Afterword - Harrison and Dava Sobel

Unfair! Unfair!

material for a debate


Dava Sobel's hugely successful book, "Longitude", was wonderful in that it introduced the history of the Longitude problem to a wide audience. It was not, however, very fair to Nevil Maskelyne and to the lunar distance method of navigation which he pioneered.

Put yourself in Maskelyne's position, in the middle of the eighteenth century.

Clocks at that time meant pendulum clocks - completely unviable at sea. The possibility that a chronometer (spring-powered, with a double escapement mechanism,) could function accurately at differing temperatures when bucketing around in an ocean-going ship was virtually inconceivable. No sensible person would have suggested it.

Harrison was not, in those terms, sensible. He made a good clock out of lignum vitae wood. He was a carpenter (and a genius).

Of course Nevil Maskelyne assumed Harrison was mistaken.

Captain Cook did not. Within ten years of the chronometer's first proving voyages, he was using a copy in anger, relying on it, testing it, adopting that very pragmatism which (I believe) was utterly characteristic of men who went all over the world in wooden vessels held together and powered by string.

But how did he check his chronometer?

By calculating lunar distance measurements, of course. For more than a century, from Cook's time until 1906, it was axiomatic that a diligent navigator would use lunar distance methods as well as the chronometers to verify a vessel's longitude. The moon was reliable. Its mechanism would not break or run down (unlike a chronometer's or a battery-powered GPS).

Maskelyne, an astronomer who had proved his method at sea, knew all that. He relied on that. He put generations of seamen in a position to capitalise on that by calculating lunar distance data and publishing it in the Nautical Almanac.

Sobel's book makes Maskelyne into a villain and I doubt that he was. His 'Mariner's Guide' is still a very good, clear read. His constancy, in publishing lunar distance data each year, from 1768 until his death, is remarkable. No computers in those days - no calculators, not even mechanical adding machines to help. Just pencils and lots and lots of paper. What a triumph! What an achievment!

The Longitude Board messed about with Harrison in unforgivable ways, as Dava Sobel has recorded. He didn't get all the money he deserved for ages - in fact, it took a petition from the King and an Act of Parliament to achieve recognition for him and for his son, who took over his work.

Harrison eventually got more than £22,000 (several million in today's currency), from Parliament and the Longitude Board at the behest of the King himself.

Nevil Maskelyne became the Astronomer Royal but the Longitude Commissioners never awarded him a penny, despite the fact that his solution to the longitude question was viable and reliable and became the foundation stone of British naval power in the nineteenth century.

Harrison's chronometers (or the authorised copies of them which became the standard items carried on board British naval ships) were useless on very long voyages unless you could check them regularly with reference to lunar distance observations and calculations. This was before Mr. Marconi had discovered how to broadcast time signals from Greenwich around the world. This was before the ceremony of the 'noon gun' in Jersey, Hong Kong and Cape Town (and almost everywhere else where British ships called for trade, adventure and to protect British dominions and the Empire), a ceremony which allowed visiting ships to regulate their time pieces accurately before they moved on.

A chronometer, unless wound regularly and treated very, very carefully, would fail within a few days. A single chronometer, if you trusted it wholeheartedly over a period of weeks and months, could develop a major error and lead you seriously astray.

By the late nineteenth century, chronometers were cheap enough for a serious ocean going vessel to boast at least three and Claud Worth, a celebrated fore-runner of current cruising yachtsmen, relates in his book "Yacht Cruising" how a notice reading "have you wound up the chronometers?" was placed on the breakfast table each morning on his yacht in the early twentieth century.

Breakfast would not be served unless the chronometers had been properly tended.

None of the clock-copies based on Harrison's chronometers would have survived the extended duration of Cook's voyages without being checked and rated by comparison with lunar distance observations, probably once or twice per month or towards the point of expected landfall.

Only by reference to lunar distance navigation and to Maskelyne's blue-print for it, was it possible to keep chronometers working effectively. And lunars (rather than chronometers) were the most crucial aid to navigation even when Dana wrote, in the 1830s.

