Tomorrow, Tuesday, October 13, the 28th General Conference on Weights and Measures opens in Versailles, a gathering comprising 69 member states and 34 associated states and economic entities that meets every four years to decide on matters related to the International System of Units and the measurement of time.
The question of defining an international lunar time reference scale will be put to a vote during the conference.
- Member states of the CGPM will debate and vote on a resolution proposal, presented in January by the International Committee for Weights and Measures, which aims to define a single, international lunar time reference scale linked to Coordinated Universal Time (UTC).
- Resolution D, titled “On the definition of an international lunar time reference scale and its traceability to UTC,” states that there should be only one lunar reference time scale to avoid the ambiguity that would arise from multiple competing scales1.
- It relies on Lunar Coordinate Time (LCT), defined in 2024 by the International Astronomical Union, without prescribing a specific choice.
The topic is as much physical as it is geopolitical: the Moon’s weaker gravitational field alters how time flows compared to Earth—an advanced lunar clock runs about 56 microseconds per day. Defining a lunar time scale would thus provide a chronometric reference for lunar spacecraft and satellites, which require extreme precision for their missions.
- The United States and China are both racing to send crews to the Moon (as early as 2028 for Washington, before 2030 for Beijing) and to begin constructing infrastructure to establish a permanent lunar presence.
- For Washington, the 2028 deadline within the Artemis program is all the more crucial as it corresponds to the final year of Trump’s second term—and, constitutionally, the last one.
- In April 2024, under the Biden administration, the White House had already tasked NASA with developing by the end of 2026 a strategy to establish a lunar time reference framework for future missions.
Despite the navigation errors risk associated with using two different time systems—a mere microsecond difference can skew positioning by hundreds of meters—the positions of Beijing and Washington diverge.
- The two powers, both members of the International Bureau of Weights and Measures (BIPM), are indeed at odds over which standard should govern lunar time.
- While the White House has charged NASA with establishing a “Lunar Time Coordinate” (LTC), intended to serve as the reference for its future LunaNet navigation network, China, which has not adopted the LTC, has presented its own tool, the LTE4402.