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    To Gordon Talge Re:Lunars
    From: Mike Burkes
    Date: 2002 Oct 15, 17:25 -0700
    From Mike: Hi George! In response to your statement regarding modern Lunar Distance explanations. The Navigation Foundation Articles 54 to 56 by John Luykx has info on Lunars with theory and a sample problem with solutions. If you need more info my email:m_burkes---.com. Thanks!
     
    ----- Original Message -----
    From: Arthur Pearson
    Sent: Sunday, October 13, 2002 7:02 PM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude
     
    Bruce,

    Thanks for your comments on this procedure; they are specific enough
    with respect to the sequence of tasks to allow us to try it the next
    time the opportunity presents itself. Your tables take care of the lunar
    and the almanac converts arc to time, the next challenge for me is
    developing the formulas and a form with which to derive LAN from the
    timed sun sight.

    I look forward to reading your method for calculating the altitudes
    without exact time.

    Regards,
    Arthur

    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM] On Behalf Of Bruce Stark
    Sent: Saturday, October 12, 2002 1:35 PM
    To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    Subject: Re: Use of Sun Sights for Local time, and Lunars for Longitude

    Good posting from Arthur and George. Anyone interested in the navigation
    of
    that era should go over that posting several times to absorb not only
    what
    they've said, but the implications.

    The procedure Arthur suggested will work today for someone who's lost
    Greenwich time and is uncertain of the dead reckoning. I'll lay that out
    in
    more detail:

    Take and work a time sight in the morning. You won't get accurate time
    from
    it, not unless the sun was due east, or the DR latitude correct, but
    you'll
    get it close enough to know, within a few minutes, when to start
    monitoring
    the sun's altitude for the noon latitude. Anyone who's tried to get noon
    latitude with only vague idea of when the sun will "dip" will appreciate
    the
    value of this first, approximate, working of the time sight.

    Once you've got the correct latitude, work the time sight again to find
    exactly how fast or slow the watch is on local apparent time. Be sure to
    write that down because, until you get the next time sight, it will be
    the
    basis of your calculations.

    That's all the regular "sight reduction" you'll have to do, and the only
    thing you took from the Almanac was the sun's declination. At the very
    most,
    that never changes more than 1' per hour. Your estimated GMT would have
    to be
    a long way from the truth to get you in trouble.

    Now let's say you get a set of distances of the sun from the moon. If
    you are
    able to take the altitudes before and after the set of distances you can
    go
    ahead and clear it, find, from the Almanac, the GMT that fits, and apply
    the
    equation of time to convert GMT to GAT.

    To the average watch time of the lunar observation apply the correction
    you
    found with the time sight: so many hours minutes and seconds to be added
    or
    subtracted to convert watch time to local apparent time. The difference
    between the LAT and GAT of the lunar is the longitude of the place where
    you
    took the time sight.

    Keep that in mind. The time you're using is specific to the meridian
    where
    you took the time sight, so the longitude you find is specific to that
    meridian also.

    You've found latitude and longitude, and the lack of accurate GMT was no
    hindrance whatever in working the observations. Besides the noon
    latitude and
    lunar, which took no more work than if you'd had accurate GMT, you've
    worked
    a time sight twice, using different latitudes. That's exactly what was
    required in order to plot one Sumner line, using an accurate
    chronometer.

    Modern navigators find this hard to swallow. In the system they've been
    taught, everything is founded on, and must begin with, accurate GMT.
    They've
    come to accept, as a bedrock truth, that to work observations
    successfully
    you have to have accurate GMT. If you don't have it the only hope, in
    their
    view, is to flounder toward it by iteration.

    So much for now.

    In case a list member is wondering what to do when he can't measure
    altitudes
    for the lunar, I recently stumbled on a way of calculating them that
    fits
    present procedures, but is no more dependent on accurate GMT than the
    method
    posted under "It Works." Given time, I'll explain it soon.

    Bruce
       
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