[AstroPy] Problems with solar system ephemerides
Michael Brewer
brewer at astro.umass.edu
Fri May 1 10:50:43 EDT 2020
Paolo and Stuart,
void iauLd(double bm, double p[3], double q[3], double e[3],
double em, double dlim, double p1[3])
** Given:
** bm double mass of the gravitating body (solar masses)
** p double[3] direction from observer to source (unit vector)
** q double[3] direction from body to source (unit vector)
** e double[3] direction from body to observer (unit vector)
** em double distance from body to observer (au)
** dlim double deflection limiter (Note 4)
The main issue here is that the functions atciqz() and aticq() that
call ld() are treating the arguments p and q as identical (q = p). This
is true for a source at infinity, but not for a source within the solar
system. In that case, the argument q should be the appropriately light
delayed position vector of the source as seen from the center of the Sun.
Regards,
Michael Brewer
On 05/01/2020 06:14 AM, Paolo Tanga wrote:
>
> Stuart
>
> ok thanks! I will do, and try to figure out a test for this case
>
> Paolo
>
> On 01/05/2020 12:07, Stuart P Littlefair wrote:
>> Please do!
>>
>> If it is at all possible to provide some test data that would allow
>> us to check the implementation is correct along with the issue that
>> would be fantastic - although I appreciate this may depend upon
>> implementation of frames that do not currently exist in astropy.
>>
>> Stuart
>>
>> On Fri, 1 May 2020 at 11:01, Paolo Tanga <Paolo.Tanga at oca.eu
>> <mailto:Paolo.Tanga at oca.eu>> wrote:
>>
>> Hi Stuart
>>
>> I investigated a little further, and it turns out that SOFA
>> already include that option! Provided that the iauLd function is
>> exploited. Here is the description from the SOFA document:
>>
>>> The function iauLd is quite general. The algorithm is based on
>>> Eq. (70) in Klioner (2003) and Eq. (7.63) in Urban & Seidelmann
>>> (2013), with some rearrangement to minimize the impact of
>>> machine precision. Its arguments include (among other things)
>>> the mass of the body and full details of the geometrical
>>> arrangement. The latter means the function is valid for
>>> solar-system targets as well as distant objects, which is not
>>> strictly true for either iauLdsun or iauLdn.
>>>
>> So this implies that if one uses iauLd everywhere (with a
>> possibility to fall back to iauLdsun or iauLdn for distant
>> sources) the consistency is ensured!
>>
>> Should I rise an issue on this - maybe somebody is able to fix?
>>
>> Paolo
>>
>> On 01/05/2020 11:00, Stuart P Littlefair wrote:
>>> Hi Paolo,
>>>
>>> Your suspicion is correct; the light deflection from the Sun is
>>> calculated assuming a distant source.
>>>
>>> Fixing this is not trivial, from a quick look. It might be easy
>>> enough to turn light deflection off for nearby bodies, but
>>> anything more sophisticated than that will probably require a
>>> better model of light deflection than is implemented in SOFA/ERFA.
>>>
>>> Stuart
>>>
>>> On Fri, 1 May 2020 at 09:14, Paolo Tanga <Paolo.Tanga at oca.eu
>>> <mailto:Paolo.Tanga at oca.eu>> wrote:
>>>
>>> Hi Michael - and the list
>>>
>>> I noticed this:
>>>
>>> On 29/04/2020 17:47, Michael Brewer wrote:
>>> >
>>> > Issue #3: This is a fairly minor quibble, but the
>>> functions atciqz()
>>> > and aticq() are calculating the gravitational light
>>> deflection from
>>> > the Sun incorrectly. The third argument of erfa.ld()
>>> should be the
>>> > time delayed heliocentric position vector of the target
>>> body. I do
>>> > realize that SOFA has this problem also.
>>>
>>> I suspect (hope) this has really a very minor impact
>>> (micro-as level?).
