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Relativistic time scales in the Solar system

T0 review · 0 major / 3 minor · reviewed 2026-05-10 · grok-4.3

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Pith's one-line read The IAU 2000 framework already defines relativistic local coordinate times for each body in the Solar system, making additional scalings unreasonable.

desk verdict This paper recaps the IAU 2000 time scales, argues against extra scalings like TCL, and supplies new practical ephemerides computed from INPOP19a. read the letter →

arxiv 2604.16006 v1 submitted 2026-04-17 astro-ph.IM

Sergei A. Klioner This is my paper · ORCID
classification astro-ph.IM
keywords relativistictimescalesIAU2000coordinatetimesSolarsystemephemeridesTCBTCGGCRS
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper establishes that the IAU 2000 framework already supplies relativistic local GCRS-like reference systems and TCG-like coordinate times for every Solar system body. It explains the connections between these coordinate times and the proper time of an observer while giving practical transformation methods from the barycentric TCB to the local times. The central argument is that scaling local coordinate times, such as introducing TCL for the Moon, lacks justification within the existing definitions. This clarification supports consistent timing in precise astronomical work and space navigation by avoiding unneeded adjustments to the framework.

What carries the argument

IAU 2000 definitions of relativistic coordinate times and reference systems extended to local GCRS-like systems and TCG-like times for each Solar system body

What would settle it

A high-precision timing comparison for a lunar or planetary mission where predictions using the unscaled local coordinate times deviate from observed clock differences by more than the expected measurement uncertainty would challenge the claim.

Watch

Extended reading notes

Core claim

The IAU 2000 framework already defines relativistic local GCRS-like reference systems and the corresponding TCG-like coordinate times for each body of the Solar system. Any scaling of the local coordinate times like TCL for the Moon is unreasonable. Practical recipes of the transformations between TCB and the local coordinate time scales (TCG, TCL, etc) are then discussed. Time ephemerides giving the transformation between TCB and the local coordinate times at the center of mass of the corresponding body are computed for all major bodies of the Solar system using INPOP19a.

Load-bearing premise

The IAU 2000 definitions of coordinate times and reference systems extend directly and without modification to local systems for every Solar system body, making additional scalings unnecessary.

Editorial extensions

If this is right

  • Transformations between TCB and the local coordinate times can be applied directly using the given recipes without introducing scalings.
  • Time ephemerides for all major Solar system bodies are available as standard Chebyshev polynomials for practical use.
  • The relations between coordinate times and an observer's proper time follow directly from the IAU definitions for consistent clock synchronization.
  • No changes to the IAU framework are required to define and use local times on individual bodies.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • This approach may simplify software for converting times across multiple bodies in space mission planning.
  • The computed ephemerides provide a concrete basis for testing timing consistency against real spacecraft data.
  • Similar local time definitions could be examined for applicability in extended operations involving multiple Solar system bodies.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

0 major / 3 minor

Summary. The manuscript summarizes the IAU 2000 framework for relativistic coordinate times and reference systems, arguing that it already supplies GCRS-like local systems and TCG-like coordinate times for each Solar System body. It therefore concludes that scalings such as TCL for the Moon are unnecessary. The paper discusses relations between these coordinate times and proper time, supplies explicit transformation recipes between TCB and the local scales, and presents Chebyshev-polynomial time ephemerides derived from the INPOP19a model for all major bodies, with the ephemerides made available online.

Significance. If the central argument is accepted, the work provides a clear, practical consolidation of the IAU framework for local time scales, which could reduce ambiguity in Solar-System ephemeris construction and dynamical modeling. The explicit transformation recipes and the release of ready-to-use Chebyshev ephemerides constitute a concrete, reproducible contribution that directly benefits practitioners working with high-precision timing in astrometry and planetary dynamics.

minor comments (3)
  1. The abstract and introduction would benefit from an explicit list of the specific IAU 2000 resolutions (e.g., B1.3, B1.4) that are invoked to define the local GCRS-like systems, so that readers can immediately locate the foundational statements.
  2. The section presenting the transformation recipes should include a short worked numerical example (e.g., TCB to local time at a given epoch for the Moon) to illustrate the practical application of the formulas.
  3. The statement that the Chebyshev ephemerides are 'available online' should be accompanied by a permanent identifier (DOI or stable URL) in the final published version.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript, the positive evaluation of its significance for consolidating the IAU 2000 framework, and the recommendation of minor revision. We note that the report does not list any specific major comments requiring point-by-point response.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity

full rationale

The paper's central argument rests on summarizing and interpreting the external IAU 2000 resolutions for relativistic coordinate times and reference systems (GCRS, TCG, etc.), which are treated as given independent definitions. It then derives interrelations with proper time and explicit transformation recipes between TCB and local scales directly from those standards, without introducing or fitting any internal parameters. The time ephemerides are computed from the independent external INPOP19a ephemeris and provided as Chebyshev polynomials. No step reduces a claimed result to a self-defined quantity, a fitted input renamed as prediction, or a load-bearing self-citation chain; the derivation chain is self-contained against external benchmarks.

Assumptions & free parameters 0 free parameters · 1 assumptions · 0 invented entities

The work rests on the pre-existing IAU 2000 relativistic framework as its foundational definitions and on the external INPOP19a planetary ephemeris for all numerical content; no new free parameters, ad-hoc axioms, or invented entities are introduced.

assumptions (1)
  • domain assumption IAU 2000 resolutions defining relativistic reference systems and coordinate times (TCG, TCB, etc.)
    The paper explicitly builds its arguments and recipes on these resolutions for both global and local systems.

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Cite this review

Pith. "Pith review of Relativistic time scales in the Solar system." pith.science (2026). https://pith.science/paper/2604.16006

@misc{pith2026260416006,
  author       = {Pith},
  title        = {Pith review of: Relativistic time scales in the Solar system},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/2604.16006}},
  note         = {Machine review of arXiv:2604.16006}
}
read the original abstract

This paper summarizes theoretical definitions of the relativistic coordinate time scales introduced by the IAU 2000 framework as well as practical aspects of their use. It is argued that the IAU framework already defines relativistic local GCRS-like reference systems and the corresponding TCG-like coordinates times for each body of the Solar system. The interrelations between the coordinate times and the proper time of an observer are discussed. The arguments put forward that any scaling of the local coordinate times like TCL for the Moon is unreasonable. Practical recipes of the transformations between TCB and the local coordinate time scales (TCG, TCL, etc) are then discussed. Time ephemerides giving the transformation between TCB and the local coordinate times at the center of mass of the corresponding body are computed for all major bodies of the Solar system using INPOP19a. Those time ephemerides represented as a standard set of Chebyshev polynomials are available online.

Figures

Figures reproduced from arXiv: 2604.16006 by the authors.

Figure 1
Figure 1. The hierarchy of the relativistic reference systems in the IAU 2000 framework. In the IAU framework, each reference system is given by its metric tensor gαβ , α and β are indices running from 0 to 3: the time coordinate corresponding to index 0 and the spatial coordinates correspond to indices 1 to 3. The metric tensors allow one to derive the equations of motion (of massive bodies or light rays) in the correspondin… view at source ↗

Discussion (0). Continue with ORCID to comment.

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. The International Lunar Reference System

    astro-ph.EP 2026-07 conditional novelty 5.0 of 10

    ILuRF2026 combines DE430, INPOP21a, and EPM2021 into the first International Lunar Reference Frame, with LLR residual RMS of 1.7–3.6 cm.

Reference graph

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Reviewed May 10, 2026 · model on record in the stance chip above.