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    Re: chronometer dials: 12 or 24 hours?
    From: Clive Sutherland
    Date: 2009 Jan 19, 19:49 -0000

    Changing the clock face from 12 to 24 only substitutes one pitfall for
    Putting 24 numbers on the dial begs the question. What are you going to do
    with the Minute hand?
    If 60 minutes are to be one complete turn, which up to now is 12 hours The 1
    hour figure corresponds to 5 minutes and the 6 hours figure is the same as
    30 minutes or Half an Hour. An arrangement we and our forbears are and were
    comfortable and familiar with,
    A new arrangement of a 24 hour dial, assuming the minute train remains the
    same, would have the 1 Hour figure the same as 2 1/2 minutes and Half an
    hour being represented by 12 Hours instead of 6. 6 would be where 3 was
    before representing 'Quarter of an hour' 
    It might be done differently however. For example the minute hand being
    slowed by a factor of two so that the Half an hour would be kept at figure
    6. but there would be two half hour points on the dial, one at 90 degrees
    and another at 270 degrees.
    It could be that different ship might use different mechanisms.
    It seems to me that both these two options will incur risk of confusion. 
    Keeping the clock as it is, has a significant merit in that a 12 hour
    blunder produces such a wild position, the error is not likely to be 
    accepted and an immediate correction made. Not so with the  errors of
    wrongly reading the minutes hand.
    The only fool proof improvement would be to go to a digital dial.
    > -----Original Message-----
    > From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf
    > Of frankreed@HistoricalAtlas.com
    > Sent: 19 January 2009 04:03
    > To: NavList@fer3.com
    > Subject: [NavList 7112] Re: chronometer dials: 12 or 24 hours?
    > Paul you wrote:
    > "Why not? Chauvenet's point seems logical to me."
    > It is logical. Custom is swayed by logic only when it improves the
    > practical result. Chauvenet was a mathematician with some limited sea
    > experience, but he was not a practical navigator. I believe that the
    > concern he has would only rarely arise in practice.
    > An ocean voyage is a continuous activity, and in the 19th century, a slow
    > one. Each day at noon, you shoot your LAN sight and simultaneously record
    > your DR longitude. That DR longitude immediately tells you the approximate
    > Greenwich time, so long as you don't foul up the rules about east and west
    > longitude. So as I sail west across the Atlantic and my longitude climbs
    > to 15, then 30, 45, 60 degrees west, I know on each of those days at local
    > apparent noon that the local apparent time back at Greenwich is 1pm, then
    > 2pm, 3pm, 4pm respectively. And that will be the time on my chronometer at
    > noon near enough (even closer if I throw in the equation of time).
    > This estimation of GAT/GMT was a standard part of the noon sight in many
    > navigation methods including the ones recommended by Bowditch. Of course,
    > to calculate the noon sight correctly, you need the Sun's declination and
    > this depends on the approximate Greenwich Time. So one of the first steps
    > in working the sight is to take your longitude by account and convert it
    > to Greemwich Time. They did this every day. Assuming that the navigator
    > who does the noon Sun sight is the same as the one who works other sights
    > (morning or afternoon time sights specifically), it's hard for me to
    > imagine a case where they would look at the chronometer and get the hours
    > wrong by twelve. Additionally, if time sights are taken about the same
    > time of day, e.g. 9am local time, the corresponding Greenwich Time on the
    > chronometer will only change by a few minutes each day. There would never
    > be an opportunity to skip 12 hours. But maybe I'm just not visualizing the
    > right sort of case...
    > Losing the date, through an accounting error, strikes me as a bigger
    > problem than losing twelve hours. For a vessel that visited a lot of
    > ports, switching from "Sea days" to "Civil days" and back, maybe crossing
    > the dateline for good measure, you could end up with the wrong day of the
    > month. It would be easy to miss this until you compared notes with another
    > vessel, as they often did ("speak" other ships to compare position and
    > time information, that is).
    > -FER
    > > 
    > No virus found in this incoming message.
    > Checked by AVG - http://www.avg.com
    > Version: 8.0.176 / Virus Database: 270.10.9/1900 - Release Date: 1/18/2009
    > 12:11 PM
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