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Talk:Relativistic Celestial Mechanics - Scholarpedia

Talk:Relativistic Celestial Mechanics

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    Reviewer A

    Most of the text is certainly fine. However, a few things should be changed:

    p.2 ‘… the final goal of RCM is to answer the question if GRT alone is capable to explain all observed motions of celestial bodies.’ It is obvious that in many dynamical problems of CM non-gravitational forces are of importance (friction, radiation pressure, Yarkovsky effect etc.);

    the sentence should be modified according to that.

    p.3 Possibility one to overcome the coordinate problem certainly is a bit strange and it suggests to reduce the importance of an ephemeris. And every ephemeris is a coordinate picture of the motion.

    p3: The sentence ‘But this pragmatic option has nothing to do with the general line of researches in GRT and RCM’. This sentence is polemic and contra-productive it leaves a bad smell toward IAU activities; it should be removed from the text.

    P3: The part ‘Moreover, as was shown long ago ….. level of accuracy’ is very misleading;

    e.g., for the problem of timescales etc. the time-time component of the metric is needed to order 4 in 1/c. This part should be dropped.

    P4: Why the author avoids the abbreviations BCRS and GCRS, which are widely used among

    Experts is unclear.

    P4: Maybe in the last section the problem of cosmological expansion and its influence on local physics should be mentioned.

    P4: W.r.t. the list of references it is not well balanced; the author itself cites himself in a non justified manner. Many other authors provided significant contributions to RCM.

    Reviewer B

    The article gives a very interesting and clear presentation of what relativistic celestial mechanics (RCM) is, including what are the differences with respect to Newtonian celestial mechanics, what are the RCM bases, developments, methods and scientific applications.

    However, two parts of the article need to be modified/improved (major corrections), i.e. (i) the sentence about the IAU recommendation on page 3, which is not correct and moreover controversial, and (ii) the list of references on page 5, which has to be completed. Other corrections are also suggested below.

    1. Major corrections

    (i) Saying that IAU recommended harmonic coordinates in 1991 is not correct. The IAU recommendation for the choice of harmonic coordinates for both the geocentric and the barycentric reference systems was in fact in IAU 2000 Resolution B1.3. The mention to the IAU recommendation on page 3 should therefore be modified with taking this into account. Moreover, saying that the IAU selected option "has nothing to do with the general line of research in GRT or RCM" is irrelevant and appears as a controversial comment that has nothing to do with an encyclopedia. This should be removed.

    (ii) The list of references is not sufficient. Significant contributions to the RCM field by other authors should be added, such as Soffel ("Relativity in astrometry, celestial mechanics and geodesy", Springer Verlag, 1989), Damour, Soffel, Xu ("General relativistic celestial mechanics", Phys. Rev. 1991 (I); 1992 (II); 1993 (III); 1994 (IV))), etc..

    2. Suggestions for other corrections

    page 1: The expression "astronomy of motion" for aspect (4) of RCM is not an usual expression. According to the explanations that are provided for the meaning of that expression, it seems to be for "astrometry of celestial bodies".

    page 2: The paragraph describing the properties of Newtonian mechanics mixes two issues: the use of curvilinear coordinates and the fact that the inertial coordinates can be regarded as measurable quantities. This should be clarified.

    page 3: The accent above the e of "Painlevé" is not readable; this appears as a "?".

    page 3: To clarify the reasoning of the section on "The problem on coordinate-dependent quantities in RCM", it would be better to say e.g.: "The general line of researches in GRT and RCM is based on using three main possibilities to overcome the problem of coordinate-dependent quantities:"

    and then, after the description of those possibilities:

    "In 2000, the IAU (International Astronomical Union) recommended the use of the harmonic coordinates, which greatly simplifies the use of GRT in practical problems of ephemeris astronomy demanding high–level standardization. An explanatory supplement (2003) provides details on and explanations for all the relativistic aspects of the IAU 2000 resolutions.",

    and putting the sentence saying that "harmonic coordinates are determined by four specific non–tensorial differential relations to be added to the GRT tensorial field equations" either as a note or as an additional possibility (i.e. N° 4).

    page 4: After "present level of accuracy", it should be necessary to add the value of that present level of accuracy.

    page 4: In the section entitled "The hierarchy of reference systems in RCM", where many acronyms are used (e.g. BRS, TRS, SRS, etc.), it would be better to make explicit "Special relativity theory" instead of using the abbreviation SRT in order to avoid being confused with e.g. SRS (Satellite reference system).

    page 4: In the section entitled "The hierarchy of reference systems in RCM", it is necessary to mention the Barycentric Celestial Reference System (BCRS) and the Geocentric Celestial Reference System (GCRS) that were adopted by the IAU (cf. IAU 2000 Resolution B1.3).

    page 4: "best appropriate" would be better than "adequate".

    page 4: "vanishing in the spatial origins" should be "vanishing at the spatial origins".

    page 4: The sentence "In some cases it is sufficient to consider just the formally Newtonian equations in GRS to get the main relativistic perturbations." is misleading. It is difficult to understand how Newtonian equations alone can allow us to obtain relativistic perturbations.

    page 5: "The techniques of RCM are used for the objects outside the solar system" should be "The techniques of RCM are also used for the objects outside the solar system".

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