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    Re: Interpolation to latitude
    From: Henry Halboth
    Date: 2009 Nov 12, 22:19 -0800

    Did you really have to ask this one, or perhaps I have just misunderstood your intent?? Regardless, I will answer - if just to show that I am still alive.

    Aboard any ship with which I have been familiar, since 1936, or thereabouts - and that includes American, British, German, Japanese, Chinese, Cuban, Chilean, South African, Panamanian, Liberian, and some that I have long since forgotten, both MM and Navy - GMT was kept by the Chronometer(s), from which time relating to Celestial Navigation was obtained, whether by use of deck watch, stop watch or just plain counting. The ship's daily activities and all clocks associated therewith were regulated to Zone Time, generally advanced or retarded 20-minutes per night watch when passing from one zone to another, so as to maintain some semblance of normalcy in daily living and to be in coincidence with the time being kept at the port of intended destination on arrival there.

    I did sail on one Panamanian Bark in the 1940s, from Panama City to Durban, aboard which LMT was kept and all daily activity clocks were reset at LAN, necessitating resetting to Zone Time on arrival at the port of destination. GMT was, however, kept by Chronometer when it was running. This was an unusual circumstance for the time.

    You can readily appreciate, I am sure, that any other method of time keeping could well result in Lunch in the dark, work in the dark, and Dinner at Meridian Transit, as well as possibly large time changes to coincide with Local Time on arrival at the port of destination.



    --- On Thu, 11/12/09, George Huxtable <george@hux.me.uk> wrote:

    From: George Huxtable <george@hux.me.uk>
    Subject: [NavList 10608] Re: Interpolation to latitude
    To: navlist@fer3.com
    Date: Thursday, November 12, 2009, 7:09 PM

    Joe Schultz wrote-

    "...2. The daily pages of a paper nautical almanac are very elegant.  Enter
    the daily pages with approximate time (left side) or position (right side),
    then extract position (left) or time (right) data.  Then apply some form of
    latitude and longitude corrections to get "exact" position or time.  And the
    daily pages operate in a zone time world, just like we do in our daily


    No. The Nautical Almanac works throughout in GMT, not in Zone Time.

    And that leads me to a question, about the practice of American navigators.
    What do they use, at sea, to take their time from, for navigating?

    Me, I've never been an ocean traveller. My cruising has never taken me out
    of the Greenwich time zone. Mostly, it's been done in Summer, when daylight
    saving puts us an hour ahead of GMT. If I cross the Channel, local time is
    an hour ahead again. But my boat has a bulkhead clock that is always kept at
    Greenwich Time, summer and winter, to be used for all navigational purposes.
    Do Americans do the same, at sea? Do they keep a clock going, for
    navigation, that keeps GMT, and never gets adjusted, running several hours
    ahead of zone time? Not changed when the clocks change, spring and autumn;
    nor when a time-zone boundary is crossed, and always conformimg with times
    that the Almanac uses.

    However, my wristwatch does get switched to conform with zone time. I need
    to know which timepiece to look at, for which purpose.

    I really don't know how many UK sailing people do things the same way as I
    do. Presumably, only those interested in celestial matters. There are no

    There are some awkwardnesses about doing things that way. You can't use the
    bulkhead clock to decide when the news is due on the radio, or when the pub
    will close. Nor for working out the tide predictions, which confusingly (and
    annoyingly) switch by an hour when the clocks do.

    I've noticed that Jeremy's interesting postings contain many references to
    Zone Time, so it would be interesting to discover what US merchant vessel
    practice is, and exactly what happens when they shift into a different time
    zone. I can't say what the practice is in British merchant vessels; does
    anyone know? My guess would be that two chartroom clocks are kept running,
    one at GMT, one at Zone Time.

    Anyway, all that long ramble was for is to point out that to obtain the GMT
    of local sunrise and sunset, from the GMT given in the Almanac, just apply
    longitude at 15 degrees per hour. And if you want it as Zone time, apply the
    appropriate zone correction, in integer hours.


    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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