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Re: How Many Chronometers?
From: George Huxtable
Date: 2009 May 7, 10:32 +0100
From: George Huxtable
Date: 2009 May 7, 10:32 +0100
Jim Wilson wrote- "... But I'm a bit hazy about your reference to error and rate. I think of error as the total change, and rate as the daily change. If the rate is known, than the error can be calculated, and therefore eliminated. I note that Cook's chronometer had a quite constant rate between mid-1773 and mid-1775. Knowing that the rate of change was constant, he could have made corrections based on elapsed time from the last calibration. I assume that was the normal practice to minimize uncertainty." comment by George- Jim has it right. That's how it was done. But how accurately could it be done? that's the question. The problem is this: you could measure the rate before departing, ideally over a few days; a week, perhaps. All right if you had clear skies at the right times. It was a process that could be done very precisely on land , particularly by timing stars crossing some sight-line, which always happened at intervals of a sidereal day, 3 minutes 56 seconds short of a mean solar day . It needed no special equipment, Harrison timed stars passing the edge of the window frame of his workshop to disappear behind a neighbour's chimney. But it needed some cooperation from the weather, unreliable in these parts. And it couldn't be done at sea. But having got a figure for rate, how reliable would it continue to be? The vessel is likely to be setting towards a climate that differed greatly from the test conditions. Although chronometer makers would try to make the rate independent of temperature, there was always some remaining effect. Harrison, for the Barbados test, supplied with his chronometer a table showing how a final adjustment should be made to the stated rate depending on temperature measued in the cabin, over the voyage. That called for regular monitoring and recording. But it was also affected by other factors. Barometric pressure was one. Arnold's early chronometers were found to be susceptible to corrosion, by sea-air, of the steel balance-spring, a minute loss of metal which would affect its stiffness and so, the rate. The natural oils that were used for lubrication would dry out and harden, over time. Worst, a minute speck of dust might work its way into a sensitive spot, which could make a sudden difference, and lead a vessel into danger. After all, trusting to a longitude that's wrong is far more dangerous than not knowing what it is at all. That was why, from the 19th century, the standard "fit" for a well-found vessel was to carry three chronometers. If one suddenly changed its rate, it was obvious which was at fault. Remember, just a second a day, over a two-month ocean passage, added up to an error of a minute of time, or 15' of longitude: not a lot better than you could hope for from a lunar distance. Over a long journey, precision was vital. For an explorer, setting off away from known longitudes for years at a time, some way to correct for error became vital. Eventually, radio met that need. Before then, lunar distance, at sea; perhaps Jupiter satellites, on land. Jim states- "I note that Cook's chronometer had a quite constant rate between mid-1773 and mid-1775. Knowing that the rate of change was constant, he could have made corrections based on elapsed time from the last calibration." Yes, that was exactly the procedure. But there was no way to KNOW that there had been no change since its last rateing. Cook would spend many weeks ashore, at places such as Venus point, Tahiti, or Snip Cove, Queen Charlotte Sound, New Zealand, and an important part of the work was a repeated astronomical determination of longitude, averaged over many observations over a long period. This would help to even out some of the regular monthly cyclic errors in the almanac, about which he was at the time unaware. Knowing a precise longitude, on a return visit it became a quick and simple matter to deduce a new value for error of his chronometer. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---