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I am attempting to obtain the x, y, z coordinates of the Sun-asteroid vector using Horizons.

The code I wrote is:

jd_tdb = Time(2457171.6650230, format='jd', scale='utc').tdb.value
obj_sun = Horizons(id=21, location='500@10',
        epochs=jd_tdb, id_type='smallbody')
vec_sun = obj_sun.vectors()

The obtained values are: enter image description here

However, the Horizons website and/or the 'raw_response' in Astroquery provide:

enter image description here

So, there is a small difference (but important for astrometric purposes) between the outputs.

enter image description here

Does anyone have insight into the issue?

Thank you.

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    $\begingroup$ I dont know Astroquery (but I do know Horizons, and Python). I think the difference is due to a time scale issue. The Horizons query is using TDB, but your original Astroquery call looks like it's using UTC. $\endgroup$
    – PM 2Ring
    Commented Apr 15 at 14:24
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    $\begingroup$ I think that I am converting to tdb here: Time(2457171.6650230, format='jd', scale='utc').tdb.value. So, the inputs should be the same as far as I understand. $\endgroup$
    – Milagros Colazo
    Commented Apr 15 at 14:28
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    $\begingroup$ Yes, but your original call uses JD 2457171.6650230 UTC, but the Horizons output is for JD 2457171.6650230 TDB. $\endgroup$
    – PM 2Ring
    Commented Apr 15 at 14:40
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    $\begingroup$ This does the resolution of this astroquery issue help at all? $\endgroup$
    – Roy Smart
    Commented Apr 15 at 15:44
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    $\begingroup$ @Roy Interesting. The Horizons record for 21 Lutetia has the Solution date: 2024-Feb-23_20:13:24 ssd.jpl.nasa.gov/api/… $\endgroup$
    – PM 2Ring
    Commented Apr 15 at 17:35

1 Answer 1

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As @PM 2Ring pointed out in the comments, there was a problem between TDB and UTC times (the difference between them is small but impacts the x,y, and z results).

Now, comparing the solution and the raw response, we get the same: enter image description here

Thank you so much for your useful comments!

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  • $\begingroup$ Happy ending, yay! You can also check the "accept" button so future readers know the problem was solved. $\endgroup$
    – uhoh
    Commented Apr 18 at 11:34

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