>>>
>>> But there is another concern here. As far as I can
>>> understand from the
>>> source code of the functions that you mention (and the
>>> function they
>>> call) the gravitational light bending is ALWAYS computed for
>>> a source
>>> assumed to be "far" (=a star). This means that for a Solar
>>> System object
>>> (asteroid for intance) the result is wrong. It is easy to
>>> understand the
>>> physics: for a light beam that travels for a shorter
>>> distance in the
>>> gravity field of the Sun, bending is smaller.
>>>
>>> For typical Main Belt asteroids, the difference is of the
>>> order of 50%
>>> of the stellar light bending in the same direction (a few
>>> mas), of
>>> course larger at small Solar elongations.
>>>
>>> So, my questions are:
>>>
>>> - Can you confirm that astropy does not use a "planetary"
>>> version of the
>>> light bending, but only its "stellar" version ?
>>>
>>> - Would it be possible by having this feature corrected?
>>>
>>> (I would really like to be a SOFA expert and be able to
>>> contribute
>>> myself...)
>>>
>>> Paolo
>>>
>>> --
>>> ---------------------------------------------------------------------
>>> Paolo Tanga Astronomer
>>> Deputy director of Laboratoire Langrange / UMR 7293
>>> Observatoire de la Côte d'Azur Tel +33(0)492003042
>>> Bv de l'Observatoire - CS 34229 Fax +33(0)492003121
>>> 06304 Nice Cedex 4 - France http://www.oca.eu/tanga
>>> https://twitter.com/ziggypao
>>>
>>> _______________________________________________
>>> AstroPy mailing list
>>> AstroPy at python.org <mailto:AstroPy at python.org>
>>> https://mail.python.org/mailman/listinfo/astropy
>>>
>>>
>>>
>>> --
>>> Stuart Littlefair
>>>
>>> -------------------------------------------------------
>>>
>>> */I don't expect you to respond to my email outside your working
>>> hours. /*
>>> /
>>> /
>>> /At the University of Sheffield we value and encourage flexible
>>> working patterns, so please be assured that I respect your
>>> working pattern and I am looking forward to your response when
>>> you are next working. /
>>> /
>>> /
>>> -------------------------------------------------------
>>>
>>> Dept. of Physics & Astronomy,
>>> Univ. of Sheffield, Sheffield, S3 7RH.
>>>
>>> email: S.Littlefair at sheffield.ac.uk
>>> <mailto:S.Littlefair at sheffield.ac.uk>
>>> phone: +44 114 2224525
>>
>> --
>> ---------------------------------------------------------------------
>> Paolo Tanga Astronomer
>> Deputy director of Laboratoire Langrange / UMR 7293
>> Observatoire de la Côte d'Azur Tel +33(0)492003042
>> Bv de l'Observatoire - CS 34229 Fax +33(0)492003121
>> 06304 Nice Cedex 4 - Francehttp://www.oca.eu/tanga
>> https://twitter.com/ziggypao
>>
>>
>>
>> --
>> Stuart Littlefair
>>
>> -------------------------------------------------------
>>
>> */I don't expect you to respond to my email outside your working
>> hours. /*
>> /
>> /
>> /At the University of Sheffield we value and encourage flexible
>> working patterns, so please be assured that I respect your working
>> pattern and I am looking forward to your response when you are next
>> working. /
>> /
>> /
>> -------------------------------------------------------
>>
>> Dept. of Physics & Astronomy,
>> Univ. of Sheffield, Sheffield, S3 7RH.
>>
>> email: S.Littlefair at sheffield.ac.uk
>> <mailto:S.Littlefair at sheffield.ac.uk>
>> phone: +44 114 2224525
> --
> ---------------------------------------------------------------------
> Paolo Tanga Astronomer
> Deputy director of Laboratoire Langrange / UMR 7293
> Observatoire de la Côte d'Azur Tel +33(0)492003042
> Bv de l'Observatoire - CS 34229 Fax +33(0)492003121
> 06304 Nice Cedex 4 - Francehttp://www.oca.eu/tanga
> https://twitter.com/ziggypao
>
>
> _______________________________________________
> AstroPy mailing list
> AstroPy at python.org
> https://mail.python.org/mailman/listinfo/astropy
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