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    Re: Interpolation to latitude
    From: Gary LaPook
    Date: 2009 Nov 13, 03:16 -0800

    I use my Torgoen T5 watch. It gains four second a year. It has the
    normal watch hands plus a separately settable 24 hour hand and a 24 hour
    clock face. You can reset the 24 hour hand without affecting the setting
    of the other hands, the watch keeps running while resetting the 24 hour
    hand so doesn't lose its accuracy. I normally keep the watch set to zone
    time and the 24 hour hand set to Zulu. If traveling for just a few days
    I will leave the watch set to my home zone and set the 24 hour time to
    the zone in which I am traveling. On my recent voyage across the
    Atlantic, since I would be crossing many zones and doing celnav, I set
    the watch to Zulu and then adjusted the 24 hour hand to the zone times
    being kept on board since I could make these adjustments without losing
    Zulu time on the other hands. Here is a picture of the watch:
    George Huxtable wrote:
    > 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
    > lives."
    > ===================
    > 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
    > rules.
    > 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.
    > George.
    > 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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