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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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uhoh
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1 Answers1

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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
David Hammen
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