Taking the Long Way Home : It took centuries to devise a method for calculating longitude. Even the most famous sailors routinely wandered the seas aimlessly.

As children, we learn the difference between latitude and longitude: The latitude lines, the parallels, really do stay parallel to each other as they girdle the globe from the equator to the poles in a series of shrinking concentric rings.

The meridians of longitude go the other way: They loop from the North Pole to the South Pole and back again in great circles of the same size, so they all converge at the ends of the Earth.

What most of us did not learn was that for centuries, sailors were unable to measure their longitude at sea. The dilemma, which cost thousands of lost lives, baffled some of science’s greatest minds--from Galileo to Sir Isaac Newton--until an unknown clockmaker named John Harrison finally came up with a solution that changed the world of navigation, and consequently, the world.

Lines of latitude and longitude began crisscrossing our worldview in ancient times, at least three centuries before the birth of Christ. By AD 150, the cartographer and astronomer Ptolemy had plotted them on the 27 maps of his first world atlas.

The equator marked the zero-degree parallel of latitude for Ptolemy. He did not choose it arbitrarily, but took it on higher authority from his predecessors, who had derived it from nature while observing the motions of the heavenly bodies. The sun, moon and planets pass almost directly overhead at the equator.

Ptolemy was free, however, to lay his prime meridian, the zero-degree longitude line, wherever he liked. He chose to run it through the Fortunate Islands off the northwest coast of Africa.

Later map makers moved the prime meridian to the Azores and to the Cape Verde Islands, as well as to Rome, Copenhagen, Jerusalem, St. Petersburg, Pisa, Paris, and Philadelphia, among other places, before it settled at last in London in the late 1700s.

Historically, it was this shifting zero-degree meridian that defined the hard-core difference between longitude and latitude. While finding latitude was child’s play, determining longitude, especially at sea, was an adult dilemma that stumped the wisest minds of the world for the better part of human history.

Any sailor worth his salt can gauge his latitude well enough by the length of the day, or by the height of the sun or known guide starts above the horizon. Christopher Columbus followed a straight path across the Atlantic when he “sailed the parallel” on his 1492 journey, and the technique would doubtless have carried him to the Indies had not the Americas intervened.

In comparison, to learn one’s longitude at sea, one needs to know what time it is aboard ship and also the time at the home port or another place of known longitude--at that very same moment.

The two clock times enable the navigator to convert the hour difference into a geographical separation. Because the Earth takes 24 hours to complete one revolution of 360 degrees, one hour marks 1/24 of a spin, or 15 degrees. And so each hour’s time difference between the ship and the starting point marks a progress of 15 degrees of longitude east or west.

Precise knowledge of the hour in two different places at once--a longitude prerequisite so easily accessible today from any pair of cheap wristwatches--was utterly unattainable up to the end of the era of pendulum clocks. Such clocks were too vulnerable to the hazardous movements of a ship on the open sea, temperature changes, the rise or fall of barometric pressure, or subtle changes in the Earth’s local gravity.

For lack of a practical method of determining longitude, at some point every great captain in the Age of Exploration became lost at sea despite the best available charts and compasses. From Vasco da Gama to Vasco Nunez de Balboa, from Ferdinand Magellan to Sir Francis Drake--they all got where they were going willy-nilly, by forces attributed to good luck or the grace of God.

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As more sailing vessels set out to conquer new territories, to wage war, or to ferry gold and commodities between foreign lands, the wealth of nations floated upon the oceans. And still no ship owned a reliable means for establishing its whereabouts.

As a consequence, untold numbers of sailors died when land suddenly loomed out of the sea and took them by surprise. In a single such accident, on October 22, 1707, at the Scilly Isles near the southwestern tip of England, four homebound British warships ran aground and 2,000 men lost their lives.

The active quest for a solution to the problem of longitude persisted over four centuries and across the whole continent of Europe.

Seafaring men such as Capt. William Bligh of the Bounty and the great circumnavigator Capt. James Cook, who made three long voyages of exploration and experimentation before his violent death in Hawaii, took the more promising methods to sea to test their accuracy and practicability.

Renowned astronomers approached the longitude challenge by appealing to the clockwork universe: Galileo Galilei, Jean Dominique Cassini, Christian Huygens, Sir Isaac Newton, and Edmund Halley, of comet fame, all entreated the moon and stars for help. Palatial observatories were founded at Paris, London and Berlin for the express purpose of determining longitude by the heavens.

Meanwhile, lesser minds devised schemes that depended on the yelps of wounded dogs, or the cannon blasts of signal ships strategically anchored--somehow--on the open ocean.

As time passed and no method proved successful, the search for a solution to the longitude problem assumed legendary proportions, on a par with discovering the Fountain of Youth.

The governments of the great maritime nations periodically roiled the fervor by offering jackpot purses for a workable method. The British Parliament, in its famed Longitude Act of 1714, set the highest bounty of all, naming a prize equal to a king’s ransom (several million dollars in today’s currency) for a “practicable and useful” means of determining longitude.

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Harrison, a mechanical genius who pioneered the science of portable precision timekeeping, devoted his life to this quest. He accomplished what Newton had feared was impossible: In 1759 he invented a clock that would carry the true time from the home port, like an eternal flame, to any remote corner of the world.

Harrison, a man of simple birth and high intelligence, crossed swords with the leading lights of his day, who contested his claim to the coveted prize money. He ultimately claimed his rightful monetary reward in 1773--after years of struggling with political intrigue, international warfare and academic backbiting.

All these threads entwine in the lines of longitude. To unravel them now--to retrace their story in an age when a network of orbiting satellites can nail down a ship’s position within a few feet in just a moment or two--is to see the globe anew.

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Where in the World

In the days before global positioning satellites, the prerequisite for determining longitude was knowing the precise hour in two different locations at once. Here is a simplified example:

1. The naviagotor of a ship traveling to Hawaii from Los Angeles calculates its longitude by marketing the time aboard ship each day when the sun is at its highest point in the sky, known as “local noon.”

2. He then checks his chronometer, observing that it is 2 o’clock back in Los Angeles, a 2- hour difference.

3. Since the Earth takes 24 hours to complete one full revolution of 360 degrees, one hour marks 1/24 of a spin, or 15 degrees. And so each hour’s difference between local noon and the time where the ship started marks a progress of 15 degrees of longitude. In this case, the difference is 30 degrees (2 times 15).

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Marking the Time

Lines of latitude and longitude began crisscrossing our worldview in ancient times, at least three centuries before the birth of Christ.

But until the invention of English clockmaster John Harrison’s chronometer, time- keeping technology was unreliable. Pendulum clocks on the deck of a rolling ship would speed up, slow down or stop altogether. Changes in temperature thinned or thickened other clocks’ lubricating oil, affecting how they worked. Also, shifts in barometric pressure and even gravity affected a clock’s ability to keep accurate time.

Harrison’s chronometer was virtually friction- free and kept its moving parts perfectly balanced in relation to one another, regardless of how the world beneath pitched or tossed about.

Most present- day mariners use the Global Positioning System, whose satellites pinpoint their location within a few yards.

Source: Dava Sobel