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    Astronomy: Calculating Positions
    From: Marcel Tschudin
    Date: 2008 Apr 6, 16:45 +0300

    Dear Group Members
    
    I have some questions related to Parallax and flattened earth shape
    when calculating positions of astronomical objects (sun, moon and
    planets).  If I remember right there are certain members in this group
    having their own programs for such calculations. I start to pose my
    question here but would be prepared to continue off list if the
    subject should be beyond the general interest of the group.
    
    My question relates to the conversion from geocentric to topocentric
    coordinates. The program which I'm using for the sun claims an
    accuracy of about 1 sec of arc. Comparing apparent geocentric sun
    positions with those of CalSky (www.calsky.com) show a standard
    deviation of about 2.5 sec of arc. This indicates that what I'm doing
    is about right until the conversion from geocentric to topocentric
    cooordinates.
    
    For converting geocentric to topocentric coordinates one has to allow
    also for the difference in parallax and the difference in latitude due
    to the flattened earth. The function which performs the coordinate
    transformation does this for a sphere, it thus does not consider those
    two effects. To my understanding the two corrections have therefore to
    be made either before or after the transformation. Unfortunately I
    couldn't find in any of my textbooks how the two corrections have to
    be considered in the context of this transformation. The textbooks
    only indicate how to calculate the (maximal?) size of the effects by
    showing a figure with a cross-section corresponding to the object
    (sun) passing the meridian; but how is the situation with the object
    at an othe altitude, like near the horizon (sunrise or sunset)? I
    therefore presume that the corrections have to be applied before doing
    the transformation but I don't know how. May be correcting the
    declination with the Parallax and the observer's latitude with the
    difference to the flattened earth before entering the transformation?
    
    Any help or a hint to where I might find help is welcome. Thank you.
    
    Marcel
    
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