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 · UTC19:33:08.699Coordinated 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)19:33:09.714model offset +1.015744 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.

+1.0157 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 18 131.81 days since the TCL epoch of 1977 January 1 [2]Official — 1 015 744.26 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

Five organisations 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 [8]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.

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

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, [9]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 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.

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. 28th meeting of the CGPM (2026) — Bureau International des Poids et Mesures, . Official document. Verified .
  9. Resolution on the establishment of a coordinated lunar time standard by international agreement — International Astronomical Union, Commission A3 — XXXII General Assembly, . Official document. Verified .

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