lunartime.org

Lunar time

This site is a reference and tracker for Coordinated Lunar Time. No lunar time scale has been realized yet, so every clock value published here is a model value, computed from the published relativistic definitions rather than read from a clock standing on the Moon.

  1. EARTH · UTC05:37:25.438Coordinated Universal Time, read from the browser clock. Milliseconds are where a browser clock stops being honest, so this line has no microsecond field.
  2. MOON · LTC (model)05:37:26.453model offset +1.015880 sCoordinated Lunar Time as a model: UTC plus the accumulated 56.02 µs/day by which a selenoid clock runs ahead of a geoid clock.

+1.0159 s since 1977-01-01Why?

The microsecond field shows a modelled offset, not a measurement. Browser clocks are accurate to milliseconds at best.

The counter is arithmetic, not a reading: 56.02 µs/day, the rate at which a clock on the Moon's selenoid runs ahead of a clock on Earth's geoid[1], multiplied by the 18 134.23 days since the TCL epoch of 1977 January 1 [2]Official — 1 015 879.81 µs of modelled drift so far. [3]

Key facts

  • A clock near the Moon's selenoid ticks about 56.02 microseconds per day faster than a clock near Earth's geoid. [1]
  • Draft Resolution D on an international lunar reference time scale is on the agenda of the 28th CGPM, 13–15 October 2026 in Versailles. [4]Official
  • No lunar time scale has been realized. Every clock value on this site is a model. [3]

Where the standard stands

Six organisations and forums are working on lunar time. None of them has a running scale: the furthest any of them has gone is a vote on a definition.

Next: 28th CGPM, Versailles, 13–15 Oct 2026 [9]Official

The full board, with the stage of each organisation and its next dated milestone, is on the lunar time standard tracker. The same decisions in the order they happened are on the timeline. What the next decision actually is, and what each outcome would change, is covered in the lunar time news hub. The published work the standard rests on is read paper by paper in the papers hub. Which body holds which decision — a definition, a vote, a rate figure, a deadline — is set out in the file on the organisations deciding lunar time. The missions and programmes whose own documents ask for one lunar clock are listed in where lunar time is needed. When one of those lines moves, we report the move by email in the Lunar Time Brief, at most weekly.

Start here

Four articles cover what people ask first: what the standard is, why lunar clocks run faster, whether the Moon gets zones, and which rate figure is the right one.

  1. What is Coordinated Lunar Time (LTC)?

    Coordinated Lunar Time is a proposed time standard for the Moon. It does not exist yet. Here is what has been proposed, by whom, and what is still open.

  2. Why clocks run faster on the Moon

    Two effects, one number: weaker gravity speeds lunar clocks up, orbital motion slows them down, and the net result is about 56 microseconds per day.

  3. Will the Moon have time zones?

    The proposals on the table put the whole Moon on one scale, not on zones. Here is what NASA, the IAU and the BIPM actually propose, and what is still open.

  4. 56 vs 58.7 microseconds: two different pairs of clocks

    Both are right. 56.02 µs/day compares clocks on two surfaces; 58.7 µs/day compares Lunar Coordinate Time with Terrestrial Time. The gap is the Moon itself.

All nine articles, including how long a day on the Moon lasts, are in the Learn hub. The glossary defines the abbreviations they use.

The mechanics behind those answers have articles of their own: the family tree of time scales, how UTC is calculated, why microseconds matter on the Moon and how a lunar time scale would be realized.

Primary documents

Five texts carry the whole subject. Each has its own page with the issuer, the date, what it decides and what it leaves open.

  1. OSTP Celestial Time Standardization memo (2024)

    White House Office of Science and Technology Policy, [5]Official

    Names the proposed standard Coordinated Lunar Time, abbreviated LTC.

  2. IAU 2024 Resolution II: LCRS and TCL

    International Astronomical Union, Commission A3, [2]Official

    Builds the Lunar Celestial Reference System with the methods used for the GCRS.

  3. IAU 2024 Resolution III: a coordinated lunar time

    International Astronomical Union, Commission A3, [10]Official

    Records that a coordinate time is not read by any real clock.

  4. NIST's relativistic framework for lunar clocks

    National Institute of Standards and Technology / The Astronomical Journal, [1]

    Gives a method for estimating clock rates on the Moon and at the Lagrange points to a few nanoseconds per day.

  5. CGPM 2026 Draft Resolution D on lunar time

    Bureau International des Poids et Mesures / CIPM, [4]Official

    Nothing yet. The text is a draft and the conference has not met.

All five in one table, with type and status: the primary documents hub.

How we calculate this

Every lunar value here is arithmetic on a published rate, not a measurement. We multiply 56.02 µs per day, the rate of a selenoid clock against a geoid clock[1], by the time since the TCL epoch of 1977 January 1[2]Official. Nothing is fitted and nothing is smoothed: the formula and every constant are printed inhow we calculate lunar time[3].

The same arithmetic is available as data. The JSON API returns the model offset and the drift between any two dates, and every response carries the sources and the reminder that the values are modelled. The same clock goes on someone else's page as one iframe: embed the lunar clock, with the word model and the link back built into the widget.

The full clock puts four scales side by side and names the pair of scales behind every number. Short answers to the questions people ask first are in the lunar time FAQ. Who publishes this and how corrections are handled is set out in about lunartime.org. Reporting on this? The press kit holds the quotable figures with their sources, the dates ahead and the diagrams under CC BY 4.0.

Sources

  1. A Relativistic Framework to Estimate Clock Rates on the Moon — Neil Ashby and Bijunath R. Patla, NIST — The Astronomical Journal 168:112, . Peer-reviewed. Verified .
  2. Resolution to establish a standard Lunar Celestial Reference System (LCRS) and Lunar Coordinate Time (TCL) — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .
  3. Our calculation: the lunartime.org clock model — Lunartime Editorial, . Our calculation. Verified .
  4. Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001) — Bureau International des Poids et Mesures / CIPM, . Official document. Verified .
  5. Policy on Celestial Time Standardization — White House Office of Science and Technology Policy, . Official document. Verified .
  6. Telling time on the Moon — European Space Agency, . Official document. Verified .
  7. Lunar Time Ephemeris LTE440: definitions, algorithm and performance — Xu Lu, Tian-Ning Yang, Yi Xie, Purple Mountain Observatory, Chinese Academy of Sciences — Astronomy & Astrophysics 704, A76, . Peer-reviewed. Verified .
  8. 2nd Joint ICG-IOAG Multilateral Cislunar PNT Workshop — Interagency Operations Advisory Group (IOAG) with the UN ICG, . Official document. Verified .
  9. 28th meeting of the CGPM (2026) — Bureau International des Poids et Mesures, . Official document. Verified .
  10. Resolution on the establishment of a coordinated lunar time standard by international agreement — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .

Last verified