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:

enter image description here

enter image description here

enter image description here


1 Answer 1


Change the "Table Settings". You want to select

reference plane: Earth mean equator and equinox of reference epoch
reference system: ICRF/J2000.0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.