Longitude. Dava Sobel

numerous astronomers found gainful employment observing the moons and improving the accuracy of the printed tables. In 1668, Giovanni Domenico Cassini, a professor of astronomy at the University of Bologna, published the best set yet, based on the most numerous and most carefully conducted observations. Cassini’s well-wrought ephemerides won him an invitation to Paris to the court of the Sun King.

      Louis XIV, despite any disgruntlement about his diminishing domain, showed a soft spot for science. He had given his blessing to the founding, in 1666, of the French Academie Royale des Sciences, the brainchild of his chief minister, Jean Colbert. Also at Colbert’s urging, and under the ever-increasing pressure to solve the longitude problem, King Louis approved the building of an astronomical observatory in Paris. Colbert then lured famous foreign scientists to France to fill the ranks of the Académie and man the observatory. He imported Christiaan Huygens as charter member of the former, and Cassini as director of the latter. (Huygens went home to Holland eventually and traveled several times to England in relation to his work on longitude, but Cassini grew roots in France and never left. Having become a French citizen in 1673, he is remembered as a French astronomer, so that his name today is given as Jean Dominique as often as Giovanni Domenico.)

      From his post at the new observatory, Cassini sent envoys to Denmark, to the ruins of Uraniborg, the “heavenly castle” built by Tycho Brahe, the greatest naked-eye astronomer of all time. Using observations of Jupiter’s satellites taken at these two sites, Paris and Uraniborg, Cassini confirmed the latitude and longitude of both. Cassini also called on observers in Poland and Germany to cooperate in an international task force devoted to longitude measurements, as gauged by the motions of Jupiter’s moons.

      It was during this ferment of activity at the Paris Observatory that visiting Danish astronomer Ole Roemer made a startling discovery: The eclipses of all four Jovian satellites would occur ahead of schedule when the Earth came closest to Jupiter in its orbit around the sun. Similarly, the eclipses fell behind the predicted schedules by several minutes when the Earth moved farthest from Jupiter. Roemer concluded, correctly, that the explanation lay in the velocity of light. The eclipses surely occurred with sidereal regularity, as astronomers claimed. But the time that those eclipses could be observed on Earth depended on the distance that the light from Jupiter’s moons had to travel across space.

      Until this realization, light was thought to get from place to place in a twinkling, with no finite velocity that could be measured by man. Roemer now recognized that earlier attempts to clock the speed of light had failed because the distances tested were too short. Galileo, for example, had tried in vain to time a light signal traveling from a lantern on one Italian hilltop to an observer on another. He never detected any difference in speed, no matter how far apart the hills he and his assistants climbed. But in Roemer’s present, albeit inadvertent, experiment, Earthbound astronomers were watching for the light of a moon to reemerge from the shadow of another world. Across these immense interplanetary distances, significant differences in the arrival times of light signals showed up. Roemer used the departures from predicted eclipse times to measure the speed of light for the first time in 1676. (He slightly underestimated the accepted modern value of 300,000 kilometers per second.)

      In England, by this time, a royal commission was embarked on a wild goose chase—a feasibility study of finding longitude by the dip of the magnetic compass needle on seagoing vessels. King Charles II, head of the largest merchant fleet in the world, felt the urgency of the longitude problem acutely, and desperately hoped the solution would sprout from his soil. Charles must have been pleased when his mistress, a young Frenchwoman named Louise de Keroualle, reported this bit of news: One of her countrymen had arrived at a method for finding longitude and had himself recently arrived from across the Channel to request an audience with His Majesty. Charles agreed to hear the man out.

      The Frenchman, the sieur de St. Pierre, frowned on the moons of Jupiter as a means of determining longitude, though they were all the rage in Paris. He put his personal faith in the guiding powers of Earth’s moon, he said. He proposed to find longitude by the position of the moon and some select stars—much as Johannes Werner had suggested one hundred sixty years previously. The King found the idea intriguing, so he redirected the efforts of his royal commissioners, who included Robert Hooke, a polymath equally at home behind a telescope or a microscope, and Christopher Wren, architect of St. Paul’s Cathedral.

      For the appraisal of St. Pierre’s theory, the commissioners called in the expert testimony of John Flamsteed, a twenty-seven-year-old astronomer. Flamsteed’s report judged the method to be sound in theory but impractical in the extreme. Although some passing fair observing instruments had been developed over the years, thanks to Galileo’s influence, there was still no good map of the stars and no known route for the moon.

      Flamsteed, with youth and pluck on his side, suggested that the king might remedy this situation by establishing an observatory with a staff to carry out the necessary work. The king complied. He also appointed Flamsteed his first personal “astronomical observator”—a title later changed to astronomer royal. In his warrant establishing the Observatory at Greenwich, the king charged Flamsteed to apply “the most exact Care and Diligence to rectifying the Tables of the Motions of the Heavens, and the Places of the fixed Stars, so as to find out the so-much desired Longitude at Sea, for perfecting the art of Navigation.”

      In Flamsteed’s own later account of the turn of these events, he wrote that King Charles “certainly did not want his ship-owners and sailors to be deprived of any help the Heavens could supply, whereby navigation could be made safer.”

      Thus the founding philosophy of the Royal Observatory, like that of the Paris Observatory before it, viewed astronomy as a means to an end. All the far-flung stars must be cataloged, so as to chart a course for sailors over the oceans of the Earth.

      Commissioner Wren executed the design of the Royal Observatory. He set it, as the King’s charter decreed, on the highest ground in Greenwich Park, complete with lodging rooms for Flamsteed and one assistant. Commissioner Hooke directed the actual building work, which got under way in July of 1675 and consumed the better part of one year.

      Flamsteed took up residence the following May (in a building still called Flamsteed House today) and collected enough instruments to get to work in earnest by October. He toiled at his task for more than four decades. The excellent star catalog he compiled was published posthumously in 1725. By then, Sir Isaac Newton had begun to subdue the confusion over the moon’s motion with his theory of gravitation. This progress bolstered the dream that the heavens would one day reveal longitude.

      Meanwhile, far from the hilltop haunts of astronomers, craftsmen and clockmakers pursued an alternate path to a longitude solution. According to one hopeful dream of ideal navigation, the ship’s captain learned his longitude in the comfort of his cabin, by comparing his pocket watch to a constant clock that told him the correct time at home port.

       4. Time in a Bottle

      There being no mystic communion of clocks it hardly matters when this autumn breeze wheeled down from the sun to make leaves skirt pavement like a million lemmings.

      An event is such a little piece of time-and-space you can mail it through the slotted eye of a cat.

      —DIANE ACKERMAN, “Mystic Communion of Clocks”

      

      Time is to clock as mind is to brain. The clock or watch somehow contains the time. And yet time refuses to be bottled up like a genie stuffed in a lamp. Whether it flows as sand or turns on wheels within wheels, time escapes irretrievably, while we watch. Even when the bulbs of the hourglass shatter, when darkness withholds the shadow from the sundial, when the mainspring winds down so far that the clock hands hold still as death, time itself keeps on. The most we can hope a watch to do is mark that progress. And since time sets its own tempo, like a heartbeat or an ebb tide, timepieces don’t really keep time. They just keep up with it, if they’re able.

      Some clock enthusiasts suspected that good timekeepers might suffice to solve the longitude problem, by enabling mariners