balboa, from Ferdinand Magellan to Sir FrancisDrake--they all got where they were going willy-nilly,by forces attributed to good luck or the grace of God.

As more and more sailing vessels set out to conqueror explore new territories, to wage war, or toferry gold and commodities between foreign lands,the wealth of nations floated upon the oceans. Andstill no ship owned a reliable means for establishingher whereabouts. In consequence, untold numbers ofsailors died when their destinations suddenly loomedout of the sea and took them by surprise. In a singlesuch accident, on October 22, 1707, at the Scilly Islesnear the southwestern tip of England, four homeboundBritish warships ran aground and nearly twothousand men lost their lives.

The active quest for a solution to the problem oflongitude persisted over four centuries and across thewhole continent of Europe. Most crowned heads ofstate eventually played a part in the longitude story,notably King George III of England and King LouisXIV of France. Seafaring men such as Captain WilliamBligh of the Bounty and the great circumnavigatorCaptain James Cook, who made three long voyagesof exploration and experimentation before his violentdeath in Hawaii, took the more promising methods tosea to test their accuracy and practicability.

Renowned astronomers approached the longitudechallenge by appealing to the clockwork universe:Galileo Galilei, Jean Dominique Cassini, ChristiaanHuygens, Sir Isaac Newton, and Edmond Halley,of comet fame, all entreated the moon and stars forhelp. Palatial observatories were founded at Paris,London, and Berlin for the express purpose of determininglongitude by the heavens. Meanwhile, lesserminds devised schemes that depended on the yelpsof wounded dogs, or the cannon blasts of signalships strategically anchored--somehow--on the openocean.

In the course of their struggle to find longitude,scientists struck upon other discoveries that changedtheir view of the universe. These include the first accuratedeterminations of the weight of the Earth, thedistance to the stars, and the speed of light.

As time passed and no method proved successful,the search for a solution to the longitude problem assumedlegendary proportions, on a par with discoveringthe Fountain of Youth, the secret of perpetualmotion, or the formula for transforming lead intogold. The governments of the great maritime nations--includingSpain, the Netherlands, and certaincity-states of Italy--periodically roiled the fervor byoffering jackpot purses for a workable method. TheBritish Parliament, in its famed Longitude Act of1714, set the highest bounty of all, naming a prizeequal to a king's ransom (several million dollars in today'scurrency) for a "Practicable and Useful" meansof determining longitude.

English clockmaker John Harrison, a mechanicalgenius who pioneered the science of portable precisiontimekeeping, devoted his life to this quest. Heaccomplished what Newton had feared was impossible:He invented a clock that would carry the truetime from the home port, like an eternal flame, to anyremote corner of the world.

Harrison, a man of simple birth and high intelligence,crossed swords with the leading lights of hisday. He made a special enemy of the Reverend NevilMaskelyne, the fifth astronomer royal, who contestedhis claim to the coveted prize money, and whose tacticsat certain junctures can only be described as foulplay.

With no formal education or apprenticeship to anywatchmaker, Harrison nevertheless constructed a seriesof virtually friction-free clocks that required nolubrication and no cleaning, that were made from materialsimpervious to rust, and that kept their movingparts perfectly balanced in relation to one another, regardlessof how the world pitched or tossed aboutthem. He did away with the pendulum, and he combineddifferent metals inside his works in such a waythat when one component expanded or contractedwith changes in temperature, the other counteractedthe change and kept the clock's rate constant.

His every success, however, was parried by membersof the scientific elite, who distrusted Harrison'smagic box. The commissioners charged with awardingthe longitude prize--Nevil Maskelyne among them--changedthe contest rules whenever they saw fit, so asto favor the chances of astronomers over the likes ofHarrison and his fellow "mechanics." But the utilityand accuracy of Harrison's approach triumphed in theend. His followers shepherded Harrison's intricate,exquisite invention through the design modificationsthat enabled it to be mass produced and enjoy wideuse.

An aged, exhausted Harrison, taken under thewing of King George III, ultimately claimed his rightfulmonetary reward in 1773-after forty strugglingyears of political intrigue, international warfare, academicbackbiting, scientific revolution, and economicupheaval.

All these threads, and more, entwine in the linesof longitude. To unravel them now--to retrace theirstory in an age when a network of geostationary satellitescan nail down a ship's position within a few feetin just a moment or two--is to see the globe anew.

Continues...
Excerpted from Longitudeby Dava Sobel Copyright © 2007 by Dava Sobel. Excerpted by permission.
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