IAU 2024 Resolution III: a coordinated lunar time

In plain language

Resolution II defines a coordinate. Resolution III, adopted at the same General Assembly, says that a coordinate is not enough ^[1]Official.

The gap it names is practical. A coordinate time belongs to a reference system; it is not the reading of any instrument ^[1]Official. Missions need a scale that a receiver can display and that everyone recognizes, and that scale has to be related to the coordinate time and to Coordinated Universal Time in a way all parties accept. The entry for coordinate time draws the same distinction in fifty words.

This is the shortest of the five documents on this site and the one with the least machinery in it. It is a request for agreement, addressed to nobody in particular ^[1]Official.

What it decides

• That a coordinate time is not a clock reading. The resolution states the distinction plainly, which is why an operational scale is needed at all ^[1]Official. The definition of TCL is the coordinate in question ^[3]Official.
• That a practical lunar scale is needed. The need is recorded as a need, not as a decision to build one ^[1]Official.
• That the relations must be agreed collaboratively. The recommendation is quoted verbatim below ^[1]Official.

What it leaves open

• Who leads. No organization is named, so the request applies to all of them and binds none of them ^[1]Official.
• In what form. A resolution of the CGPM, a technical recommendation of the CCTF and a bilateral arrangement between agencies would all satisfy the wording ^[4]Official.
• By when. No date appears in the text. The only dated commitments on the subject are national ones, such as the 31 December 2026 deadline in the OSTP memorandum ^[5]Official.

Key passages

The operative sentence is a recommendation, not an instruction, and its subject is a relationship rather than a scale ^[1]Official.

the relationships between the possible versions of a lunar reference time scale and other time scales, in particular a lunar coordinate time and UTC, are pursued in collaborative agreement among the relevant international organizations

Two years later the phrase relevant international organizations still has no official membership. The table below lists the bodies that have done public work on the question since the resolution was adopted, and what that work is. None of them was appointed by this resolution ^[1]Official.

Body	Public work on a lunar time scale since 2024	Source
BIPM and the CGPM	Lunar time is Draft Resolution D and item 17 on the provisional agenda of the 28th CGPM, 13–15 October 2026	[2]Official
CIPM	Decisions CIPM/115-6 and CIPM/115-14 moved an updated draft to the CGPM and recorded that one scale is needed	[6]Official
CCTF	Work with space agencies on a lunar reference time scale, with workshops held before the 2026 conference	[4]Official
ICG and IOAG	Joint cislunar positioning, navigation and timing workshops, and a working group on lunar PNT	[7]Official
NASA and OSTP	A national deadline: a finalized implementation strategy for LTC by 31 December 2026	[5]Official
ESA	Agencies meeting at ESTEC in 2022 agreed that a common lunar reference time was urgent	[8]Official

The standards tracker keeps the current stage of each of these tracks with its next dated milestone, and the timeline shows the order in which the steps happened ^[2]Official. The nearest scheduled test of this recommendation is Draft Resolution D, which is on the agenda for October 2026 and has not been voted on ^[2]Official.