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    Long and Time at Sea
    From: Gordon Talge
    Date: 2005 Jun 4, 11:52 -0700

    According to J.W. Norie's "Complete Epitome of Practical Navigation"
    1844, there are several ways of finding Longitude at sea.
    A short listing of the table of contents shows:
    On finding Longitude by Observation
    To find the apparent or mean time by an Altitude of the Sun
    "                                                  " a star.
    "                                                 " a planet.
    "                                                 " the moon.
    To find the error of a watch or chronometer by equal altitudes
    of the Sun.
    On Finding Longitude by Chronometers
    On Finding Longitude by Lunar Observation
    To find the apparent Altitude of the Sun, Moon, Star, or Planet.
    So they had various ways to find their Longitude.
    It seems to me that at that time (1844) and for years to come,
    finding Latitude and Longitude were related, but separate problems.
    There wasn't any "line of position" as we know it today.
    The "Noon Sun Shot" was a staple. However, due to the fact that the
    Sun tends to "hang" in the sky on the meridian with little apparent
    change in altitude means that finding longitude by noting the time
    of passage, while in theory should work, in reality is not practical.
    One method has the observer, taking a shot at maybe 10 minutes before
    LAN and noting the altitude and the watch time, doing a noon shot
    and resetting the sextant to the altitude that was noted 10 minutes
    before and watching the Sun until it is at this same altitude and
    noting the watch time. Since the Sun must have been at LAN midway
    between these two times, you then calculate that time and using the
    equation of time from the NA get your longitude.
    There is more to it then just time, but it is the basic idea.
    In the tropics with the Sun at close to 90 degrees in altitude at
    LAN is would seem to me to almost impossible to get an Azimuth or
    decent altitude. Maybe a Moon, planet or Star shot would work
    I also tend to thing that unless you are close to land, that knowing
    your exact position is really not that big of a deal. If you are
    out in the middle of no where, an error of 20 or 30 miles would be
    no big deal. Close to land, however would be another story.
    Voyages took weeks and there were plenty of opportunities for
    and corrections if the DR was kept up.
    My $.02
    -- Gordon
                                 (. .)
    | Gordon Talge WB6YKK         mail: gtalge AT silcon DOT com |
    | (o-    Debian / GNU / Linux                                |
    | //\    The Choice of the GNU Generation                    |
    | v_/_                  .oooO                                |
    |    - E Aho Laula -     (  )   Oooo. - Wider is Better -    |
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                               \_)  ) /

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