Reaching for the moon

13 Jan 2011

To make sense of the novel eighteenth-century methods for longitude determination at sea, whether chronometric or through the use of lunar distances, we need also to recognize the unevenness of these new methods' use on board ship. Historians often seem to spend much more time marking heroic innovations than in tracing mundane adoption and use. Recall how common and trusted was the method of dead reckoning among expert mariners. Two examples from the period when the marine chronometer and the Nautical Almanac were introduced remind us of how these puzzles worked. The examples are also interesting because they involved vessels of the East India Company, an early adopter of the newfangled techniques.

At the end of 1770, the sixteen-year-old William Marsden, future eminent orientalist and Royal Society treasurer, shipped from England to the East India Company base at Bencoolen in Sumatra. He was carried on the Company's ship Seahorse, captain Edward Dampier, then on its second voyage to south China. The ship soon lost its entire crew to a press gang at Spithead, and was only able to set out for the East Indies in mid-January 1771.  In a voyage to Sumatra of 14000 miles without landfall, with the crew suffering from scurvy and short of water, position finding mattered. Here's Marsden's reminiscence about the longitude method used on board:

'During the course of the voyage, I was led by the example of some of the officers to pay attention to the method, then recently coming into practice, of ascertaining the longitude of the ship's place by observation of the apparent distance of the sun or moon and certain stars; which, under the particular circumstances of our case, proved to be of much importance; for in consequence of our not seeing any land since leaving England, by which the dead-reckoning as it is termed might be corrected, the progressive amount of error became very great'.

Marsden then recalls that thanks to a lunar eclipse he and his colleagues were able successfully to check their observations and sums. But, he continues, 'the captain, indeed, was not a convert to this new process of determining what ought to be supposed the true place of the ship, and laying down on his chart the daily run by the log, his tract led him into the continent of New Holland [Australia], and the aggregate amount of our error amounted to no less than twenty-five degrees of longitude'.  Their first landfall was at the Sumatra coast, 'the first land that blessed the sight and revived the drooping spirits of the crew'.

Fig. 1: Portrait of an East India Company Captain, c.1800 (NMM: BHC3127)

The second case is from the voyage of the astronomer and mathematician Reuben Burrow from Southampton for Bengal in autumn 1782.  Burrow was already an expert surveyor and hydrographer, a grumpy collaborator of Astronomer Royal Nevil Maskelyne. His East India Company ship, the General Coote, captain Daniel Hoare, initially sailed in convoy with the main Royal Navy fleet through fear of French attack. Burrow's notoriously irascible character had already made trouble for the ship's ability to navigate. The great clockmaker John Arnold refused to provide Hoare with a marine chronometer just because of a remark by Burrow: in reply, the astronomer sent Arnold 'a most bloody letter'.  Once under sail, in the mid-Atlantic approaching Brazil, Burrow 'attempted the method by the Moon, but not having a watch that could be depended upon, and having nobody on board capable of helping me, I never got a good observation. I took the distance of the Moon from the Sun without using the telescope, but the Moon's altitude was very bad, owing to a ship being in the way of the horizon.'  Burrow's sums gave a longitude of 17 degrees; dead reckoning gave 14 or 15 degrees.

Burrow is characteristically eloquent about the difficulties of the lunar distance method, and the corresponding incompetence of his shipmates. 'I took the [lunar] distance and two of the Mates took the altitudes, but out of three sets of observations only one was anything like right, for some of the Altitudes were palpably erroneous, owing to the stupidity of the observers'.

Then Burrow explained the problem. Even though the East India Company had ordered the adoption of the new longitude methods at sea by their officers, 'except the Captain I did not find any one that had the least knowledge of such matters...and they were likewise so conceited and ignorant as to be above being shown. I endeavoured to teach them better, but they only made ridicule of it and pretended they could carry a ship to India without it'. In Burrow's view the second mate Stephen Newton 'might be the means of destroying the lives of thousands if there was nobody on board better informed than himself'.  Before the end of the voyage, at least on his own telling, Burrow was giving Captain Hoare highly accurate measures of longitude from lunar distances.

We see in these cases, and others, how the adoption of such complex and novel methods was neither self-evident nor easy. Discovery needed much more than great insight.