A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How was GMT originally established ?
From: George Huxtable
Date: 2004 Jan 29, 21:37 +0000
From: George Huxtable
Date: 2004 Jan 29, 21:37 +0000
Patrick Stanistreet said- > Will have to investigate measuring the transit >of the sun as a method for setting a clock. and Thomas Smith added- >> I seem to remember dimly that Harrison observed the sun in >> order to calibrate his clocks, but I don't have a source >> handy. That is a poor way to determine the accuracy of a clock. The Sun does not keep a regular time, it appears to speed up and slow down for two main reasons, which combine together. 1. The eccentricity of the Earth's orbit around the Sun. 2. The tilt of the Earth's Equator with respect to the plane of its orbit around the Sun The result is a complex double-humped curve of the error compared with the steadily changing time that an accurate clock could measure; the difference is confusingly known as the Equation of Time. There is no way of making an irregular clock that could follow the variations of Sun time, which can amount over the year to about 15 minutes error each way. Even the Greeks, I think, were aware of the irregular timekeeping of the Sun (though I can't quote references), but it was accurately measured by a Dutchman, Huygens, who invented the first accurate pendulum clock in 1657, intended for use by astronomers. He also tried to adapt pendulum clocks for use at sea, which all failed. He published a rather accurate table of the equation of time. Any subsequent use of the Sun to determine GMT relies on the precision of previous determinations of the equation of time. Star time, however, is constant, to a very close approximation: varying only slightly as the earth's rotation gradually slows. As discussed in this thread, it was what Harrison used to adjust his master-clock (his "regulator") against which other timepieces could be compared. A star crosses a pair of vertical markers indicating a particular azimuth, at intervals of precisely one sidereal day. And the disappearance of a star behind even an improvised azimuth, such as the Harrison chimney, is an instantaneous event: not so for the Sun. So, although Thomas Schmidt suggests that Harrison used Sun transits, I think it's probable that he relied entirely on stars. Patrick Stanistreet asked- >Assuming timing of a star one ends up with a sidereal >clock but when did land based clocks achieve sufficient >accuracy to time a star over 24 or more hours so as >to differentiate between sidereal and solar time? Well, the ratio between sidereal day and mean-solar day could have been known to quite high accuracy by the Greeks (and before them, the Egyptians), though they would not have used those terms. It doesn't need a clock to measure it, because it falls out immediately from the number of days (N) in the year. The ratio of the length of the sidereal day to the mean solar day is N / (N+1). Various references can be found in "Astronomy before the Telescope", ed. C.Walker, BCA, 1996. It followed from the realisation that the Sun took a path between the stars over a year, which was complicated by the fact that the Sun and the stars were never visible together. Patrick asked- >Was the land based authority setting GMT associated >with the Almanac office? and Thomas Schmidt replied- >> At Harrison's time (17xx) there was no GMT (at least not >> in the sense of a zone or even universal time) and no >> associated authority. To start with, it must have been a private arrangement between observatories, which developed into consensus. Huygens' original table for equation of time did not give mean time from apparent time, but by an amount between 0 and 33 minutes, which always had to be ADDED to Apparent (i.e. sundial) Time to give a regular, constant (but not mean) solar time. This was later superseded in favour of Mean Time. And the French (of course) continued to use Paris Mean Time for many years. Until 1834, the Nautical Almanac continued to use Greenwich Apparent Time (Sundial Time) as its "argument" (that is, the time-scale on which its tables were based). The Equation of Time was tabulated so that chronometers, which insist on Mean Time, could be used. The change was made to using Mean time as the argument in 1834, in deference to the increasing number of chronometer-users. But that was for mariners, and astronomers, and didn't make GMT the legal basis of civil time in Britain. Different towns in Britain used a wide range of different times to suit themselves, some Local Apparent Time, some Local Mean Time, some Greenwich Apparent Time. Chaos. As the railways spread, so did "railway time", which was effectively GMT, and one by one, usually under protest, the towns along the lines would adopt it. But GMT didn't get legal sanction as the official time in the UK until 1880. and- >Any book recommendations that cover this topic in detail? Yes. Derek Howse's "Greenwich Time and the Longitude", 1997, pub Philip Nelson. This is recommended over the 1980 ed., which was titled "Greenwich Time and the discovery of the Longitude". And a volume that I keep plugging on this list, "The Quest for Longitude", ed William J H Andrewes, Harvard, 1996. This is the proceedings of a symposium in 1993. Such proceedings are often boring, but this one is NOT. Lots of detailed and interesting stuff about lunars, timekeeping, Harrison, beautifully produced and illustrated. A real pleasure to own, or borrow. My perceptive wife presented me with it as a birthday present soon after it appeared. Very highly recommended, but probably expensive. Brooke Clarke said- >A pendulum clock could be compared to astronomical events and it's rate >determined. This rate would apply after the clock was moved to some >other location as long as the temperature was the same as when it was >calibrated. Not so. A pendulum clock depends on gravity, which varies from place to place, because of the rotation of the Earth and its ellipsoidal shape, among other reasons. Patrick asked- > Could it be that each clockmaker >independently set their own clocks and that any ship's >clock was somewhat relative in time. I would guess not >as to take sights one would need to use a astronomical >almanac using some time standard of the era. Setting the chronometer to read Greenwich Time was not normally done until departure from the port of embarkation, and was the navigator's responsibility, not the clockmaker's. He would usually have been handed the clock in a stopped state. It involved timing transits (ideally of stars) over several nights, from on-land, to establish the rate. Then its indicated time was compared with a standard clock at a local observatory (if such existed), which itself had been set by a transit instrument, or by a set of "time-sights, perhaps on board. Note that the chronometer would not necessarily be set to show GMT exactly: it was sufficient to note the time-error, and the rate. In many cases, the biggest unknown was the exact longitude, with respect to Greenwich, of the local observatory. Later on, the job would be made much easier by the introduction of a dropping time-ball or the one o'clock gun. Some of the stuff in this paragraph was later practice, before the days of the sea-testing of the Harrison clocks. In the trial of Harrison's No 4, to Jamaica via Madeira, in 1761, his son William had determined its rate at Portsmouth, before departure, after 9 days of equal-altitude observations, to be two and two-thirds seconds per day. (see Rupert Gould, "The Marine Chronometer", (1923 and many later reprints), page 56, footnote). George. ================================================================ contact George Huxtable by email at firstname.lastname@example.org, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================