How a lunar time scale would be realized
Key facts
- No lunar scale is being computed today: no clock ensemble on or around the Moon produces one, and no agency has announced that it will. [1]
- The 2024 policy memorandum says an ensemble of clocks on the Moon might set lunar time, the way an ensemble on Earth sets Terrestrial Time. [2]Official
- A clock on a proposed time aligned orbit, with a semi-major axis of about 1.5 lunar radii, would drift from selenoid proper time by up to 190 ns in a year. [3]
- The LTE440 ephemeris converts between lunar coordinate time and the barycentric scales with an accuracy better than 0.15 ns before 2050. [4]
- The CCTF is working with space agencies on a lunar reference time scale and has held workshops with lunar system providers. [5]Official
A definition is not a time scale. The IAU has defined a coordinate time for the lunar reference system, and a policy memorandum has given an operational scale a name, but neither of those is a clock.
Realizing a scale means running hardware and publishing a result on a schedule. This page collects what has been proposed for each layer of that job, and marks how far each proposal has got.
What realized means
On Earth the answer is concrete. Terrestrial Time is the ideal, and International Atomic Time realizes it as a weighted average of hundreds of atomic clocks around the world [2]Official.
Nothing of the kind runs for the Moon. Every lunar value on this site is a model value computed from published rates, and it will stay that way until an ensemble exists [1].
Clocks on the surface
NovaMoon is a proposed geodetic and timing station for ESA’s Argonaut lander. It would combine laser retroreflectors, VLBI transmitters, navigation signal receivers, atomic clocks and radio links at a single site in the Moon’s south polar region [6].
Its stated aim is sub-metre to decimetre positioning, local differential corrections for lunar users, and a stable realization of position and time [6]. That is one station, on one lander, and it has not flown.
A surface clock also inherits a problem the Earth equivalent does not have. In the option that ties the scale to the selenoid, lunar surface topography itself causes a frequency difference between clocks [3].
A clock in orbit
Yang and colleagues propose sidestepping that. They describe a time aligned orbit whose ideal clock reads selenoid proper time directly, and whose readings convert to Lunar Coordinate Time by a known linear transformation [3].
The orbit has a semi-major axis of about 1.5 lunar radii, depending slightly on inclination [3]. In simulation the clock desynchronizes from selenoid proper time by up to 190 ns after a year, a frequency offset of 6 × 10⁻¹⁵, which is 3.75 % of the frequency difference caused by lunar surface topography; accounting for the deviation of the mean orbits brings those figures to 13 ns and 4 × 10⁻¹⁶ [3]. Our read of that paper sets out the rest.
The weighted average
The 2024 policy memorandum makes the analogy explicit: just as Terrestrial Time is set through an ensemble of atomic clocks on Earth, an ensemble of clocks on the Moon might set lunar time [2]Official.
That is the whole published instruction. No number of clocks, no weighting algorithm, no operator, no schedule.
The committee work is further along than the hardware. The CCTF 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 [5]Official.
Five layers, and how far each one has got
Read the table downwards: each row depends on the one above it, and no row is operating.
| Layer | What it would be | Earth counterpart | Status today | Source |
|---|---|---|---|---|
| Clocks on the surface | atomic clocks at a lunar geodetic and timing station | clocks in national laboratories | proposed payload, not flown | [6] |
| A clock in orbit | a clock on a time aligned orbit at about 1.5 lunar radii | satellite navigation clocks | published proposal, simulated only | [3] |
| A weighted average | an ensemble of lunar clocks | TAI, hundreds of clocks averaged | named in policy, no operator | [2]Official |
| A transformation to coordinate time | the LTE440 ephemeris, better than 0.15 ns before 2050 | ephemeris-based time transformations | software published | [4] |
| Traceability to UTC | a published relation to UTC | UTC itself | required by policy, relations not agreed | [2]Official |
The last row is the one with an owner and no answer. IAU Resolution III asks international organizations to agree how a lunar reference time scale, a lunar coordinate time and UTC relate to each other, and that agreement does not exist [7]Official.
Which coordinate time it would sit on
A trade-off study of the options concludes that Lunar Coordinate Time is the best practical reference on the Moon, with no need to define a new scale by rescaling it [8].
The Metrologia treatment reaches the same conclusion from the other end: it examines the options for a lunar reference timescale and proposes realizations that stay traceable to UTC [9]. Its surface-to-surface rate, about 56 µs per day, matches the figure everyone else quotes [10], which is the same 56.02 µs per day a clock at rest on the selenoid gains on a clock at rest on Earth’s geoid [11].
What has not been decided
- Who computes it. No agency has said it will run the ensemble or publish the result [1].
- How many clocks, and where. Surface, orbit or both is an open design question; the orbital proposal exists only in simulation [3].
- The tie to UTC. The standing IAU request is unanswered [7]Official.
No date has been announced for any of it. What the proposals are and who made them is in what Coordinated Lunar Time is; the recipe being copied is in how UTC is calculated; and the standards tracker records where each body stands. The Learn hub has the rest.
Sources
- Our calculation: the lunartime.org clock model
- Policy on Celestial Time Standardization
- Two birds with one stone: simultaneous realization of both Lunar Coordinate Time and lunar geoid time by a single orbital clock
- Lunar Time Ephemeris LTE440: definitions, algorithm and performance
- Latest Updates from 2025 CCTF Meeting
- NovaMoon: A Strategic Lunar Reference Station for Positioning, Timing, and Largely Enhanced Science in the Earth-Moon System
- Resolution on the establishment of a coordinated lunar time standard by international agreement
- Lunar Time
- Lunar Reference Timescale (preprint)
- Lunar reference timescale
- A Relativistic Framework to Estimate Clock Rates on the Moon
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