IAU 2024 Resolution II: LCRS and TCL

In plain language

The International Astronomical Union adopted this resolution at its XXXII General Assembly in 2024 ^[1]Official. It does the work that a policy memorandum cannot do: it says what lunar time is measured in, and from which reference system.

Two objects come out of it. The first is the Lunar Celestial Reference System, a coordinate system centred on the Moon. The second is its time coordinate, Lunar Coordinate Time, abbreviated TCL ^[1]Official.

Neither object is a clock. TCL is a coordinate of a reference system, so no instrument displays it and no bulletin publishes it ^[2]Official. Everything an operational lunar standard would need is built on top of this definition, not instead of it. The glossary entry for TCL states the definition in fifty words.

What it decides

• A lunar reference system, built like the Earth one. The Lunar Celestial Reference System follows the methods already used for the Geocentric Celestial Reference System ^[1]Official. Nothing new is invented; the terrestrial construction is transferred to the Moon. The entry for the LCRS is the short version.
• A name for its time coordinate. The time coordinate of that system is Lunar Coordinate Time, TCL ^[1]Official.
• The same metric machinery as in 2000. The metric and the transformation between the barycentric system and the lunar one follow IAU 2000 Resolution B1.3, with lunar quantities replacing the terrestrial ones ^[1]Official. That is why the relativistic literature could pick the definition up immediately ^[4].
• A unit that matches the SI second. The unit of TCL is chosen to agree with the SI second ^[1]Official, which is what makes rate comparisons with terrestrial scales meaningful at all ^[5].
• An epoch, fixed against TCB. The resolution ties the reading of TCL to the reading of Barycentric Coordinate Time at the centre of the Moon, quoted in full below ^[1]Official. The entry for TCB explains the scale it is tied to.

What it leaves open

• The realization. A definition is not a time scale: somebody has to run clocks and publish a result. The resolution does not say who, how or when ^[1]Official.
• The tie to UTC. How TCL relates to Coordinated Universal Time is left to the companion resolution and to the organizations it addresses ^[2]Official.
• What users would read. Whether a crew on the surface would read a scale derived from TCL, or a scale defined separately and merely traceable to it, is not decided here ^[6]Official.

Key passages

The epoch is the part of the resolution that later work has to reuse verbatim, because it fixes where the two scales are set equal ^[1]Official.

the reading of TCL be 1977 January 1, 0h 0m 32.184s exactly when the reading of TCB be 1977 January 1, 0h 0m 32.184s at the center of the Moon

The reading 0h 0m 32.184s is the same offset that ties Terrestrial Time to International Atomic Time, and 1977 January 1 is the epoch already used for TCB and TCG ^[7]. The lunar scale was deliberately started from the terrestrial convention rather than from a new zero point ^[1]Official.

The lunar branch of that tree is one edge long today. What would hang below it — an operational scale, an ensemble, a published offset — is the subject of Resolution III and of the OSTP memorandum ^[2]Official. Our own arithmetic on top of this definition is written out on the methodology page ^[3], and what Coordinated Lunar Time is puts the definition next to the proposed standard.