How UTC is calculated, and what a lunar scale would copy
Key facts
- TAI is the primary realization of Terrestrial Time, produced as a weighted average of hundreds of atomic clocks around the world. [1]Official
- UTC differs from TAI by an integer number of leap seconds, inserted to keep it aligned with Earth solar days. [1]Official
- TAI minus UTC is 37 s today, and TT is TAI plus 32.184 s exactly. [2]
- The CCTF is working with space agencies on a lunar reference time scale and has held workshops with lunar system providers. [3]Official
- The CIPM has recorded that one lunar reference time scale is needed even if it ends up with several realizations. [4]Official
UTC is not measured by any single clock. It is computed, from hundreds of clocks, in steps that are written down and repeated every month.
That matters for the Moon, because the lunar proposals copy the recipe rather than invent one. Four of the five steps have a stated lunar counterpart. One does not.
What UTC actually is
The chain starts with a theoretical ideal. Terrestrial Time is an analytical definition of time at mean sea level, and International Atomic Time is its primary realization, produced as a weighted average of hundreds of atomic clocks around the world [1]Official.
UTC then departs from TAI by a whole number of leap seconds, inserted from time to time to keep the scale aligned with Earth solar days despite changes in the rate of the Earth’s rotation [1]Official.
The two offsets are fixed numbers you can carry in your head. TT = TAI + 32.184 s exactly, and TAI − UTC is 37 s today [2].
Step by step, with the lunar counterpart
Each row is one step of the Earth recipe and whatever has been proposed in its place for the Moon. The status column says how far that proposal has actually got, which for now is never further than paper.
| Step on Earth | What it produces | Proposed lunar counterpart | Status today | Source |
|---|---|---|---|---|
| Atomic clocks are run in national laboratories | local realizations of UTC | atomic clocks on the lunar surface and in lunar orbit | proposed, none operating | [5] |
| Hundreds of those clocks are combined as a weighted average | TAI | an ensemble of clocks on the Moon | named in policy, no operator named | [1]Official |
| The ideal is defined on a chosen surface | TT at mean sea level | the same construction on the selenoid, worth 56.02 µs per day against the geoid | published in the literature | [6] |
| Whole leap seconds tie the scale to the planet’s rotation | UTC | nothing proposed | open | [7]Official |
| The result is disseminated and stays traceable | UTC in national laboratories | traceability to UTC, required of any lunar standard | required by policy, relations not agreed | [1]Official |
The step with no lunar counterpart
Leap seconds exist because UTC has a second job: it has to stay close to the Sun in the sky. The Earth’s rotation is not uniform, so whole seconds are inserted to keep the two together [1]Official.
Nothing in the lunar proposals does that job. No published option ties a lunar scale to the Moon’s rotation, and the question of how a lunar reference time scale, a lunar coordinate time and UTC relate to one another is exactly what IAU Resolution III asks international organizations to settle [7]Official.
This is the one place where the Moon cannot simply copy the recipe. It is also the reason our own model ignores leap seconds and says so [2].
Who is doing the work
The CCTF, the committee that prepares time questions for the metre convention, has been working with space agencies on a reference time scale for lunar positioning, navigation and timing, and has held workshops with lunar system providers [3]Official.
Above it, the CIPM has recorded that a single lunar reference time scale is needed even if it ends up with several realizations [4]Official.
The metrology literature has gone further than the committees. The Metrologia treatment of a lunar reference timescale examines the options and proposes realizations that stay traceable to UTC [5], and a later trade-off study concludes that Lunar Coordinate Time is the best practical reference on the Moon [8].
What is still open
- Who runs the clocks. No agency has announced that it will compute and publish a lunar scale on a schedule [2].
- How the tie to UTC is expressed. The relations between the three scales are the subject of a standing IAU request [7]Official.
- How the rate correction is applied. The surface-to-surface figure is about 56 microseconds per day [9]Official, and an independent derivation in Metrologia reaches the same result [10]. Every lunar value we print from it is a model value [2].
The scales themselves, and how they hang together, are drawn in the family tree of time scales. What would have to be built to make any of this run is in how a lunar time scale would be realized, and the Metrologia paper has its own read on the papers hub. The Learn hub lists everything else.
Sources
- Policy on Celestial Time Standardization
- Our calculation: the lunartime.org clock model
- Latest Updates from 2025 CCTF Meeting
- Meetings of the CIPM: Outcomes
- Lunar Reference Timescale (preprint)
- A Relativistic Framework to Estimate Clock Rates on the Moon
- Resolution on the establishment of a coordinated lunar time standard by international agreement
- Lunar Time
- What Time Is It on the Moon?
- Lunar reference timescale
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