Lunar time: frequently asked questions
Ten questions people actually ask about lunar time, answered in two to four sentences, each with its source and a link to the page that goes deeper.
Key facts
- No lunar time scale has been realized, so every clock value in these answers is a model value computed from the published definitions. [1]
- A clock on the Moon's selenoid gains about 56.02 µs per day on an identical clock on Earth's geoid. [2]
- The 28th CGPM meets from 13 to 15 October 2026 in Versailles, where lunar time is Draft Resolution D. [3]Official
- The 2024 OSTP memorandum sets three separate deadlines for NASA, not one. [4]Official
Ten questions, ten short answers. Each answer names its source and links to the page that treats the question properly. How we pick and check those sources is set out in about this site.
What time is it on the Moon right now?
No lunar time scale is being computed and published by anyone [5]Official. So there is no time on the Moon in the sense that there is a time in Paris. What the lunar clock shows is a model: UTC plus the offset a clock on the lunar surface would accumulate at 56.02 µs per day against a clock on Earth’s geoid [2]. The arithmetic is printed in how we calculate lunar time.
Does Coordinated Lunar Time exist yet?
No. Coordinated Lunar Time (LTC) is a name and a set of deadlines, given in a United States policy memorandum of 2 April 2024 [4]Official. No agency computes it, no bulletin publishes it, and no document says how it would be realized from real clocks. What Coordinated Lunar Time is lists what has been proposed, and by whom.
Why do clocks run faster on the Moon?
Two effects pull in opposite directions. The Moon’s weaker gravitational potential makes a lunar clock tick faster; its motion relative to Earth makes the same clock tick slower. The first effect wins, and about 56 µs per day is what survives for a selenoid clock against a geoid clock [6]Official. Why clocks run faster on the Moon works through both terms.
Is it 56 or 58.7 microseconds per day?
Both, because they compare different pairs of clocks. 56.02 µs per day compares a clock near the Moon’s selenoid with a clock near Earth’s geoid [2]. 58.7 µs per Earth-day is the figure the OSTP memorandum gives for a lunar coordinate time against Terrestrial Time [4]Official. The gap between them is the Moon’s own gravitational potential, and 56 vs 58.7 microseconds shows the subtraction.
Will the Moon have time zones?
Probably not, but nothing is settled. Every proposal published so far puts the whole body on one scale rather than on a set of zones. The IAU resolution of 2024 defines a single coordinate time for the lunar reference system [7]Official, and the case for and against zones is set out in will the Moon have time zones.
How long is a day on the Moon?
It depends which day you mean. The Moon turns on its axis once every 27.322 Earth days, while sunrise to sunrise at a fixed point on the surface takes about 29.5 Earth days [8]Official. Neither figure sets the length of a lunar second: nothing in the proposed work redefines the second or ties a scale to sunrise [7]Official. How long is a day on the Moon puts the three answers side by side, and why lunar time is not a calendar separates counting days from the rate of a clock.
Who decides lunar time?
Nobody decides it alone. The IAU has defined the coordinate time [7]Official. The BIPM has put a lunar reference time scale on the agenda of the 28th CGPM [3]Official, and a United States memorandum directs NASA to deliver an implementation strategy [4]Official. Where each organisation stands today is on the lunar time standard tracker.
What is the difference between LTC and TCL?
TCL, Lunar Coordinate Time, is the time coordinate of the Lunar Celestial Reference System, defined by the IAU in 2024 [7]Official. It is a coordinate time, so no physical clock reads it directly. LTC, Coordinated Lunar Time, is the proposed operational standard that missions would use, named in the OSTP memorandum [4]Official. Both have glossary entries: TCL and Coordinated Lunar Time.
When does the CGPM vote on lunar time?
The 28th General Conference on Weights and Measures is scheduled for 13 to 15 October 2026, at the Palais des Congrès de Versailles [9]Official. Lunar time comes to it as Draft Resolution D, item 17 on the provisional agenda [3]Official. What that text would and would not settle is on CGPM Draft Resolution D.
What is the deadline for NASA?
There are three deadlines in that memorandum, not one, and they are easy to confuse. NASA is to deliver a finalized implementation strategy by 31 December 2026. Two earlier ones fell on 31 December 2024: LTC in the annual Moon-to-Mars Architecture Concept Review, and a milestone-based approach delivered to OSTP [4]Official. All three are read line by line in the OSTP memorandum of 2024.
Sources
- Our calculation: the lunartime.org clock model
- A Relativistic Framework to Estimate Clock Rates on the Moon
- Convocation of the 28th meeting of the CGPM (O/Ref AK-CFA-2026-1001)
- Policy on Celestial Time Standardization
- Resolution on the establishment of a coordinated lunar time standard by international agreement
- What Time Is It on the Moon?
- Resolution to establish a standard Lunar Celestial Reference System (LCRS) and Lunar Coordinate Time (TCL)
- Moon Phases
- 28th meeting of the CGPM (2026)
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