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    Re: Tinyac almanac program for Windows
    From: Paul Hirose
    Date: 2010 May 18, 13:37 -0700

    Dave Walden wrote:
    > More playing: Set long&lat = 0 0 0, (lat seems to effect geocentric LHA? 
    > thinking about that one.) coord sys to "apparent geocentric geodetic 
    > (LHA)", gregorian, UT1, de405, and for 9 may 2009 18h, I get back NA 
    > values to NA accuracy.
    
    When comparing Tinyac values against an almanac, it's not necessary to
    set the observer's position. This omission will disable some time and
    coordinate frame options, but they aren't needed.
    
    For example, to read the Greenwich hour angle of a body, select the 
    "terrestrial" button in the Coord Sys dialog. This aligns the coordinate 
    frame to the geodetic latitude / longitude system. (Perhaps this would 
    have been clearer if I'd marked the button "geodetic".) By default the 
    angle goes from -180° to +180° (east positive), but the Coord Sense 
    sub-dialog lets you change the sense of the angle to 0 - 360°, 
    increasing west.
    
    That setting is used to compute GHA Aries too. There's no way to read 
    GHA Aries directly, but you can find it by adding the GHA and RA of the 
    same body.
    
    You also have the option to select hours as the unit of measure
    instead of degrees. One use for GHA in time units is the determination
    of equation of time. For example, what is the equation of time at 1798 
    Jan 1 12 h GAT? Set time to 12h UT1 (not GAT!), read GHA Sun = 
    +0h04m14.7s (select the east+/west- button to read GHA in that format). 
    That's a first approximation to the equation of time. Change the time to 
    12h04m14.7s UT1, read GHA Sun = +0h00m00.2s. Add .2s to UT1, and this 
    time the result is perfect. The equation of time is 4m14.9s, UT1 ahead 
    of GAT. The 1798 Nautical Almanac is online, and its value agrees within 
    .2s:
    
    http://books.google.com/books?id=1PcNAAAAQAAJ&pg=PR2
    
    That's better than I expected. Of course the precise definition of 
    Greenwich mean time (what we now call UT1) has changed many times over 
    the centuries, and I've read warnings that historic investigators need 
    to take that into account. In the case of Tinyac, for instance, even 
    when you input time in the Greenwich apparent time scale, GAT is 
    converted to UT1 by iteratively computing the equation of time. The 
    algorithm is similar to the one I used above. Then delta T is applied to 
    obtain Terrestrial Time. When UT1 and TT are known, the program has what 
    it needs to compute the position of a body.
    
    In my 1798 example, it looks like the modern UT1 implementation has 
    practically perfect continuity with the old mean time. But one data 
    point means little. I'll investigate this further.
    
    -- 
    I filter out messages with attachments or HTML.
    
    
    
    

       
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