The International Space Station is seen in silhouette as it transits the moon at roughly five miles per second, Sunday, Aug. 2, 2015, Woodford, VA. Photo Credit: (NASA/Bill Ingalls)
While it might sound like an easy question to know: what time is it on the Moon? It is far more difficult to answer than you might think.
On Earth, the planet is divided into twenty-four world time zones based on lines of longitude. Essentially, every time zone is a number of hours offset from Universal Coordinated Time (UTC) – where UTC is the same as Greenwich Mean Time (GMT), meaning the UK has UTC in the winter, but not in the summer.
As more plans are made for lunar settlements and potential industrial developments on the surface, the importance of having an agreed method for measuring the passage of time becomes critical. For example, both NASA and the European Space Agency (ESA) are planning to create satellite navigation systems to support the communication and navigation processes required for reliable and accurate positioning.
Ideally, there should be a single agreed system to maximise system interoperability and facilitate cross-organisational activities, and a proliferation of time scales must be avoided as it risks ambiguity when exchanging time-tagged data.
In addition, because the Moon and Earth have different gravitational fields, time dilation, as described by Albert Einstein’s theory of general relativity, also needs to be addressed. Time passes about 57 microseconds faster each day on the Moon, and while this is tiny, it will build up and could affect GPS and navigation systems used on the Moon.
Lunar Time Proposals
As you might imagine, there are different proposals to resolve these issues; for example, known proposals include:
- United States’ Office of Science and Technology Policy has suggested one standard time zone for the Moon, known as Coordinated Lunar Time, which is traceable to UTC.
- International Astronomical Union has suggested one coordinate time, Lunar Coordinate Time (TCL), for the lunar reference system.
- International Bureau for Weights and Measures (BIPM) has suggested one lunar reference time scale, possibly with several realisations.
- ESA has suggested a common lunar reference time agreed between agencies.
In addition, it is known that China is working on another proposal and there may be others. While, for those that have comment, there seems to be a common thread to only have one time zone on the Moon, rather than the 24 on Earth, there is work to do to agree on what it should be.
To address this issue, discussions are taking place at the 28th General Conference on Weights and Measures between the 13th and 15th October 2026 at the Palais des Congrès de Versailles in France. At this stage, it is not proposed to reach a conclusion in October, but rather the draft resolution describes space agencies and communities working together to discuss a TCL and to offer their knowledge and practical experience on the subject. The specific draft recommendation to be voted on by members is:
if a scaled version of TCL corresponding to the lunar geoid is considered, its frequency offset with respect to TCL is chosen according to a conventional fixed value of the gravitational potential, established by the relevant international organizations
The paper also notes that the BIPM’s International Committee for Weights and Measures will take into account the discussion and experience of space agencies and then take action, as necessary, to define a lunar reference time scale and its relationship to UTC with the involvement of the various relevant communities.
This resolution mirrors the BIPM’s role in determining UTC on Earth, which was undertaken based upon data gathered by member states. In addition, the BIPM wants this system to be transferable to other planets as well; if missions are sent to Mars, they can duplicate the approach used to measure time on the Moon and apply it to a new planet.
Conclusion
Currently, most stakeholders are working together and broadly want to achieve the same outcome, but there is still work to do to get there. It will be fascinating to watch in the coming months as we all measure time on the Moon.
