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The British Longitude Act Reconsidered

Was it responsible governance or was it subterfuge?

William E. Carter, Merri Sue Carter

Altruism or Other Motives?

Forming the Board of Longitude to search for a solution to the longitude problem benefitted members of Parliament and the Admiralty. By shifting the longitude problem to the scientific community, they stifled public criticism of their inability to solve the problem. After all, if the leading scientists of the age, including even Isaac Newton, had no solution, what could anyone expect from them? Passage of the Longitude Act also benefitted the scientific community, especially astronomers. By taking on the task of finding a solution, astronomers assured themselves of continuing funding to collect and analyze observations, to travel to distant lands to observe events such as lunar and solar eclipses and to develop new instruments. Members of the Board of Longitude received reimbursement for expenses associated with carrying out their duties and appointment to the body became a coveted honor.

2012-03MacroCarterFB.jpgClick to Enlarge ImageBut was it responsible governance to pass the Longitude Act without other efforts to protect British seamen? Or might it have been subterfuge—a disingenuous attempt to shift attention away from the realities of their life at sea. Conditions were horrendous aboard most British naval vessels at the time. Scurvy and other diseases ran rampant, killing more seamen each year than all other causes combined, including combat. Few experts felt that a solution to the longitude problem would be found quickly, so there was little chance that the prize moneys would be paid out for decades. If someone did solve the longitude puzzle, it would have been well worth the prize moneys and not only for the lives saved. Improved navigation would enable British naval vessels to accurately map the shorelines of continents and islands as they sailed the globe. While doing so, Britain could record the precise locations of distant ports belonging to nations such as France and Spain, with whom Great Britain so often found itself at war.

With the approval of the Board of Longitude, astronomers set about collecting the many thousands of observations needed to determine accurately the right ascensions and declinations of bright stars covering the full sky, including the increasingly important southern hemisphere. And they worked to improve their ability to predict the motions of the Moon, the Sun and the planets relative to the background of the stars, which from Earth appear to be unmoving. Being able to accurately predict the ever-changing declinations of the Moon and Sun was particularly valuable for nautical navigation because measuring the altitudes of these bodies near transit (as they crossed the observer’s meridian) was a convenient and accurate way to determine latitude. Little was needed beyond knowing the approximate day of the year. Combining reliable measurements of a ship’s latitude with estimates of its longitude based on dead reckoning helped navigators recognize landmarks as they approached continental coastlines and identify specific islands among others of a group.

Newton had considered the problem of measuring the altitudes of celestial bodies from the deck of a ship at sea as early as 1699 and communicated his ideas for a reflecting quadrant to Edmund Halley. Unfortunately neither Newton nor Halley published the design and it was another three decades before John Hadley independently developed an octant of similar design. Hadley’s octant soon became the standard instrument for celestial navigation at sea before being replaced by the sextant, an instrument of essentially the same design but with a larger range of observable angles—120 degrees rather than 90 degrees. The sextant made determining latitude at sea faster, easier and more accurate. It also made possible the first practical solution to the longitude problem, because it could be used to measure the angular separations, or distances, between celestial bodies—most importantly between the Moon and stars, the planets and Sun, measurements referred to as lunar distances.

Lunar distances were the key to determining longitude at sea without a clock that could carry Greenwich time accurately for months or years while exposed to changes in gravity and temperature and to the continual slap of waves. As seen from Earth, the Moon moves relatively rapidly against the background of fixed stars—approximately 13 degrees per day, or roughly one lunar diameter per hour. By measuring the lunar distances of known stars, planets or the Sun and comparing them to tables of predicted values for Greenwich (corrected for local refraction and parallax), it was possible to accurately compute Greenwich time. A pocket watch could then be used to keep time well enough during the interval required to observe altitudes of one or more celestial bodies in order to determine the difference between Greenwich and local time—that is, the observer’s longitude. However, there was one catch. The motion of the Moon is anything but simple. It is so complex that Isaac Newton is said to have told his contemporary, John Machin, that his head never ached except with his study of the Moon.

Fortunately, in 1753 the German astronomer Tobias Mayer published a new set of highly accurate lunar tables. In 1765, after 11 years of negotiations, the British Parliament awarded Mayer’s heirs £3,000 for use of the tables. Nevil Maskelyne, the fifth Astronomer Royal of England, set about developing step-by-step instructions and ancillary tables. A ship’s captain or navigator, without extensive mathematical skills, could follow the tables to “reduce” raw lunar distance observations and obtain his vessel’s longitude. Maskelyne’s tables became the basis for the Nautical Almanac published in England annually beginning in 1767.

2012-03MacroCarterFC.jpgClick to Enlarge ImageCaptain James Cook took along a set of Maskelyne’s tables on his first exploratory voyage to the South Pacific, from 1768 to 1771, on board the HMS Bark Endeavour. Cook and Charles Green, an astronomer aboard the ship as a member of the Transit of Venus observing party, made frequent determinations of the ship’s longitude along the way. Cook reported in his journals that “By these [Maskelyne’s] tables the calculations are rendered short beyond conception and easy to the meanest capacity.”

Cook was exuberant in praising the lunar distance method of determining longitude, saying that it could “never be enough recommended to the attention of all sea officers, who now have no cause left for not making themselves acquainted with this useful and necessary part of their duty. Much credit is also due to the Mathematical Instrument makers for the improvements and accuracy with which they make their Instruments for without good Instruments the Tables would loose [sic] part of their use.” Still, the measurement of lunar distances required more skill than the simpler measurements of the altitudes of celestial bodies, usually requiring anchoring in a well-sheltered harbor or going ashore to collect the necessary observations.

On his second voyage of exploration, from 1772 to 1775, Cook took four sea clocks aboard the Resolution. The primary clock was a copy of John Harrison’s clock H4 (which had performed well in sea tests conducted in 1763), made by Larcom Kendall, and designated K1. The other three clocks were of different design and made by John Arnold. Cook reported that one of Arnold’s clocks “kept time in such a manner as not to be complained on.” However, he was much more enthusiastic about K1, stating that “it had been found to answer beyond all expectation.” Cook was particularly pleased to have a clock that kept time well enough to determine longitude accurately from simple altitude observations, which could be accurately performed at sea. In his journal he noted that “Even the situation [latitude and longitude] of such islands as we past [sic] without touching at are by means of Mr. Kendall’s Watch determined with almost equal accuracy.”

Confident that he could determine his position generally to within a few minutes of arc, Cook could devote less time to finding his way. He could give more attention to surviving wind and ice storms, avoiding being driven onto leeward shorelines and reaching protected areas of anchorage. His journals contain entry after entry about his decisions to skip anchorage spots completely or to spend days waiting for the right combination of wind, tides and currents to enter sheltered bays, and the mouths of rivers to resupply with fresh food, water and wood. He was often forced to resort to towing his ship with rowboats, or hauling anchors out, placing them and then “warping” his ship to an anchorage by pulling in the anchor lines. It would have been so much quicker and safer if he had had a self-contained, on-demand source of propulsion, the simplest mechanical engine, rather than only the capricious wind.

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