As Dana's ship approached Cape Horn, it was important to speak with another vessel, to learn where the ice was and to check longitude: “for we had no chronometer and had been drifting about so long that we had almost lost our reckoning; and opportunities for lunar observations are not frequent or sure in a place like Cape Horn.”'

Patrick O'Brian was, as everyone now knows, a fabulous chronicler of life at sea during the Napoleonic wars and accurately tells of the exploits of British sailors when they tried to confront the elements, the enemy and the technologies of the time.

In his 'Blue at the Mizzen' (around page 148) two ships meet at sea and their Masters consult and quarrel about the longitude. One of them has lost the use of his chronometer. The phrase ‘triangle of uncertainty’ is used.

"There would be no problem," he says, "if I could only get a decent lunar."

But the skies have been cloudy. It was not the right time of the month.

(This incident is so very like one in Dana's ‘Two Years before the Mast’ that I wondered if O’Brian had cribbed it, as he copied and honourably re-invented many incidents from accounts in the Naval Gazette, of which he was a constant student.)

From Dana's book, again, around page 276:

“We had evidently made great progress and had good hope of being soon up with the Cape, if we were not there already. We could put but little confidence in our reckoning, as there had been no opportunities for an observation, and we had drifted too much to allow of our dead reckoning being anywhere near the mark. If it would clear off enough to give a chance for an observation, or if we could make land, we should know where we were; and upon these, and the chances of falling in with a sail from the eastward, we depended almost entirely.”

What a world! What a way to live and sail, chancing everything on finding another vessel in the wastes of the Southern Ocean, risking a landfall on a barren, rocky coast, without any clear way of establishing - even within 100 miles - where you might be.

They were brave men, those sailors. No hot showers for them. No lavatories below the waterline. No breathable waterproof clothing or thermal underwear. Wickedly bad food, unyielding danger, the necessary disciplines of a ship at sea, the constant uncertainties about position, direction, landfall and survival.

Francis Chichester was a great and renowned airman before he became a renowned sailor and he taught and studied navigation for many years.

In his autobiography, The Lonely Sea And The Sky, he writes: (p. 297) “ if one could rely on accurate information, navigation would be a simple science, whereas the art and great fascination of it lies in deducing correctly from uncertain clues.”

In spite of Chichester’s amazing knowledge of navigation, with the widest possible experience of teaching it, writing textbooks about it and using it as a flyer and as a seaman, in spite of his access or potential access to the most up-to-date Ocean pilots, pilot charts and other reference sources, he is continually taken aback by ocean currents and eddies which upset his Dead Reckoning. There are examples of his difficulties on pages 291 and 295 of his book and there are other, similar complaints elsewhere. We are talking of 50 and more miles of error in a single day when Dead Reckoning has been carefully worked up by an acknowledged expert who spends up to two hours a day on his navigation (page 328).
This is very significant.

Chichester (p. 304) claims almost phenomenal expertise (in particular, he says he can estimate the speed of his vessel to one quarter of a knot).

If Chichester could not get Dead Reckoning right all the time, then no one could or can.

Thank heaven for the Global Positioning System and its eventual successors. Long may they help to protect us from our own inefficiency on the oceans.

But it is worth sounding a loud cautionary note.

In May, 2,000, GPS SA (Selective Availability) was removed and GPS became suddenly much more accurate.

How kind of the American Government, which controls this system completely and unilaterally, to cease to degrade the signals and the system.

How very (apparently) warm-hearted!

from the Journal Of Navigation, Sept. 2001, p. 439

Selective Availability was removed because the “capacity to deny GPS signals on a regional basis was achieved at the beginning of 2000.”

In other words, the Americans can switch GPS on or off, at any time, in any region of the world if they so choose.

Keep your sextants handy and practice Dead Reckoning whenever you can.

I certainly intend to.

You may need those skills badly one day.