Navigation: The Science of Commerce

Sources

Practicality. The majority of European Americans in the eighteenth century lived within one hundred miles of the seacoast. Virtually everyone living there was linked inescapably to the great Atlantic commercial routes, from the urban merchants to the inland farmers, from governors to slaves. Manufactures of every sort—clothing, tools, pottery—came from Europe, paid for with American timber, tobacco, fish, and grain. And none of this extensive trade could take place without the ability to get a ship safely and efficiently across three thousand miles of trackless ocean. The science of navigation made American trade possible. It was an ancient art, but significant advances in astronomy and instrumentation in the eighteenth century transformed it into a science, one in which mathematics and the latest astronomical information were tools of the trade.

One’s Place in the World. For centuries seafarers had used maps that placed the known world on a grid of imaginary lines, one set running east-west, parallel to the equator, and another set running north-south, converging at the north and south poles—the lines of latitude and longitude. Even before the first settlement of America, mariners knew how to find their latitude thanks to the never-moving North Star at night and the Sun’s zenith position at noon every day. Determining longitude, however, was always more complicated. By the mid eighteenth century making one’s “westing” accurately required solving the “celestial triangle.” One point of this triangle was formed by the north or south pole. The second point was a position on earth directly under another known star; this was determined from published astronomical tables. The last point was the ship’s (unknown) degree of longitude. To solve the triangle the navigator had to make careful measurement of the angle of the known star and know the time as accurately as possible. Using spherical trigonometry the navigator could then determine the final point of the triangle and thus his position at sea.

Improvements. For measuring angles above the horizon crude cross staffs and plumb-line contrivances gave way to sophisticated optical instruments. About 1731 the navigator’s quadrant made possible much more accurate measurements of angles above the horizon, and later the sextant refined measurements still further. As warships extended the British empire around the world, they brought back data that aided in making accurate astronomical tables for every season in every ocean. Perhaps most importantly, Englishman John Harrison developed the chronometer, essentially a highly accurate seaborne clock, with intricate mechanisms to compensate for a ship’s movement, changes in temperature and humidity, dust, and other variations. Chronometers were expensive, as were sextants, but the advantage they gave American shipmasters made them prized possessions. But whether equipped with the latest English instruments or the more common quadrant and hourglass, American trade undoubtedly benefited from the great improvements in astronomical measurement, and the value they placed on learning the science of navigation is suggested by the frequent advertisements in colonial newspapers for training in celestial navigation.

Sources

Brian Lavery, Nelson’s Navy: The Ships, Men, and Organization, 1793-1815 (Annapolis, Md.: Naval Institute Press, 1994);

Dava Sobel, Longitude (New York: Walker, 1995).