Ecliptic coordinate transformation precision #6461

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1. astrofrog

   added

   Bug

   coordinates

   Priority-High

   on Aug 20, 2017
2. astrojuanlu commented on Sep 1, 2017

   

   astrojuanlu

   on Sep 1, 2017

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   I started using the Ecliptic reference frames from Astropy when loading asteroid data from the JPL Small-Body Database Browser and found some discrepancies. By suggestion of @mhvk I copy my conclusions here:

   Although it was stated in the wiki that the results from Horizons and the Small Object Browser are not exactly the same, in fact all the differences stay within the declared uncertainties (1 sigma), so we can consider them equal in some sense 🙂

   And on the other hand, this is the information Horizons gives about the reference frame:

   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
             Note: obliquity of 84381.448 arcseconds wrt ICRF equator (IAU76)
   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.
   

   With respect to the obliquity, this is the result of using Astropy functions:

   
   In [8]: from astropy.time import Time
   
   In [9]: equinox = Time("J2000")
   
   In [11]: from astropy.coordinates.builtin_frames.utils import get_jd12
   
   In [12]: jd1, jd2 = get_jd12(equinox, 'tt')
   
   In [13]: from astropy import units as u
   
   In [14]: obl = erfa.obl06(jd1, jd2)*u.radian
   
   In [15]: obl
   Out[15]: <Quantity 0.40909260059596453 rad>
   
   In [20]: obl.to(u.arcsec)
   Out[20]: <Quantity 84381.4059990474 arcsec>```
   

   which is slightly different from the announced value of 84381.448 arcseconds. This is because Horizons uses the ICRF equator of IAU76!

   
   In [24]: obl80
   Out[24]: <Quantity 0.40909280421771277 rad>
   
   In [25]: obl80.to(u.arcsecond)
   Out[25]: <Quantity 84381.44799904784 arcsec>```
   

   So we need a mean ecliptic of IAU76, which is not present in Astropy.
3. astrojuanlu

   added a commit that references this issue on Sep 3, 2017

   Remove nutation from ecliptic transformation

   ...

   This issue will close once commit fb8efaa is merged into the 'main' branch.

   fb8efaa
4. astrojuanlu

   added a commit that references this issue on Nov 28, 2017

   Remove nutation from ecliptic transformation

   ...

   This issue will close once commit 3219570 is merged into the 'main' branch.

   3219570
5. eteq

   added a commit that references this issue on Dec 6, 2017

   Remove nutation from ecliptic transformation

   ...

   This issue will close once commit 728dedf is merged into the 'main' branch.

   728dedf
6. eteq

   closed this as completedin 728dedfon Dec 6, 2017