Читать книгу Longitude - Дава Собел - Страница 8

One night I dreamed I was locked in my Father’s watch

With Ptolemy and twenty-one ruby stars Mounted on spheres and the Primum Mobile Coiled and gleaming to the end of space And the notched spheres eating each other’s rinds To the last tooth of time, and the case closed.

—JOHN CIARDI, “My Father’s Watch”

As Admiral Shovell and Commodore Anson showed, even the best sailors lost their bearings once they lost sight of land, for the sea offered no useful clue about longitude. The sky, however, held out hope. Perhaps there was a way to read longitude in the relative positions of the celestial bodies.

The sky turns day to night with a sunset, measures the passing months by the phases of the moon, and marks each season’s change with a solstice or an equinox. The rotating, revolving Earth is a cog in a clockwork universe, and people have told time by its motion since time began.

When mariners looked to the heavens for help with navigation, they found a combination compass and clock. The constellations, especially the Little Bear with the North Star in its handle, showed them where they were going by night—provided, of course, the skies were clear. By day, the sun not only gave direction but also told them the time if they followed its movements. So they watched it rise orange out of the ocean in the east, change to yellow and to blinding white as it gained altitude, until at midday the sun stopped in its tracks—the way a ball tossed in the air pauses momentarily, poised between ascent and descent. That was the noon siren. They set their sandglasses by it every clear day. Now all they needed was some astronomical event to tell them the time somewhere else. If, for example, a total lunar eclipse was predicted for midnight over Madrid, and sailors bound for the West Indies observed it at eleven o’clock at night their time, then they were one hour earlier than Madrid, and therefore fifteen degrees of longitude west of that city.

Solar and lunar eclipses, however, occurred far too rarely to provide any meaningful aid to navigation. With luck, one could hope to get a longitude fix once a year by this technique. Sailors needed an everyday heavenly occurrence.

As early as 1514, the German astronomer Johannes Werner struck on a way to use the motion of the moon as a location finder. The moon travels a distance roughly equal to its own width every hour. At night, it appears to walk through the fields of fixed stars at this stately pace. In the daytime (and the moon is up in the daytime for half of every month) it moves toward or away from the sun.

Werner suggested that astronomers should map the positions of the stars along the moon’s path and predict when the moon would brush by each one—on every moonlit night, month to month, for years to come. Also the relative positions of the sun and moon through the daylight hours should be similarly mapped. Astronomers could then publish tables of all the moon’s meanderings, with the time of each star meeting predicted for one place—Berlin, perhaps, or Nuremberg—whose longitude would serve as the zero-degree reference point. Armed with such information, a navigator could compare the time he observed the moon near a given star with the time the same conjunction was supposed to occur in the skies over the reference location. He would then determine his longitude by finding the difference in hours between the two places, and multiplying that number by fifteen degrees.

The main problem with this “lunar distance method” was that the positions of the stars, on which the whole process depended, were not at all well known. Then, too, no astronomer could predict exactly where the moon would be from one night or day to the next, since the laws that governed the moon’s motion still defied detailed understanding. And besides, sailors had no accurate instruments for measuring moon-to-star distances from a rolling ship. The idea was way ahead of its time. The quest for another cosmic time cue continued.

In 1610, almost one hundred years after Werner’s immodest proposal, Galileo Galilei discovered from his balcony in Padua what he thought was the sought-after clock of heaven. As one of the first to turn a telescope to the sky, Galileo encountered an embarrassment of riches there: mountains on the moon, spots on the sun, phases of Venus, a ring around Saturn (which he mistook for a couple of close-set moons), and a family of four satellites orbiting the planet Jupiter the way the planets orbit the sun. Galileo later named these last the Medicean stars. Having thus used the new moons to curry political favor with his Florentine patron, Cosimo de’ Medici, he soon saw how they might serve the seaman’s cause as well as his own.

Galileo was no sailor, but he knew of the longitude problem—as did every natural philosopher of his day. Over the next year he patiently observed the moons of Jupiter, calculating the orbital periods of these satellites, and counting the number of times the small bodies vanished behind the shadow of the giant in their midst. From the dance of his planetary moons, Galileo worked out a longitude solution. Eclipses of the moons of Jupiter, he claimed, occurred one thousand times annually—and so predictably that one could set a watch by them. He used his observations to create tables of each satellite’s expected disappearances and reappearances over the course of several months, and allowed himself dreams of glory, foreseeing the day when whole navies would float on his timetables of astronomical movements, known as ephemerides.

Galileo wrote about his plan to King Philip III of Spain, who was offering a fat life pension in ducats to “the discoverer of longitude.” By the time Galileo submitted his scheme to the Spanish court, however, nearly twenty years after the announcement of the prize in 1598, poor Philip had been worn down by crank letters. His staff rejected Galileo’s idea on the grounds that sailors would be hard-pressed just to see the satellites from their vessels—and certainly couldn’t hope to see them often enough or easily enough to rely on them for navigation. After all, it was never possible to view the hands of the Jupiter clock during daylight hours, when the planet was either absent from the sky or overshadowed by the sun’s light. Nighttime observations could be carried on for only part of the year, and then only when skies were clear.

In spite of these obvious difficulties, Galileo had designed a special navigation helmet for finding longitude with the Jovian satellites. The headgear—the celatone—has been compared to a brass gas mask in appearance, with a telescope attached to one of the eyeholes. Through the empty eyehole, the observer’s naked eye could locate the steady light of Jupiter in the sky. The telescope afforded the other eye a look at the planet’s moons.

An inveterate experimenter, Galileo took the contraption out on the harbor of Livorno to demonstrate its practicability. He also dispatched one of his students to make test runs aboard a ship, but the method never gained adherents. Galileo himself conceded that, even on land, the pounding of one’s heart could cause the whole of Jupiter to jump out of the telescope’s field of view.

Nevertheless, Galileo tried to peddle his method to the Tuscan government and to officials in the Netherlands, where other prize money lay unclaimed. He did not collect any of these funds, although the Dutch gave him a gold chain for his efforts at cracking the longitude problem.

Galileo stuck to his moons (now rightly called the Galilean satellites) the rest of his life, following them faithfully until he was too old and too blind to see them any longer. When Galileo died in 1642, interest in the satellites of Jupiter lived on. Galileo’s method for finding longitude at last became generally accepted after 1650—but only on land. Surveyors and cartographers used Galileo’s technique to redraw the world. And it was in the arena of mapmaking that the ability to determine longitude won its first great victory. Earlier maps had underestimated the distances to other continents and exaggerated the outlines of individual nations. Now global dimensions could be set, with authority, by the celestial spheres. Indeed, King Louis XIV of France, confronted with a revised map of his domain based on accurate longitude measurements, reportedly complained that he was losing more territory to his astronomers than to his enemies.

The success of Galileo’s method had mapmakers clamoring for further refinements in predicting eclipses of the Jovian satellites. Greater precision in the timing of these events would permit greater exactitude in charting. With the borders of kingdoms hanging in the balance, 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.