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  • $\begingroup$ My trepidation comes from the use of "frame" in relativity, where we also have time to consider, but then again perhaps time is a coordinate? Also, aren't precession, nutation, and polar motion simply behaviors that "coordinate systems centered on the geocenter... rotating with the planet" would exhibit and not really separate things from them? $\endgroup$
    – uhoh
    Commented Nov 7, 2020 at 0:58
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    $\begingroup$ Most orbit mechanics problems don't worry about special relativity :D. For ECEF precession, nutation, and define the axis of rotation of the frame (coordinate system?). Polar motion defines the alignment of the physical body, and hence the coordinate frame, relative to the axis of rotation. And the UT1 offset defines the variability of the rotation rate. $\endgroup$
    – Carlos N
    Commented Nov 7, 2020 at 1:03
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    $\begingroup$ Certainly in the engineering world of shuttle ops "frame" = "coordinate system" so my experience agrees with yours. $\endgroup$
    – Organic Marble
    Commented Nov 7, 2020 at 1:15
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    $\begingroup$ The SPICE frame tutorial and Navipedia have useful definitions of frames and coordinates systems, particularly slide 4 of the SPICE tutorial. I also recommend the SOFA cookbook on Earth Attitude for precession, nutation and earth orientation. The modern way is to think of these as a matrix, changing with time, which describes where Earth's pole is right now $\endgroup$
    – astrosnapper
    Commented Nov 7, 2020 at 1:56
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    $\begingroup$ Not sure, the use of ECI and ECEF seems to be much more prevalent in the near-Earth/satellite communities - in astronomy, I've never really come across them being used, only ITRF/ITRS. But as far as tags go, I would consider eci and ecef as examples of frames. Maybe frame, frame: eci and frame: ecef tags ? $\endgroup$
    – astrosnapper
    Commented Nov 7, 2020 at 23:54