<p>I want to convert some Cartesian vectors between the Geocentric Celestial Reference Frame (GCRF) and Earth-centered J2000 frame. I know that these two frames are nearly identical and, for most purposes, can be treated as the same, but I'm still interested in the conversion.</p>
<p>What code/tool/other thing can I use to do this conversion? I tried astropy, starting with the GCRS frame, but I couldn't find something for the Earth-centered J2000 frame.</p>

CC BY-SA 4.0
2024-11-15T02:20:40.157Z
From https://ssd.jpl.nasa.gov/horizons/manual.html#frames "The ICRF was constructed to closely align with the older FK5/J2000 dynamic reference frame – within the uncertainties of that system – while replacing dynamical definitions based on intersecting and moving planes with ones based on the more precisely determined and fixed radio source positions [...] the ICRF is thought to differ from the previous FK5/J2000 dynamical system by at most 0.02 arcseconds".
PM 2Ring
CC BY-SA 4.0
2024-11-15T02:26:18.437Z
So it's tricky. ;) You need to know exactly how your alleged J2000 frame data defines the equatorial plane / polar direction (as well as the equinox direction), but older definitions weren't totally unambiguous. There's a lot of related info (with links) on https://astronomy.stackexchange.com/q/53193/16685 Also see https://space.stackexchange.com/a/55177/38535 Sorry, that doesn't answer your question, but at least it explains why there is no easy answer.
PM 2Ring
<positional-astronomy><astropy><frame-of-reference>
Tool to convert between GCRF and (Earth-centered) J2000 reference frames
NeutronStar
# Tool to convert between GCRF and (Earth-centered) J2000 reference frames

(Asked by: username_0 on 2024-11-15)

I want to convert some Cartesian vectors between the Geocentric Celestial Reference Frame (GCRF) and Earth-centered J2000 frame. I know that these two frames are nearly identical and, for most purposes, can be treated as the same, but I'm still interested in the conversion.

What code/tool/other thing can I use to do this conversion? I tried astropy, starting with the GCRS frame, but I couldn't find something for the Earth-centered J2000 frame.

**Tags:** positional-astronomy, astropy, frame-of-reference
> **Comments:**
> **username_1**: From https://ssd.jpl.nasa.gov/horizons/manual.html#frames "The ICRF was constructed to closely align with the older FK5/J2000 dynamic reference frame – within the uncertainties of that system – while replacing dynamical definitions based on intersecting and moving planes with ones based on the more precisely determined and fixed radio source positions [...] the ICRF is thought to differ from the previous FK5/J2000 dynamical system by at most 0.02 arcseconds".  
> **username_1**: So it's tricky. ;) You need to know exactly how your alleged J2000 frame data defines the equatorial plane / polar direction (as well as the equinox direction), but older definitions weren't totally unambiguous. There's a lot of related info (with links) on https://astronomy.stackexchange.com/q/53193/16685 Also see https://space.stackexchange.com/a/55177/38535 Sorry, that doesn't answer your question, but at least it explains why there is no easy answer.  

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## 0 answers

