What Time is it on the Moon?

The coming decade is set to see a surge in lunar exploration, with plans to establish permanent bases on the Moon and dozens of missions. However, this endeavor poses many challenges, including the need to determine what time it is on the Moon. Currently, the Moon doesn’t have an independent time and each lunar mission uses its own timescale that is linked to Coordinated Universal Time (UTC) on Earth. But this method is imprecise and becomes a problem when multiple spacecraft are working together, which will require precise timing signals for satellite navigation.

Representatives from space agencies and academic organizations worldwide met in November 2022 to draft recommendations for defining lunar time. Patrizia Tavella, who leads the time department at the International Bureau of Weights and Measures, warns that decisions must be made soon to avoid multiple solutions from space agencies and private companies.

The need for lunar time is most pressing for the creation of a dedicated global satellite navigation system for the Moon. From 2024, NASA and the European Space Agency (ESA) will trial deriving positions on the Moon using satellite navigation signals from Earth-based craft. The lunar satellite navigation projects plan to place dedicated satellites around the Moon, each with its own atomic clock, which will enable a receiver on the Moon’s surface to triangulate its position.

Defining lunar time is complex, as the theory of relativity dictates that clocks tick slower in stronger gravitational fields. The Moon’s weaker gravitational pull means that, to an observer on Earth, a lunar clock would run faster. Defining a lunar standard will involve installing at least three master clocks that tick at the Moon’s natural pace, which will then be combined by an algorithm to generate a virtual timepiece. The metrologists will then have to decide whether to base lunar time on UTC or create an independent time.

Finally, there is the question of whether astronauts on the Moon would use universal lunar time everywhere, or offset it in time zones linked to the Sun’s position in the sky. While the question of time zones on the Moon may be more a matter of convention, establishing a universal lunar time is crucial for ensuring cooperation and communication among missions on the Moon.