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Re: Lunars: Jupiter's BIG.
From: Fred Hebard
Date: 2003 Dec 24, 11:51 -0500
From: Fred Hebard
Date: 2003 Dec 24, 11:51 -0500
I also must thank George for his expansion of his point that lunars would be inadequate for rating chronometers. I agree with his conclusion. On Dec 24, 2003, at 11:14 AM, George Huxtable wrote: > Fred Hebarf wrote- >> In answer to various claims about the accuracy of lunars, it would >> seem >> to me that the error in lunars should approach the precision of the >> sextant, given enough measurements of decent quality and decent >> reduction procedures. That would be 0.1 to 0.2' of arc, or 12-24 >> seconds. > > That may be what Fred can reliably achieve on land, with a stable > platform > beneath his feet. It's rather better than could be expected at sea, in > the > size of vessel that was used in the heyday of lunars. > > > Replying to Fred's comment that- > >>> "I believe that to _rate_ a chronometer one needs at least three >>> lunars spread over at least three days. " > > Frank Reed replied- > >>> Just one lunar will do. When you leave port, you know your >>> chronometer's error (assuming it's a port with a well-established >>> longitude). Let's suppose it's sixty seconds slow as you depart. >>> After >>> five months at sea, you get some measure of your longitude. This >>> could >>> be from speaking another ship, from visiting a port, OR from shooting >>> a lunar. Suppose your chronometer now appears to be 4 minutes fast. >>> That means it's gaining 1 minute per month. That's the rate. > > Finding the current rate isn't quite as simple as that. Let's say that > at > departure the rate had been determined to be zero; neither gaining nor > losing, on the basis of, say, time-gun signals over a day or perhaps a > few > days. And the error in the chronometer time, at departure, happened to > be > 60 seconds behind Greenwich. And after five months, as Frank presumes, > the > time error is found to be 4 minutes ahead of Greenwich. We can agree, > then, > that the AVERAGE rate over that 5-month period has been 1 minute per > month, > gaining. But that's no more than history, water under the bridge. What > we > need to know is what is the rate NOW, in order to extrapolate > chronometer > error into the future. > > If the rate had been changing smoothly and steadily, from zero at > departure, and the average rate over the period was 1 minute per month > (gaining), then its current rate at the end of the period would be 2 > minutes per month (gaining). It's a big assumption, of course, that the > rate would change smoothly and steadily. > > The point I am trying to make is that in order to RATE a chronometer, > rather than simply establish its error, one needs to find the rate AT > OR > NEAR THAT MOMENT, not what it has been averaging over a previous > passage. > For this to be done, one needs to be back in harbour, with a > time-source > that's precise to the second or better, and the longitude held > constant. It > doesn't even require a harbour with known longitude:just any old > anchorage > will do, using successive time-sights over a day or days, to determine > the > RATE, though to find the clock-error then requires a spot with known > longitude. > > The trouble with using a lunar to determine rate is that because each > measurement is so inaccurate (to a minute or two of time) then any > determination of rate over a short interval is hopelessly imprecise. > > George > > ================================================================ > contact George Huxtable by email at george@huxtable.u-net.com, by > phone at > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > ================================================================ >