What Time Is It On the Moon?

Satellite navigation systems for lunar settlements will require local atomic clocks. Scientists are working out what time they will keep. From a report: It's not obvious what form a universal lunar time would take. Clocks on Earth and the Moon naturally tick at different speeds, because of the differing gravitational fields of the two bodies. Official lunar time could be based on a clock system designed to synchronize with UTC, or it could be independent of Earth time. Representatives of space agencies and academic organizations worldwide met in November 2022 to start drafting recommendations on how to define lunar time at the European Space Research and Technology Centre of the European Space Agency (ESA) in Noordwijk, the Netherlands.

Decisions must be made soon, says Patrizia Tavella, who leads the time department at the International Bureau of Weights and Measures in Sevres, France. If an official lunar time is not established, space agencies and private companies will come up with their own solutions, she says. 'This is why we want to raise an alert now, saying let's work together to take a common decision.' The most pressing need for lunar time comes from plans to create a dedicated global satellite navigation system (GNSS) for the Moon, similar to how GPS and other satellite navigation networks enable precise location tracking on Earth.

Space agencies plan to install this lunar GNSS from around 2030. ESA approved a lunar satellite navigation project called Moonlight at its ministerial council meeting on 22 and 23 November 2022 in Paris, and NASA established a similar project, called Lunar Communications Relay and Navigation Systems, last January. Until now, Moon missions have pinpointed their locations using radio signals sent to large antennas on Earth at scheduled times. But with dozens of missions planned, 'there's just not enough resources to cover everybody,' says Joel Parker, an engineer who works on lunar navigation at the Goddard Center.

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