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    Re: Parallax of the Moon
    From: Gary LaPook
    Date: 2015 Feb 7, 08:13 -0800
    Hey Stan, don't be too hard on yourself, we've all been there.


    From: Stan K <NoReply_StanK@fer3.com>
    To: garylapook@pacbell.net
    Sent: Saturday, February 7, 2015 2:11 AM
    Subject: [NavList] Re: Parallax of the Moon

    Please ignore all messages from me with the subject "Parallax of the Moon".  To start with, I did not give all the information required in the original question.  Then.more mistakes.  I should not be doing this when I should be sleeping.  Now I almost have my head removed from my butt, but not quite.


    -----Original Message-----
    From: slk1000 <slk1000---.com>
    To: slk1000 <slk1000---.com>; navlist <navlist@fer3.com>
    Cc: ronandsandy <ronandsandy{at}olympus.net>
    Sent: Sat, Feb 7, 2015 4:44 am
    Subject: Re: Parallax of the Moon

    Uh oh...  Now I'm not so sure.

    -----Original Message-----
    From: slk1000 <slk1000---.com>
    To: navlist <navlist@fer3.com>
    Cc: ronandsandy <ronandsandy{at}olympus.net>; slk1000 <slk1000---.com>
    Sent: Sat, Feb 7, 2015 2:47 am
    Subject: Re: Parallax of the Moon

    I asked this question a couple of days ago and I cannot believe I have not received one response.  Dozens of responses on "Irregular Quadrilateral Center" and "Japanese Sextant 1979", but none on this question.  Well, last night I was sitting on the couch watching Hawaii Five-0 when I had an epiphany.  I was using the formula on page 280 of the Nautical Almanac, PA = HPcosH+OB, where HP is the horizontal parallax, H is the apparent altitude (sextant altitude corrected for index error and dip, H = Hs + I - D), and OB is the correction for the oblateness of the Earth.  It might be said that the first part of the formula, HPcosH, is just an approximation, but it is a very good approximation.  The problem is much more basic than that.  THE FORMULA IS INCORRECT!  Now I will play Frank Reed and challenge you to tell me why.  It is really pretty simple once you give it a little thought.


    -----Original Message-----
    From: slk1000 < slk1000---.com>
    To: navlist < navlist@fer3.com>
    Cc: slk1000 < slk1000---.com>
    Sent: Thu, Feb 5, 2015 6:07 pm
    Subject: Parallax of the Moon

    The attachment is part of an output from the USNO Celestial Navigation Data web page.  Can anyone tell me how the USNO gets a parallax value of 15.6'?  Every other source I tried got about 15.27', but 15.6' is the only one that makes the numbers agree with the Moon Altitude Correction Tables in the Nautical Almanac.


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