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    Re: Interpolation to latitude
    From: George Huxtable
    Date: 2009 Nov 13, 00:09 -0000

    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.
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
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