Why the Moon gains 56 or 58.7 µs a day, and what CGPM votes on in October
Contents
Kyodo News reported on August 30, 2026 that the effort to fix a time scale for the Moon, separate from Earth’s, is picking up speed.
The concrete item behind that is Draft Resolution D, which goes to the 28th CGPM (General Conference on Weights and Measures) in Versailles on October 13 to 15, 2026. It is voted on in the same session as Draft Resolution C, which stops leap second insertions.
What Draft Resolution D asks for, though, is that there be exactly one lunar reference time scale. It does not go as far as picking which one.
What Draft Resolution D says
The title of Draft Resolution D is “On the definition of an international lunar reference time scale and its traceability to UTC”.
It sits in version 5 (July 13, 2026) of the draft resolutions for the 28th CGPM, published by the BIPM (International Bureau of Weights and Measures).
Traceability here means the property that you can follow a clock’s reading back through an unbroken chain and arrive at an international reference time.
The first thing it raises is that space agencies are interested in lunar infrastructure, including PNT (positioning, navigation, and timing) systems.
Making those systems work with each other requires reference frames and a reference time that are recognized and adopted internationally.
On top of that it points to the IAU (International Astronomical Union) defining the LCRS (Lunar Celestial Reference System) and its coordinate time TCL (Lunar Coordinate Time) at its 32nd General Assembly in 2024, along with the equations for the relativistic transformation from TCB (Barycentric Coordinate Time) to TCL.
It also notes that the IAU, the IAG (International Association of Geodesy), and the ICG — the international committee for GNSS, the umbrella term for satellite positioning systems like GPS — have engaged with the BIPM on the definition of a lunar reference time scale and its traceability to UTC.
There are two notes on the other side.
One is that high-accuracy lunar applications would be greatly complicated by the direct use of UTC, because of the effects of general relativity.
The other is that a proliferation of reference time scales risks ambiguity when exchanging time-tagged data, and must be avoided.
Carrying UTC over as-is, and inventing a separate time scale per celestial body, are both off the table as far as Draft Resolution D is concerned.
Draft Resolution D has been rewritten twice since it was published on January 13, 2026.
Version 3 (March 30, 2026) added the condition that conventional values fixed by international organizations be used. How far the BIPM’s own role extends was written in at the same time.
Then version 4 (June 30, 2026) swapped “the realization of lunar reference time scales” in the recommending section for “the realizations of the lunar reference time scale”. It looks like nothing more than moving the plural around.
The version history, though, gives the reason: to reflect the aim of the draft resolution, which is that there may be several realizations but there must be a single reference time scale.
TCL, defined by the IAU in 2024
IAU 2024 Resolution 2 recommended building the LCRS for the Moon with the same techniques used to construct the GCRS (Geocentric Celestial Reference System) for Earth, and designating its coordinate time TCL.
A coordinate time is a time axis spread across a reference system as a whole, and it is a different thing from the time an actual clock sitting at a given place ticks. What the clock itself ticks is called proper time.
The LCRS metric tensor (the quantity that sets how distances in spacetime are measured) and gravitational potentials, and the transformation to the BCRS (Barycentric Celestial Reference System), are defined exactly as in IAU Resolution B1.3 from 2000, with quantities related to the Moon substituted for those related to the Earth. The Earth framework is read across to the Moon unchanged.
| Reference system | Origin | Coordinate time |
|---|---|---|
| BCRS (Barycentric Celestial Reference System) | Solar system barycenter | TCB |
| GCRS (Geocentric Celestial Reference System) | Center of mass of the Earth | TCG |
| LCRS (Lunar Celestial Reference System) | Center of mass of the Moon | TCL |
One second of TCL is made consistent with the SI second.
For the origin, the reading of TCL is exactly 1977 January 1, 0h 0m 32.184s when TCB reads the same at the center of the Moon. That date and time is the value TT (Terrestrial Time), TCG, and TCB share.
Per the note in the resolution, though, TCL has no historical relation to the other time scales, so this origin is entirely arbitrary and is set only for specificity. It means nothing more than lining the number up with the Earth side.
