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Re: How Many Chronometers?
From: Gary LaPook
Date: 2009 May 11, 11:17 +0200
From: Gary LaPook
Date: 2009 May 11, 11:17 +0200
Frank wrote: N=2: Some folks argue logically that two chronometers is no better than one since, if they disagree, you have no way of knowing which one is wrong. That's perfectly true, in theory, but in practice two chronometers back each other up. If they agree, you very likely have nothing to worry about, even if you have no time check for months on end. And if they suddenly disagree, THEN you have good cause to use other means to get a time check. Hail another vessel, or get on the radio and ask, or listen for a radio station with an established reputation for an accurate time tone at the top of the hour (e.g., I can pick up WCBS in New York from hundreds of miles away at night). First, of course, you must apply the known rate of each chronometer before making the comparison. If they suddenly start to disagree then you know that one of the rates has changed since it is unlikely that both decided to go off at the same time. But which one? They way to handle this is to work the sight using one of the chronometers and then to also plot the fix after adjusting its longitude for the difference in the two times. Then plan your next course and actions based on which of the two fixes places you closest to danger. Then plan the landfall as "latitude sailing" aiming far enough off so that even the most critical fix does not place you in danger. And there is much to be gained in peace of mind when they do agree since it is very unlikely that they both failed in the same way allowing them to stay in step even though they are both wrong. gl frankreed@HistoricalAtlas.com wrote: > Greg, in the opening of this thread, you wrote: > "When a time tick or other master reference is not available then by what means can a navigator at sea use to determine whether a chronometer is maintaining a consistent rate. If two chronometers are carried then an inconsistency between the two would suggest a problem but then which chronometer is at fault? How about a third chronometer to help the navigator determine which chronometer is behaving badly. Carrying three chronometers does triple the chances of one going down but since quartz watches are cheap why not carry three?" > > My interpretation of your question may be a little different from the other replies you received. I read this message to mean that you were asking about the general issue of N chronometers. Here's some thoughts... > > N=1: > You can't trust it! Some people in the early days of chronometers came very close to arguing just that. But that's surely over-kill. A better policy would be to bear in mind that there is some steadily increasing probability that the chronometer has gone wrong. How much probability? Well, that depends on the chronometer and the user. Modern quartz watches have such high reliability that the probability is negligible on any voyage less than many months away from land or contact with other mariners, and when would that every happen in this century? But 150 years ago, even after a month, you might want to consider a significant probability that the rate has changed. > > N=2: > Some folks argue logically that two chronometers is no better than one since, if they disagree, you have no way of knowing which one is wrong. That's perfectly true, in theory, but in practice two chronometers back each other up. If they agree, you very likely have nothing to worry about, even if you have no time check for months on end. And if they suddenly disagree, THEN you have good cause to use other means to get a time check. Hail another vessel, or get on the radio and ask, or listen for a radio station with an established reputation for an accurate time tone at the top of the hour (e.g., I can pick up WCBS in New York from hundreds of miles away at night). > > N>=3: > Things get interesting as we go to higher numbers. You can trust the group in the set that agree, right? But the probability of a failure of one chronometer increases with each additional instrument that you carry. So if we have two that agree within a second, and one that is ten seconds different, what should we do? With traditional mechanical chronometers, you might get a sudden fixed jump in rate which would be detectable as a steady drift in the error of that aberrant chronometer as the days pass. We can then toss that one. Or should we assume random-walk errors and just average up all the indicated times? That might be a good policy, but then again, if the chronometer stops dead, you know you wouldn't average in its reading. So there's a limit to the averaging policy, too... > > This is perhaps all "academic" today since quartz watches are so excellent, but I still agree with your comment that 'since they're cheap, why not carry more?' The probability of a change in rate of a single quartz watch may be so very low that it's not worth noticing, but there's still a chance you could drop your sextant on it. > > -FER > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---