[AstroPy] Problems with solar system ephemerides

Fri May 1 06:07:16 EDT 2020

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> 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> 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
>> 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
> 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 - France              http://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
phone: +44 114 2224525
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