What CGPM Draft Resolution D proposes
Key facts
- Draft Resolution D covers the definition of an international lunar reference time scale and its traceability to UTC. [1]Official
- Decision CIPM/115-14 refined the draft so that it asks for one scale, while allowing more than one realization of it. [2]Official
- IAU Resolution II of 2024 already defines Lunar Coordinate Time (TCL) as the time coordinate of the Lunar Celestial Reference System. [3]Official
- IAU Resolution III of 2024 asks international organizations to agree the relations between a lunar reference time scale, a lunar coordinate time and UTC. [4]Official
- The draft names no organization to compute the scale and no date on which one would start. [1]Official
Draft Resolution D is the lunar time item at the 28th CGPM. Its subject, as the convocation of 13 January 2026 states it, is short enough to quote in full [1]Official.
On the definition of an international lunar reference time scale and its traceability to UTC
Two things are worth noticing in that sentence. It asks for a definition, not for a running scale. And it ties the new scale to an existing one, UTC, rather than letting it stand alone.
One scale, several realizations
The distinction the draft rests on comes from the CIPM. Decision CIPM/115-14 refined the wording so that the conference is asked for a single scale, while leaving room for more than one way of producing it [2]Official.
there must be a unique lunar reference time scale, even if there may be several realizations
That is how terrestrial timekeeping already works. Many laboratories keep their own local approximations of Coordinated Universal Time, and one scale is computed from them [5]Official. The glossary entry on weighted clock ensembles sets out the arithmetic, and the entry for UTC describes the terrestrial arrangement it copies.
Where the published texts stand
The table below is our reading of four published documents, one row per open question. Every row cites the document it rests on. None of these answers comes from the draft text of a resolution that has not been put to a vote.
| Question | What the published texts say | Source |
|---|---|---|
| One scale or several? | One scale, with several realizations allowed | [2]Official |
| Tied to what? | Traceability to UTC is part of the subject of the draft | [1]Official |
| Is a coordinate time already defined? | Yes, TCL, by an IAU resolution of 2024 | [3]Official |
| Who agrees the relations between scales? | International organizations, by agreement, per IAU 2024 Resolution III | [4]Official |
| Who would compute the scale? | Not named in any published text | [1]Official |
| When would it start? | No date is on the public record | [1]Official |
What the draft would settle
If the conference carries the draft, the definition of a lunar reference time scale stops being a proposal and becomes a resolution of the CGPM. The subject is fixed by the text quoted above: a definition, and traceability to UTC [1]Official.
That would give the technical bodies something to build against. The CCTF has been working with space agencies on exactly this question, and has held workshops with lunar system providers ahead of the conference [5]Official.
What it leaves open
A definition is not a realization. Nothing in the published material names the organization that would compute a lunar scale, the clocks that would feed it, or the date on which it would first be published [1]Official.
Traceability to UTC is named as a requirement rather than specified as a method. A working definition of traceability has to name a comparison technique and an uncertainty budget, and the published peer-reviewed work treats both as open problems [6].
The draft also says nothing about how a lunar scale would be labelled for the people using it. That question is separate, and it is covered in will the Moon have time zones.
Where to read the text itself
The full document page, with the six draft titles quoted from the convocation [1]Official and the two CIPM decisions behind this one [2]Official, is CGPM 2026 Draft Resolution D. The other two time drafts at the same meeting are covered in three time resolutions at the 2026 CGPM, and the procedure that would put this text to a vote is in how the CGPM votes on a draft resolution.
Both possible outcomes, and what each would leave unchanged, are set out in what happens after the vote. More on the proposed standard itself is in what is Coordinated Lunar Time and in the lunar time news hub.
Sources
- Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001)
- Meetings of the CIPM: Outcomes
- Resolution to establish a standard Lunar Celestial Reference System (LCRS) and Lunar Coordinate Time (TCL)
- Resolution on the establishment of a coordinated lunar time standard by international agreement
- Latest Updates from 2025 CCTF Meeting
- Lunar reference timescale
- 28th meeting of the CGPM (2026)
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