Three Theories of Longitude Solution

At this time three methods were now being investigated to find the solution to the problem of longitude. These were: "calculating the variation of the magnetic compass for true north, measuring lunar distances and developing a marine chronometer." (Berthon and Robinson, 1991:118).

Magnetic Variations - Finding True North

Magnetic variations had first been noticed in the Age of Discovery. Until then, compass bearings and charts based upon them referred to magnetic north rather than true north as measured by the position of the Pole Star. This distinction only became vital when sailors began using latitude as the way to determine their course. As they were sailing along a latitude they noted that the compass needle varied unexpectedly. It 'northeasted' or ‘northwested' that is, the position of magnetic north shifted with the position of the ship (Berthon and Robinson, 1991:118).

This discovery initially greatly dismayed the Portuguese pilots who first discovered it. Having no idea of the real reason - alterations in the magnetic field of the earth which varies greatly in direction from both decade to decade and place to place - they wrongly attributed the moves to badly hung compass needles, inferior lode stones or the leeway caused by ocean currents. Compass makers began to make their own 'allowances' for the variations by off-setting north on the compass and off-setting different points on the journey. Hence it was that Columbus found that his Flemish and Italian compasses provided different readings (Berthon and Robinson, 1991:119).

Soon the idea developed that one "true" meridian may exist along which the magnetic variation from true north was zero. On either side of this meridian, it was believed, the variation would increase uniformly in opposite directions, thus creating the effect of northeasting and northwesting. If this was reality it meant that the direction of magnetic north relative to true north could be predicted at any point around the earth's circumference and this could then be used by a ship to calculate longitude by comparison with its measured north.

Unfortunately this was not actually the case as first established by a Chief Pilot of the Portuguese Indian Fleet, John de Castro. He was the first to show that the variation did not follow any pre-determined pattern and certainly not that of a true meridian. In 1638, more than a century later Henry Gellibrand, a professor of mathematics confirmed that the variation altered over time as well as place. It was in fact even more unpredictable than sailors had feared (Berthon and Robinson, 1991:120).

Despite this finding, in 1698 Edmund Halley arranged to be commissioned captain of a ship, the Paramour. Sailing through the Atlantic he measured the magnetic variation from true north and then used these data to plot lines of equal variation for the year 1700. However the map caused much scepticism amongst sailors when Halley drew 'north' lying east-west at the coast of the now United States and a 'true' meridian "that looked more like a parabola than a straight line." (Berthon and Robinson, 1991:120). Halley’s research excluded magnetic variation as a practical solution for the calculation of longitude, although it remained theoretically possible if a great enough spread of reference values, such as those plotted by Halley, were monitored year by year. This was not a condition that was to be fulfilled until the twentieth century with the advent of satellite (Berthon and Robinson, 1991:120 and Taylor, 1956:240).

Sailors were not able to rely on magnetic variation to determine longitude. Another solution was required.

Measuring Lunar Positions and Distances

During the seventeenth and eighteenth centuries it was realised that if the changes in the position of the earth’s moon could be observed and then compared to the lunar tables prepared in Paris or London, then, theoretically, longitude could be calculated. However, science during this time was simply not sufficiently rigorous to be able to achieve the necessary accuracy required for the calculations. Newton himself observed that the "accuracy of the lunar tables was between two and three degrees of longitude - no better than dead reckoning." (Berthon and Robinson, 1991:120).

After 1767 sailors began seeking to calculate longitude at sea by measuring the distances of particular stars from the moon, that is, the lunar distance method. The greater accuracy that was required to achieve this led to the development of the sextant for use at sea after about 1770. The name of the sextant refers to the actual arc but not to the angle that can be measured. More accurate than the octant, sextants were produced in great numbers during the 1800s onwards. Navigators with the more wealthy companies such as the East India Company typically used a sextant (Turner, 1980: 34).

The Marine Chronometer - an invariant engine for time

Although the idea of using a Chronometer to calculate longitude dates from the thirteenth century the actual invention of the Marine Chronometer itself, a revolution in navigation, was not made until the eighteenth century by a skilled English clock-maker, John Harrison. Born in 1693, John Harrison was a carpenter by trade. However, like his brother James, he was particularly skilled in the art of making and repairing clocks and it was to this art that the brothers turned their attention. They were so successful at correcting the existing causes of inaccuracies in clocks that by 1726 they had manufactured two clocks which lost no more than one second a month. This was a remarkable achievement and advanced far beyond any existing technologies of the day (Quill, 1966:8).

This achievement appears to have encouraged the brothers to seek the longitude award. Their aim was to make a timekeeper, as accurate as that just produced, which could withstand the movements of the seas and the variations in temperature that it would be subjected to when travelling aboard a ship. Their cooperative efforts led to the older brother, John Harrison's eventual success.

The clocks that the Harrisons made "represented a tremendous technical advance. Each was constructed in such a way that it was unaffected by variations of temperature, and great ingenuity was shown in devices which not only reduced friction to a minimum, but also eliminated the need for oil in any part of the mechanism." (Quill, 1966:25). See Fig 13.

By 1730 the brothers had developed a new form of timekeeper which retained many of the features of the earlier wooden regulators but not the characteristic pendulum. This meant that the ‘sea-clock’, as John Harrison referred to it, was portable as never before and hence potentially able to be used at sea for navigational purposes. To withstand the rough passage of sea travel, metal such as brass replaced wood in many of the new clock’s parts. This was expensive, so external assistance would be required and the brothers eventually asked the Board of Longitude for financial assistance under the Act.

In 1730 Harrison, unable to meet with the full Board, instead met with Dr Halley, the Astronomer Royal and Commissioner on the Board of Longitude. Halley had dedicated many years to studying the problem of longitude and at the time of this meeting was engaged in creating tables by which the path of the Moon could be predicted and hence longitude calculated. Unfortunately the Moon's path is extremely complicated hence Halley’s work, was progressing only very slowly at best. Halley was interested therefore in the plans that Harrison showed him, immediately aware that if it could prove workable, then this would offer a quick and simple solution for the problem of longitude. Consequently, Halley referred Harrison to George Graham who was also a Fellow of the Royal Society. Graham considered Harrison’s work and then agreed with Halley that, if Harrison could produce a working and accurate clock, they would support him on his final approach to the Board. Further, Graham personally provided some money to Harrison to help finance researching and building the clock. (Quill, 1966:34-37).