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Does anyone know where the solar barycentre is in the old equatorial (based on Earth's position on midday, 01/01/2000) system? I want to add an ICRS option for a 3d model I'm working on. EDIT: degrees would be ideal (0h = 0 degrees, 12h = 180), but RADEC works too.

Kazon
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  • Typical SSB (Solar System Barycentre) plots are relative to the Sun, eg https://astronomy.stackexchange.com/a/28036/16685 & https://astronomy.stackexchange.com/a/44903/16685 But Horizons will happily supply vectors for the SSB relative to Earth, if you want. – PM 2Ring Jul 15 '23 at 05:14
  • Eg, https://ssd.jpl.nasa.gov/api/horizons.api?format=text&EPHEM_TYPE=V&OUT_UNITS=KM-S&REF_PLANE=E&VEC_LABELS=NO&CSV_FORMAT=YES&OBJ_DATA=NO&VEC_TABLE=2&COMMAND=0&CENTER=%40399&START_TIME=%272000-Jan-1+12%3A00+UTC%27&STOP_TIME=2001-Jan-1&STEP_SIZE=1d That query uses the J2000.0 ecliptic plane as the XY plane. If you want the equatorial plane, set REF_PLANE=F, see https://ssd.jpl.nasa.gov/horizons/manual.html#frames for details. – PM 2Ring Jul 15 '23 at 05:34
  • @PM2Ring Thank-you. Where can I enter REF_PLANE=F? I can't seem to find that option anywhere. – Kazon Jul 15 '23 at 23:57
  • Like this: https://ssd.jpl.nasa.gov/api/horizons.api?format=text&EPHEM_TYPE=V&OUT_UNITS=KM-S&REF_PLANE=F&VEC_LABELS=NO&CSV_FORMAT=YES&OBJ_DATA=NO&VEC_TABLE=2&COMMAND=0&CENTER=%40399&START_TIME=%272000-Jan-1+12%3A00+UTC%27&STOP_TIME=2001-Jan-1&STEP_SIZE=1d F is an abbreviation of FRAME. For more details on this API see https://ssd-api.jpl.nasa.gov/doc/horizons.html – PM 2Ring Jul 16 '23 at 00:03
  • Sorry, I just noticed your edit. I see you want RA & DEC, which you get from an Observer Table. With that table, you don't specify a reference plane directly. The astrometric RA & DEC essentially use the J2000.0 ecliptic & equinox, the apparent RA & DEC use the instantaneous ecliptic & equinox of the requested date. – PM 2Ring Jul 16 '23 at 01:19
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    The RA/Dec coordinates will change continuously due to Earth's motion around it. It's apparent position can be computed the same as any other solar system object by using the coordinates 0,0,0. – Greg Miller Jul 16 '23 at 06:10
  • @GregMiller Is the apparent position where it was during the J2K epoch? – Kazon Jul 16 '23 at 06:49
  • @Kazon I don't understand what you want to do with this RA/DEC data. But anyway, here it is for J2000.0 (and a few days following), using the file API. – PM 2Ring Jul 16 '23 at 08:47
  • https://sagecell.sagemath.org/?z=eJxlUVtr2zAUfjf4P2hZQQm4iuKulAYEVWIl0VZfKqmF9EWYxW1dEtuzlTFa-t93bLK-7EVwvpuk831Fm7ot3-qqQ7viUBOUxShUZfWMQhpeEBoSeuV75aGpW4fa4tex6FyHfM_3ju0eMTR6ca7p5tNp1-3Ia7MnVd7l5Ln-Pc2bcvpyyrZP5b4ggIwG68_DrreORr735exMpyvfi_kPYUW2ETHbCu17yzSOeRIx6nsDas02EyxdaKEehAJeJEYohi8pvbm4vsa-pw1XxhoZC4ZDSun5a16dz9AsnFOKjBkUafafgA6EyKyWj0DMUMS3GrC7e54YaaTQAAZh8O0SQCVWVm9BHjMsl2oF0JLf2lWqYm4YXqRmc4KG5-I1TP2FNpJraSAplsm9EX0-T9afvkj0Qp5lXMG3GOZS3Qrdq5b64VOVpACki-824oafRtjhsFFoBjb6rx_S1J0bQ0EB2uUuZ-_4qW4PucNzhF3xx-GPAPWNdMCUVXPsiTGGVnCA4Jx8THyvacvKjSGR9I7JX-dqlpw=&lang=python – PM 2Ring Jul 16 '23 at 08:47
  • @Kazon, the apparent position changes every day, just like it does for the Sun. – Greg Miller Jul 16 '23 at 16:38

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