Here is how the time scales connect.
graph TD
TCB[TCB<br/>Barycentric Coordinate Time]
TCG[TCG<br/>Geocentric Coordinate Time]
TCL[TCL<br/>Lunar Coordinate Time]
TT[TT<br/>Terrestrial Time<br/>TCG times a constant]
TAI[TAI<br/>International Atomic Time<br/>a realization of TT]
UTC[UTC<br/>Coordinated Universal Time]
TCB -->|substitute Earth quantities| TCG
TCB -->|substitute Moon quantities| TCL
TCG --> TT
TT --> TAI
TAI --> UTC56 and 58.7 microseconds a day
Both 56 and 58.7 microseconds turn up in coverage as the amount by which lunar clocks run fast against Earth.
Kyodo News and NASA use 56 microseconds, while the Celestial Time Standardization policy memo that the US OSTP (Office of Science and Technology Policy) issued on April 2, 2024 says an Earth-based clock will appear to lose an average of 58.7 microseconds per Earth-day to an observer on the Moon.
The numbers disagree because the pairs being compared are different.
A paper on the lunar reference timescale by authors from the BIPM, Paris Observatory, the Royal Observatory of Belgium and elsewhere, published in Metrologia, lays out the mean relative frequency offsets — the ratio expressing the difference in how fast clocks run — body by body.
Mars Coordinate Time (TCM), incidentally, has not been formally defined by the IAU yet, but it can be estimated the same way as for Earth and the Moon.
| Pair compared | Relative frequency offset | Per day |
|---|---|---|
| A clock on Earth’s surface vs TCG | −6.97×10⁻¹⁰ | −60.2 microseconds |
| A clock on the lunar surface vs TCL | −3.1×10⁻¹¹ | −2.7 microseconds |
| TCG vs TT | 6.97×10⁻¹⁰ | 60.2 microseconds |
| TCL vs TT | 6.8×10⁻¹⁰ | 58.7 microseconds |
| TCM vs TT | 5.8×10⁻⁹ | 501 microseconds |
The 58.7 microseconds is the difference between TCL and TT. TT is the Earth-side reference time scale obtained by multiplying TCG, the coordinate time at the center of mass of the Earth, by a constant, and TCL, the coordinate time at the center of mass of the Moon, is being compared against it.
Compare a clock sitting on Earth’s surface directly against a clock on the lunar surface, on the other hand, and the proper-time offsets at each surface are added on top of the coordinate time transformations, for a total of 6.58×10⁻¹⁰, or about 56 microseconds per day. 58.7 is between coordinate times, 56 is between clocks on the ground.
The paper cross-checks this value against results from Ashby and Patla, Kopeikin and Kaplan, and Turyshev et al.
The difference cannot be written as a single constant, though.
Between the stations the paper computes, at latitude and longitude (0°, 0°) on Earth and (0°, 0°) on the Moon, taking only the coordinate time transformations and leaving out proper time, a linear increase of 1.5 microseconds per day carries a monthly period with an amplitude of 127 microseconds. That is the part that goes out and comes back over a month.
LTE440, a software package that computes lunar coordinate time, also reports an annual term with an amplitude of 1.65 ms and a monthly term of 126 microseconds between TCL and TCB / TDB (Barycentric Dynamical Time).
The secular drift LTE440 estimates between TCL and TDB — the part that keeps growing in one direction over time — is 6.79835524×10⁻¹⁰. Two separate calculations land on the same number as the 6.8×10⁻¹⁰ the Metrologia paper gives.
TCG−TCB and TCL−TCB, in the first place, change value depending on where the transformation is evaluated, so no unique relation can be written for them.
The Kyodo News piece, for what it’s worth, frames the impact in the order of a 300-meter position error for every microsecond of error in the received time.
Whether to scale TCL, in three options
The Metrologia paper restricts the lunar reference time TL to an affine function of the coordinate time TCL — TCL plus a constant frequency offset — and lists three options from there.
Set the offset to zero and use TCL as it is; match the mean rate to the proper time of a clock on a lunar equipotential surface (the selenoid); or match it so that only periodic variations remain against TT.
