Longitude. Dava Sobel

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       3. Adrift in a Clockwork Universe

       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 СКАЧАТЬ