Nathaniel Bowditch The Practical Navigator


He had just been given command of the Henry , owned by Elias Hasket Derby [ see “To the Farthest Port of the Rich East,” AMERICAN HERITAGE , February 1955], wno engaged Prince to take her on a coffee trading voyage to the isle of Bourbon (now Réunion), in the Indian Ocean. Bowditch persuaded Prince to sign him as second mate and clerk and also to let him take along a lew pairs of shoes lor profitable trade on his own account—an “adventure” it was called in those days.

There probably has never been a more maladroit seaman than the clerk of the Henry . To cap his lack of experience and natural ineptitude, he was also inclined to seasickness, a blow to his dignity. He knew that few captains, even old friends, could afford a hand who did nothing but keep books, and pride drove him very hard, it is recorded that lie finally became a sort of “assistant” to the helmsman!

There was, however, one consolation: Prince encouraged Bowditch to experiment as much as he liked with pencil and slate. We can imagine the excitement and joy he must have had at this opportunity to verify his theories. Near shore or in dangerous waters, the Henry no doubt was navigated by the usual methods, for in spite of Prince’s desire to know if there really was anything to this business of mathematical navigation, he was too fine a seaman to take undue risks with his ship. But in the long reaches of the open sea, Bowditch had his chance. As far as he was concerned, the trip was a complete success, and Prince could not help but agree. The Henry made a phenomenally fast voyage and the reason was apparent: she had been sailed across the ocean in a straight line .

In Bowditch’s struggle to perfect his methods, the big obstacle was the determination of longitude when out of sight of land. This, in somewhat oversimplified terms, is arrived at today by calculations relying on the difference between the observed local time at the ship’s meridian and the known time at the same moment at the prime meridian in Greenwich. By expressing these degrees of arc, or time difference, in nautical miles, the ship’s exact position east or west of the prime meridian is established.

For centuries, accurate determination of a position in longitude was so vital that many maritime countries, starting with Spain in 1598 and including Portugal, Venice, France, and England, offered very substantial rewards for a solution.

It seems simple enough. You build a good watch, set it to Greenwich time when you leave port, and thus carry the time of the prime meridian with you wherever you go. The trouble was that none of the clocks or watches available in Bowditch’s day could come anywhere near the accuracy demanded; in a tossing ship at sea they could not be depended upon at all.

Great Britain had long since established a permanent Board of Longitude to examine all solutions advanced, and for decades had offered the sum of £20,000—an astounding reward for those days—to the man who could invent a reliable clock. It finally went, in 1773, to Joseph Harrison, before Bowditch. But it wasn’t very practical for many years to come.

Work continued toward developing a practical and dependable chronometer, but meanwhile eminent astronomers and experimenters devoted much effort to finding a way of telling Greenwich time at sea independent of mechanical timepieces. These ranged all the way from the outright use of black magic to improbable schemes involving the orbits of the satellites of Jupiter, or the occultation by the earth’s moon of certain stars. None of them worked.

Bowditch, however, turned the trick on the voyage of the Henry . The high point for him is very simply recorded in his journal: “… Thursday thought of a method of making a lunar observation.” And indeed he had. By means of three simultaneous observations on the moon and a fixed star (or the sun) he was able to calculate the angular distance between them and from this look up the Greenwich time in the Nautical Almanac , a work published annually by the Commissioners of Longitude in London.

Bowditch himself modestly writes in the preface to the first edition of the New American Practical Navigator : “A new method of working a Lunar Observation is given in this work … it was invented by the author … and taught by him to a number of persons in 1796 … he not having seen any method possessing the peculiar advantages of uniformity in applying the corrections …” It was a major achievement. For the first time in history men could accurately establish positions east or west of the prime meridian.•

… In passing it should be mentioned that around 1800 a reliable ship’s chronometer was finally developed. It was a fantastically expensive and complicated mechanism, however, and many years were to pass before it came into general use at sea. The determination of longitude by lunar sights is no longer used, as modern chronometers and more advanced concepts do the job better, but at the time Bowditch’s discovery was of enormous importance.

The Henry returned to Salem on January 11, 1796, exactly one year after she had sailed. Bowditch was to have three months ashore.