May 13, 2014 7:46:53 AM
When I was at the Naval Academy nearly a half century ago, one of the courses, and it was a hard one, was Celestial Navigation, the use of a sextant, star almanacs, and charts to find out where a ship was located. From what I hear, midshipmen no longer study such things; it is easier, faster, and less liable to error to ask GPS where the ship is, and it is better to have the middies studying things they will actually use. David Barrie, a British sailor of yachts, probably knows about this curriculum change, and would not be happy about it. Sextants and celestial navigation are too important historically and philosophically and practically to let go. In the informative Sextant: A Young Man's Daring Sea Voyage and the Men Who Mapped the World's Oceans (William Morrow), Barrie lets us know just how valuable sextants and star almanacs have been to him, and to humanity as it attempted the still-incomplete task of mapping our world.
If you do celestial navigation, you have to go back to the universe as it was understood before Copernicus. The Sun, stars, Moon, and planets were all circling the Earth back then. "It is easy to understand," writes Barrie, "how difficult it was for people of the sixteenth and seventeenth centuries to adapt to the heliocentric view of the universe, and for the purposes of the working navigator the Copernican Revolution might never have occurred." It is simpler to consider the Earth at the center, and though this is an oversimplification, it has not precluded the calculation of accurate angles for the Sun and the rest since the fifteenth century.
There were primitive devices like the astrolabe or the cross staff by which the mariner might measure an angle toward a star, but the Scientific Revolution brought forth improved instruments in many fields. The sextant was an improved version of the quadrant, a device which Newton himself first designed. Quadrants can measure angles up to 90 degrees, and sextants up to 120. The first sextant was made in the mid-eighteenth century, and the design from that time has been so effective that there has been little evolution of the instrument since then. The genius of the design is that looking through the eyepiece, the mariner can spy, because of a system of mirrors, both the horizon and the star in the sky whose height (altitude) is being measured. Once the index arm of the sextant is adjusted so that the star just touches the horizon, it is clamped in position, the sextant is removed from the eye, and the angle is read from the protractor-style arc at the sextant's bottom.
One of the advantages of the sextant is that it can be used to measure angles not just of star and horizon, but between two independent objects in the sky, or between two land objects seen on the horizon. Measuring the angle between the Moon and the Sun (and between the Moon and selected stars, if the Sun were too far away) was the basis for "lunars," a method of finding out what time it was. Readers of Dava Sobel's fine Longitude will remember the worldwide efforts to invent a timepiece that would work on a ship tossed by waves and weather, but there were other ways of telling time at sea. Eclipses were handy, but too rare, and even the moons of Jupiter could be used as timepieces, but seeing them reliably by telescope at sea was impossible. Even after reliable chronometers were being taken to sea, lunars were being used as backup. They were needed. A navigator cannot know position without knowing the correct time, and even into the nineteenth century, chronometers were of variable reliability. When Captain FitzRoy, commander of the Beagle whose future voyage Darwin was to make famous, set out for a surveying voyage in 1831, he took twenty-two chronometers with him and an instrument maker to keep them going smoothly. Even so, when he returned five years later, only half of the chronometers were still of any use. Lunars were dying away, though, even before time could be accurately transmitted by radio, a better way of checking if the chronometers were on time..
Throughout his book, Barrie quotes from his own sea adventure, sailing with a couple of pals across the Atlantic in 1973, when he learned many of the arts (and terrors and boredoms and deprivations) of seamanship including the use of the sextant. He also fills his book with the stories of far more famous sailors who set out to describe accurately the uncharted lands and waters. It is a surprise to find there are still uncharted regions; more than once Barrie tells us the charting is incomplete, as in reviewing the survey Captain FitzRoy attempted of Tierra del Fuego with only partial success: "Parts of the exposed southwestern coast of Tierra del Fuego remain uncharted to this day." Some of the often grueling stories are about surveying voyages captained by sailors whose names we know, like Bligh and Shackleford; one captain we know mainly because the horticulturalist who travelled with him used his name for a flower discovered on the voyage, Louis-Antoine de Bougainville, but his exploits deserve their own fame. Others were new to me, but all endured extremes of cold, heat, exhaustion, shipwreck, disease, cannibals, or scurvy just to make the world better known to its inhabitants. Lieutenant Pringle Stokes in 1827 was surveying the most dangerous parts of the Strait of Magellan, and made note of dangerous rocks, breakers, and reefs. He wrote, "The number and contiguity of the rocks, below as well as above water, render it a most hazardous place for any square-rigged vessel: nothing but the particular duty on which I was ordered would have induced me to venture among them."
Barrie, who writes with clarity and enthusiasm, is a fan of celestial navigation the old way, and he makes a good case. GPS gives us an accurate location, but distances us from knowing where we are in that we don't have to pay attention to our surroundings, the natural world, and the galaxy we live in. "By contrast," he writes, "the practice of celestial navigation extends our skills and deepens our relationship with the universe around us." To get a GPS fix, we have to have electrical power and receiving equipment that can fail. The GPS satellites themselves can be disabled and may be subject to being destroyed as an act of war. The signals can be jammed (and tracking signals are jammed sometimes by truckers who don't want the company to know where they are), and when there is one-spot jamming, it jams the system for all of those around. Sailors who get a position by pushing a GPS button, Barrie says, are "denying themselves the precious rewards of agency - the use of hand, head, and eye to solve problems and overcome difficulties." Besides, GPS can break in many different ways; we can count on Sun, Moon and stars to guide us whenever skies are clear.