Learn how lunar time works
Twelve articles on what lunar time is, why a lunar clock runs faster than an Earth clock, how the scales fit together, which decisions have not been taken yet, and how to timestamp lunar data today.
Key facts
- No lunar time scale has been realized. Every lunar clock value on this site is a model value computed from the published definitions. [1]
- A clock at rest on the Moon's selenoid gains 56.02 µs per day on an identical clock at rest on Earth's geoid. [2]
- The proposed operational standard is named Coordinated Lunar Time (LTC) in a United States policy memorandum of 2 April 2024. [3]Official
- Lunar time reaches the 28th CGPM as Draft Resolution D, item 17 on the provisional agenda. [4]Official
Start with these
Two articles carry the subject. The first says what is being proposed and by whom; the second settles the number everybody argues about.
- What is Coordinated Lunar Time (LTC)? — the names, the documents, and the deadlines that exist today.
- 56 vs 58.7 microseconds: two different pairs of clocks — why two different rate figures are both correct.
The physics
Why a clock changes rate when you move it, and why the length of a lunar day has nothing to do with it.
- Why clocks run faster on the Moon — two opposing effects, and the figure that survives them[2].
- How long is a day on the Moon? — three answers, from 27.322 Earth days to 24 hours[5]Official.
- Why microseconds matter on the Moon — what a microsecond of clock error costs a position fix, in metres[1].
The scales
Lunar time is not a new invention. It is one more branch on a tree of coordinate times that already carries TCB, TCG, TT, TAI and UTC[6]Official.
- The family tree of time scales: TT, TCG, TCB and TCL — what each scale is anchored to, and which ones a real clock reads.
- How UTC is calculated, and what a lunar scale would copy — the five steps behind UTC, and the four with a lunar counterpart.
The standard
What the bodies with a vote have actually put on paper, and what they have left open. The IAU has defined a coordinate time for the lunar reference system; nobody has yet built a scale on top of it[6]Official.
- Will the Moon have time zones? — one scale for the whole body, not a zone map.
- How a lunar time scale would be realized — the clocks, the orbit and the weighted average that would have to exist first.
- The standards tracker — where each organization stands, with the next dated milestone.
- China and lunar time: what has been published — the papers published by the Purple Mountain Observatory group, and what they state[7].
- The timeline — what happened when, and what is scheduled.
Using lunar time
Nothing has been realized, so nothing can be synchronized against. What can be done today is record UTC and carry the model offset as a separate, sourced quantity [1].
- Lunar time for developers: how to timestamp lunar data — a task-by-task table, what the JSON API returns, and what the
lunar-timepackage does.
What lunar time is not
This site is about time scales: the rate of a clock and the scale a timestamp is quoted in. One neighbouring subject is confused with it often enough to need a page of its own.
- Lunar time and the lunar calendar are not the same — which questions this site answers, and which it does not.
Everything in this hub
All twelve articles, each with the first of its key facts. Every figure on the site carries a source, and every lunar clock value is a model value: seehow we calculate lunar time and thelunar clock itself. The published work these articles rest on is read paper by paper in the papers hub.
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.
No lunar time scale has been realized. Every lunar clock value on this site is a model value computed from the published definitions. [1]
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.
A clock at rest on the Moon's selenoid gains 56.02 µs per day on an identical clock at rest on Earth's geoid. [2]
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.
56.02 µs per day is the rate of a clock near the Moon's selenoid measured against a clock near Earth's geoid. [2]
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.
Every proposal published so far puts the whole Moon on a single time scale rather than on a set of zones. [3]Official
How long is a day on the Moon?
The Moon turns on its axis in 27.3 Earth days, but sunrise to sunrise at a fixed point takes about 29.5. That gap is why a lunar day needs a definition.
The Moon travels around Earth once every 27.322 days, in an elliptical orbit. [5]Official
The family tree of time scales: TT, TCG, TCB and TCL
TCB, TCG, TT, TAI and UTC form one chain. TCL branches off at the top for the Moon. What each scale is tied to, and which one a real clock reads.
TCB and TCG are the time coordinates of the barycentric and geocentric reference systems the IAU defined in 1991 and revised in 2000. [6]Official
How UTC is calculated, and what a lunar scale would copy
UTC is TAI minus whole leap seconds, and TAI is a weighted average of hundreds of atomic clocks. A lunar scale would repeat four of those five steps.
TAI is the primary realization of Terrestrial Time, produced as a weighted average of hundreds of atomic clocks around the world. [3]Official
Why microseconds matter on the Moon
A navigation fix is a timing measurement. One microsecond of clock error is 299.79 m of light path, which is why the Moon needs a local time scale.
One microsecond of clock error is 299.79 m of light path, so every microsecond of timing error turns straight into a ranging error. [1]
How a lunar time scale would be realized
A definition is not a time scale. Somebody has to run clocks on the surface, clocks in orbit and a weighted average. Here is what has been proposed.
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]
China and lunar time: what has been published
A group at the Purple Mountain Observatory published LTE440, software that computes Lunar Coordinate Time. What its papers state, and what they do not.
Xu Lu, Tian-Ning Yang and Yi Xie of the Purple Mountain Observatory, Chinese Academy of Sciences, published the lunar time ephemeris LTE440. [7]
Lunar time for developers: how to timestamp lunar data
How to timestamp lunar data today: record UTC, keep the model offset separate, name both scales. What the JSON API and the lunar-time package actually do.
No lunar time scale has been realized, so there is no service to synchronize a device against today. [1]
Lunar time and the lunar calendar are not the same
Lunar time is a question about the rate of a clock. A lunar calendar is a question about dates and sunlight. This site covers the first and not the second.
The Moon travels around Earth once every 27.322 days, in an elliptical orbit. [5]Official
Sources
- Our calculation: the lunartime.org clock model
- A Relativistic Framework to Estimate Clock Rates on the Moon
- Policy on Celestial Time Standardization
- Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001)
- Moon Phases
- Resolution to establish a standard Lunar Celestial Reference System (LCRS) and Lunar Coordinate Time (TCL)
- Lunar Time Ephemeris LTE440: definitions, algorithm and performance
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