Peter Bunclark wrote:
Indeed isn't this Rob's ship's chronometer?
Actually, I think it was Mr. Harrison's. (And Steve Allen has been basing his arguments more recently on this distinction.) This healthy debate between astronomical time and clock time has happened before. The answer is the same as before - both types of time are needed. (Some things never change.) I'm sure Pete is more familiar with this story than I am, but others may not be.
Harrison attempted to build a perfect clock to win the Longitude Prize. Folks who haven't read Sobel's book should do so - my classmates at Villanova and I learned the story from an Augustinian priest who appeared old enough to have known Harrison personnally. Harrison's first glorious shipboard clock failed to take the prize due to a lack of compensation for centrifugal effects on a sailing vessel that must tack when sailing against the wind (or must wear through an even larger angle, bringing the wind across its stern). Compensation was needed for relativistic effects, if Newtonian rather than Einstein. (Some things never change.)
Harrison invented or improved a variety of familiar mechanical doodads like the roller bearing and bimetallic temperature compensation. He likely could have succeeded in solving this particular problem, but there would always have been another physical improvement needed. (Some things never change.) Each improvement would have made the clock more complicated and eventually too fragile to possibly work on a constantly moving platform buffeted and often bathed by the salty sea.
He created a second clock and was working on a third round of improvements when the idea we're discussing first occurred to him. He had been using a pocket watch as a mechanism to transfer time from stationary standard clocks (many built by himself) to his portable prototypes. He would reset the clock in one place and physically carry it to where the time was needed. If a roundtrip correction were needed, presumably he would note the time on either end and halve the difference. This is the "standard synchrony" or "conventionality of simultaneity" of special relativity - familiar to anyone who has looked under the hood of NTP. (Some things never change.)
What Harrison recognized was that he didn't need to build a perfect clock - he merely needed to quantify and log the error inherent in the clock. By replacing a large and finicky "better" clock, with a small and robust, but more even-tempered, one, the rate of the clock could be regularized and its random and systematic errors could be minimized. That the rate of the clock was now guaranteed not to match the rate of the spinning Earth was no longer a bug, but a feature. By carefully calibrating the clock rate before leaving on a voyage, and checking it against astronomical observations throughout the voyage, it was possible to compute the mean solar time at the home port. (Some things never change.) Comparison with the local time, measured by sextant, then recovered the longitude directly.
And, of course, a ship would not carry a single clock, but two or more. Friendly ships meeting at sea would also exchange clock readings - creating the first ensemble time scale. (Some things never change.)
Thus was the chronometer born - and thus did Britannia rule the waves.
The point is that time isn't just an unending count of seconds - it is the epoch of when the count was zero. That epoch often has significance in some periodic natural phenomena, usually related to Earth orientation.
Rob
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