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    Re: Lunar longitudes, not by lunar distance. Was- Re: Working a lunar
    From: Hanno Ix
    Date: 2009 Aug 7, 20:06 -0700
    Gentlemen:

    I got a copy of Chauvenet Vol 1,2. , 1874.

    In short:

    His goal is to establish positions of international astronomical observatories with an LAT/LOG error in the range of seconds.

    There is indeed an article, nr. 229, in Vol 1 where Chauvenet presents briefly the concept we are discussing here.

    He then goes on to describe methods to deal with a problem of his days: inaccurate ephemeris, particularly of the moon.

    Since he has no list of DT, as we imagine here, he subsequently describes methods of calculating DT as best he can. These methods include an approximate knowledge of GMT and position.

    He finally goes on with a statistical error analysis using the data of several lunations.

    So, I must bow to Chauvenet, or to whomever came before him.

    Modern navigators do not have his problems, though. Assume, a sailor has a transit instrument, a good stopwatch and an almanach that includes DT with sufficient resolution. With those tools and this method, he can find reasonable values of GMT and his position while on land, and can do so without prior knowledge of either.

    At least, that appears to me so at this time. The practical implemetation seems quite possible, however I have not shown that - yet!

    Regards, and thanks for your continued interest.

    H


    --- On Fri, 8/7/09, Brad Morris <bmorris@tactronics.com> wrote:

    From: Brad Morris <bmorris@tactronics.com>
    Subject: [NavList 9403] Re: Lunar longitudes, not by lunar distance. Was- [NavList 9393] Re: Working a lunar
    To: "NavList@fer3.com" <NavList@fer3.com>
    Date: Friday, August 7, 2009, 9:23 AM


    Hang on George,

    Hanno has required that the moon and the other object be ON the meridian at different times, and that the delta time between those meridian passages be the key component.

    I agree with your statement that it is the same for everybody on earth, should that delta time not include the meridian passage.

    Once the meridian passage is included however, then only ONE meridian has that precise delta time, the meridian of observation.  As the earth rotates and the moon changes position, then the delta time will change.

    At your meridian in the UK, the delta time will be X.  By the time those same objects individually CROSS MY MERIDIAN in the US, about 5 hours later, the position of the moon will change by about 2.5 degrees.  Thus the delta time between those objects on my meridian will be different!

    Anyone on your meridian, independent of latitude and under the assumption that they can see them, will get the same delta time.

    Best Regards
    Brad




    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of George Huxtable
    Sent: Friday, August 07, 2009 11:07 AM
    To: NavList@fer3.com
    Subject: [NavList 9402] Lunar longitudes, not by lunar distance. Was- [NavList 9393] Re: Working a lunar


    Hanno Ix wrote, in 9382,

    If I see things right, there must be a LOP which connects all locations
    on Earth with a given, fixed difference DT between the meridian
    passages of sun and moon.

    What I meant is:

    If I see things right, there must be a LOP which connects all those
    locations
    on Earth that have a given, fixed difference DT between the meridian
    passages of sun and moon.

    ================

    Is there woolly thinking here, I wonder?

    Wherever on Earth the Moon is seen from, it's phase is the same except for
    the effects of parallax, which can be corrected for. At Full Moon, it's
    always very nearly 180 º from the Sun. So that's the Sun's lunar distance,
    which is the same wherever on Earth it's observed from. And then the time
    difference between meridian passages of Sun and Moon is 12 hours, the whole
    World over. The phase of the Moon, the lunar distance, and the time
    difference between meridian passages, change in step, going through a whole
    cycle in a month, and at any moment in the month all are the same wherever
    on Earth they are observed from. Measuring a lunar distance, or the time
    between meridian passage (or even the phase, if it was possible to do that
    precisely), all provide the same message: the time of the month. From that
    time of the month, from the nautical tables, we could deduce the day (if
    that was needed) or, more importantly, the time-of-day, in terms of GMT. It
    by comparing that GMT with the local time that we deduce the longitude.

    There's no "LOP which connects all those locations on Earth that have a
    given, fixed difference DT between the meridian passages of sun and moon."
    At any moment that DT is the same everywhere.

    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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