Multiplying a coordinate time by a constant, in the first place, means the spatial coordinates and the mass parameters of celestial bodies have to be rescaled by the same ratio, so that the form of the equations of motion and light propagation is preserved.
The scalings the IAU currently recommends are one from TCB to TDB and one from TCG to TT, and adding one for the Moon, then one for Mars, means a separate set of mass and distance values per body.
The paper indeed estimates that taking the third option for Mars would imply a scaling on the order of 10⁻⁹, and that implementing it wrong would produce errors of a few kilometers on the Earth-Mars distance.
The second option has the advantage that an accurate clock on the lunar surface ticks the reference time directly.
That only holds, however, when the clock’s accuracy is better than 10⁻¹¹ and the user requirement is looser than 10⁻¹³.
The 10⁻¹³ figure is how much proper time varies from place to place across lunar topography, and for anything more precise than that, the clocks end up needing to be steered back onto the reference. On top of which, a reference gravitational potential value for the Moon does not exist yet.
The paper’s conclusion is the first option, TL = TCL.
Draft Resolution D likewise writes in its notes that thorough studies have shown TCL without any scaling would meet the scientific requirements for a lunar reference time scale.
It has not killed off the scaled version, though. If a version matched to the selenoid is considered, it recommends that the frequency offset be chosen from a conventional gravitational potential value established by international organizations such as the IAU and IAG, and that its origin agree with the TCL origin defined by the IAU.
The paper also adds a note on the abbreviation: LT is not used for lunar reference time, because that abbreviation is already taken by “Local Time”.
LTC, the US track
The name the OSTP memo settled on, on the other hand, is LTC (Coordinated Lunar Time).
The features the memo requires of each celestial body’s time standard are four: traceability to UTC, accuracy sufficient to support precision navigation and science, resilience to loss of contact with Earth, and scalability to space environments beyond the Earth-Moon system.
The memo describes LTC as the operational form of an ideal time scale corresponding to Earth’s TT.
Just as an ensemble of atomic clocks on Earth realizes TT, an ensemble of clocks on the Moon might realize Lunar Time, is how it is put.
On top of that, it points out that using UTC without correction as the local lunar time scale would introduce uncertainty into the definitions of the SI base units, since the meter and the kilogram depend on the definition of the SI second.
NASA explained in a September 12, 2024 article that 56 microseconds is enough time for light to travel about 168 football fields.
NASA will then, in coordination with the Departments of Commerce, Defense, State, and Transportation, provide a finalized strategy for implementing lunar timing standardization to the Executive Office of the President by December 31, 2026. That deadline falls after October’s CGPM.
The IAU and BIPM documents deal with the coordinate time TCL, and the OSTP memo deals with the operational time LTC.
Draft Resolution D writes that a proliferation of reference time scales must be avoided, but it does not name LTC.
NASA also writes that it coordinates with international standards organizations, and Draft Resolution D for its part points to working together with space agencies and international organizations.
The time used for talking to Earth
The Metrologia paper expects UTC to remain the common operational time scale for humans on the Moon.
Contact with operators on the Earth side will be frequent and crucial, and in many of those cases the synchronization error is completely negligible.
As an example, the paper writes that synchronizing launches for an orbital rendezvous is less error-prone if it is aligned with the control center in UTC than if different spacecraft use different time scales. Over that span of time, the error from ignoring the periodic differences in proper time is negligible against the trajectory corrections that will be necessary anyway for other reasons.
Even once TCL is settled, the clocks people carry on the Moon are not going to drift away from UTC.
Precise Earth-Moon communication is another matter, and it needs both the periodic terms between TL and TT — the part that swings back and forth — and the travel time of the communication link.
The paper therefore proposes broadcasting the periodic terms from the coordinate time transformation integrals to users in a standard way. The idea is to distribute them as polynomials, the same way GNSS navigation messages distribute the offset between UTC and each GNSS time scale.
The degree of the polynomials, their validity period, sensitivity to ephemerides, and expected accuracy are things the paper leaves to be studied in more detail.
If that broadcast happens, a device on the Moon can keep its internal reference aligned on TL and still talk to an Earth station running on UTC.