Andrew, Your information and suggestions are very valuable.
They show that, if the clock is set to local solar time, all the other indications are correct. Therefore now the clock has a perfect mechanism for - the movement of the pointer, - the movement of the little sun on the pointer, - the movement of the disk with the zodiac, - the movement of the ring with the Bohemian hours. And the clock has a correct layout of hour lines for unequal hours, hour points and datelines. It is even more regrettable that all this perfection is put aside by the clock setting to CET. Willy Leenders Hasselt in Flanders (Belgium) Visit my website about the sundials in the province of Limburg (Flanders) with a section 'worth knowing about sundials' (mostly in Dutch): http://www.wijzerweb.be Op 1-mrt-2011, om 13:17 heeft Andrew James het volgende geschreven: > -----Original Message----- > Frank King wrote: > > ... Or have I missed something crucial? > > [It is still a splendid clock, but I want to > be clear about its design limitations!] > > ---------- > > I think we must remember that the timekeeping mechanism driving the > astronomical or astrolabic gearing of the Prague clock is mid 19th > century (and it is itself very interesting - I was privileged to visit > the "insides" of the clock some years ago). By that time, a clock > mechanism kept pretty accurate mean time, so that, once regulated, its > accumulated error from mean time would not be noticeable for many days, > especially on a 24-hour dial. > > However its original predecessor(s) - 1410 and later - would have been > very poor at public timekeeping by our standards, with random errors of > the order of several minutes or tens of minutes per day. Hence it would > need regular correction; and the only instrument conveniently telling > the true time to which to correct it was a sundial. So it > *automatically* would be regulated to - and keep - (local) solar time > without any need to compute the equation of time - which although known > about by scholars was then not even tabulated as we know it. > > Sir George White gave a most interesting lecture a few years ago > expounding the proposition that the English lantern clock in the early > 17th century - a very poor timekeeper by modern standards - was in fact > excellently suited to keeping practical time from day to day (and > overnight), which anyway needed to be adjusted to stay in line with > local solar time, as that was then the only standard. > > Fortuitously the time zone GMT+1 is fairly close to local mean time for > the Prague clock as it is at 14d 25m East - so the discrepancy is about > 2 minutes 19 seconds. It would be far worse off in Seville! > > So I suggest that: > > - originally the Prague clock certainly kept local solar time, being > frequently adjusted (corrected) to that from its own (very) approximate > timekeeping: > > - when the local time standard become mean rather than solar it would > have still been frequently adjusted of necessity, but I do not myself > know whether to mean or solar time, nor do I know when mean time was > adopted in Prague: > > - after the mid-19th century work it would naturally keep mean time > quite well but could have been adjusted to local solar time if desired: > > - after the adoption of time zones it may or may not have been adjusted > to civil time: > > - and now it is adjusted to CET (UTC+1). > > I suspect the convenience and motivation for keeping civil time is > mostly so that tourists know when to expect the automata to perform, > rather than because viewers will use it to set their watches. > > Andrew James > > > > Secure Meters (UK) Ltd. is a registered company in England: 2199653 > Secure House, Moorside Road, Winchester, SO23 7RX > > This correspondence is confidential and solely for the intended recipient(s). > If you are not the intended recipient, you must not use, disclose, copy, > distribute or retain this message or any part of it. If you are not the > intended recipient please delete this correspondence from your system and > notify the sender immediately. > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial >
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