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Thread: Understanding reference planes for NASA's "Horizons" tool?

  1. #1 Understanding reference planes for NASA's "Horizons" tool? 
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    Sep 2013
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    Hi,

    I'm trying to model the solar system by sticking in the Sun at [x,y,z,vx,vy,vz] = [0,0,0,0,0,0] and sticking in all the orbiting bodies with initial data from NASA's Horizon tool. I'd then set these off like clockwork and hope the model was accurate enough to keep their positions at least mildly consistent with another Horizon reading, say, a year later. However, I'm having trouble understanding certain aspects of the system.

    I've set 'Ephemeris Type' to 'Vectors', 'Coordinate Origin' to 'Sun (Body Centre)', and will set 'Target Body' to each major body in the solar system in turn. 'Time Span' would presumably not matter as I just want a single starting point in time this year to launch my model from, so 'Table Settings' is the issue.

    I've set the 'Quantities code' in this to 'State Vector (x, y, z, vx, vy, vz)', and 'Type' to 'Geometric states'. It seems like these should be correct. The one I don't understand is 'Reference Plane'.
    'Ecliptic and mean equinox of reference epoch'
    'Earth mean equator and equinox of reference epoch'
    'Body mean equator and node of date'

    None of these seems to correspond to a simple cartesian form (I suppose because Cartesian coordinates aren't good for any reasonably complex space problem), and I'm having trouble understanding how to convert one of them to what I want, or even which one to focus on trying to understand. Wikipedia is a bit full on with the jargon (I've only very recently started studying space in my course), and I'm having a bit of difficulty trying to find simple explanations of them to help me convert them mathematically.

    So, my questions are:
    1) Which of the above three reference planes would be the one to focus on and read about so I can transform it into Cartesian coordinates?
    2) If possible, could you point me in the right direction to somewhere I can read about that reference plane and get a solid understanding of it in a self-contained explanation?
    3) Am I going about this totally the wrong way? Should I be trying to model the solar system in one of these coordinate systems instead? I plan to have a gravitational force act between each body in the solar system and model it in simple SUVAT motion steps of an hour or minute or day, and I feel like Cartesian coordinates would make this much simpler (if a little slower...)

    Thanks for your time. Sorry for not including links to Horizon & my settings, my first attempt at posting this got wiped for having URLs in it as my first post


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