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    Re: Coordinates on Cook's maps
    From: Paul Hirose
    Date: 2007 Apr 18, 11:51 -0700

    George Huxtable wrote:
    >
    > It's easy to convert between those ecliptic coordinates and the RA / dec of
    > a modern prediction, for which you need an accurate value of the Earth's
    > tilt of axis, at that date. And you need to be aware that the time scale of
    > the old almanacs (up to 1834) was in terms of apparent time not GMT, so the
    > Sun really was exactly on the Greenwich meridian at noon. For comparison,
    > modern Moon predictions have to be taken, not for noon and midnight UT, but
    > offset from that by the equation of time.
    
    JPL's online HORIZONS system can give the Moon's apparent geocentric
    ecliptical coordinates directly.
    http://ssd.jpl.nasa.gov/?horizons
    
    Copy and paste the following lines (starting with !$$SOF and ending with
    !$$EOF) into an email. Use JOB for the subject and send to:
    horizons{at}ssd.jpl.nasa.gov
    (Do not fill in the blank after EMAIL_ADDR. By default HORIZONS will
    return its result to the same address that sent the command.)
    
    !$$SOF (ssd)       JPL/Horizons Execution Control VARLIST
      EMAIL_ADDR = ' '
      COMMAND    = '301'
      OBJ_DATA   = 'NO'
      MAKE_EPHEM = 'YES'
      TABLE_TYPE = 'OBS'
      CENTER     = 'geo'
      START_TIME = '1776-Jul-4 00:00'
      STOP_TIME  = '1776-Jul-4 21:00'
      STEP_SIZE  = '3 hours'
      QUANTITIES = '31'
      ANG_FORMAT = 'deg'
      APPARENT   = 'AIRLESS'
      TIME_DIGITS = 'MIN'
      TIME_ZONE   = '+00:00'
      SUPPRESS_RANGE_RATE= 'NO'
      ELEV_CUT   = '-90'
      SKIP_DAYLT = 'NO'
      SOLAR_ELONG= "0,180"
      AIRMASS    = '38.0'
      EXTRA_PREC = 'NO'
      CSV_FORMAT = 'NO'
      R_T_S_ONLY = 'NO'
    !$$EOF++++++++++++++++++++++++++++++++++++++++++++++++++++++
    
    
    The system is fast; by the time I can get to my inbox and check mail,
    the reply is waiting.
    
    You should get back a table of the Moon's longitude and latitude for one
    day. The time scale is UT1. George says the old almanacs used apparent
    time, and I see no way to get HORIZONS to calculate with that time
    scale, or give the equation of time. However, it can give the local
    apparent solar time at Greenwich with this command:
    
    !$$SOF (ssd)       JPL/Horizons Execution Control VARLIST
      EMAIL_ADDR = ' '
      COMMAND    = 'sun'
      OBJ_DATA   = 'NO'
      MAKE_EPHEM = 'YES'
      TABLE_TYPE = 'OBS'
      CENTER     = 'Greenwich'
      START_TIME = '1776-Jul-4 00:00'
      STOP_TIME  = '1776-Jul-4 21:00'
      STEP_SIZE  = '3 hours'
      QUANTITIES = '34'
      ANG_FORMAT = 'deg'
      APPARENT   = 'AIRLESS'
      TIME_DIGITS = 'MIN'
      TIME_ZONE   = '+00:00'
      RANGE_UNITS = 'AU'
      SUPPRESS_RANGE_RATE= 'NO'
      ELEV_CUT   = '-90'
      SKIP_DAYLT = 'NO'
      SOLAR_ELONG= "0,180"
      AIRMASS    = '38.0'
      EXTRA_PREC = 'NO'
      CSV_FORMAT = 'NO'
      R_T_S_ONLY = 'NO'
    !$$EOF++++++++++++++++++++++++++++++++++++++++++++++++++++++
    
     From that you could determine the UT corresponding to a given apparent
    time, and use that to generate the Moon's position. It's not the easiest
    way to work, but at least HORIZONS is free and very accurate.
    
    If I were researching the accuracy of those almanacs, I'd write software
    to take the input directly in apparent time.
    
    --
    I block messages that contain attachments or HTML.
    
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