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In this answer I calculated the approximate angular separation between Mercury and the Sun as seen by the SOHO satellite using Skyfield's method of .ecliptic_position() because I couldn't get true ICRF coordinates for SOHO from the Horizons web interface.

Is there a way I can get ICRF coordinates for SOHO that haven't been rotated into the ecliptic?

Coordinate system description: Ecliptic and Mean Equinox of Reference Epoch

Reference epoch: J2000.0

xy-plane: plane of the Earth's orbit at the reference epoch

x-axis : out along ascending node of instantaneous plane of the Earth's orbit and the Earth's mean equator at the reference epoch

z-axis : perpendicular to the xy-plane in the directional (+ or -) sense of Earth's north pole at the reference epoch.

You can see that x,y are indeed a circle, but z is just +/- a tiny fraction of an au - basically the vertical component of the halo orbit. That's confirmed by the 6-month periodicity.

So how can I get true ICRF coordinates instead of these coordinates flattened to the particular plane the earth moves in at epoch?

Horizons web interface setup:

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Change the "Table Settings". You want to select

reference plane: Earth mean equator and equinox of reference epoch
reference system: ICRF/J2000.0
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  • $\begingroup$ OK I see - great! So Earth mean equator and equinox of reference epoch = J2000.0 would be the same as saying "ICRF coordinates"? I'm trying to learn the vocabulary here. Is "ICRF coordinates" unambiguous, or is it necessary to say that whole thing? $\endgroup$
    – uhoh
    Commented May 28, 2016 at 8:39
  • $\begingroup$ I've just asked the follow-up question Will these equations give me the approximate apparent RA and Dec using positions from Horizons? $\endgroup$
    – uhoh
    Commented May 27, 2018 at 3:12

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