The story so far: In September 2025, NASA’s four-member Artemis crew is scheduled fly around the moon in preparation for the space agency’s mission to land on the moon again. To boost such scientific missions, the White House Office of Science and Technology Policy (OSTP) on April 2, directed its space agency, the National Aeronautics and Space Administration (NASA), to establish a Coordinated Lunar Time (LTC) to standardise cislunar operations with the universal time followed on Earth.
Explaining the move, OSTP Deputy Director for National Security Steve Welby said, “A consistent definition of time among operators in space is critical to successful space situational awareness capabilities, navigation, and communications.”
Speaking about the difference between the passage of time on the moon and Earth, he said, “Time appears to pass more slowly where gravity is stronger, like near celestial bodies. As a result, the length of a second on Earth is different to an observer under different gravitational conditions, such as on the moon.”
The LTC will be the standard to measure cislunar operations — space activities between the moon and Earth — with Coordinated Universal Time (UTC), the global time used to regulate time on Earth. Roping in federal departments like the U.S. Departments of Commerce, Defense, State, and Transportation, the White House has set a deadline of December 31, 2026, for NASA and its international partners to deliver a strategy to implement LTC.
The project falls under the current administration’s National Cislunar Science and Technology Strategy. NASA has been directed to engage with the 39 nations who have signed the Artemis Accords for this project. It is expected to present its consideration of the LTC by December 31, 2024 as part of its Moon-to-Mars Architecture Concept Review cycle.
What is Coordinated Lunar Time (LTC)?
In 2023, the European Space Agency (ESA) launched a project called ‘Moonlight’ to design satellites for astronauts and robotic explorers, which will be used to support NASA’s moon mission ‘Artemis.’ While working on the project, questions arose on setting a single time zone for the moon and how to go about it.
Speaking to the publication Wired, ESA engineer Javier Ventura-Traveset said, “On Earth, we use a 24-hour day based on the planet’s rotation. However, the moon rotates much more slowly – every 29.5 Earth days.” Due to its slow rotation, it would be practical to have less than Earth’s 24 time zones — ideally, a single time zone for the moon would be natural, said Mr. Ventura-Traveset, highlighting that this would be similar to the Coordinated Universal Time (UTC).
The idea for the UTC was formulated in the 1960s. Atomic clocks — devices that measure time based on the vibration of atoms — are known for their extreme accuracy in measuring time. Meanwhile, solar time, calculating by measuring the rotation of Earth on its axis relative to the Sun, is variable in nature. A weighted average of hundreds of atomic clocks produces the International Atomic Time (TAI).
The UTC was designed as a way to accommodate the difference between solar time and atomic time, and is kept within 0.9 seconds of solar time to follow Earth’s rotation variations and within an exact number of seconds of the TAI. Currently, moon missions follow the time of the country which operates the spacecraft, while the International Space station (ISS) runs on the UTC. However, a standardised time for space and the moon is not followed.
“We would reproduce something like Coordinated Universal Time so astronauts could follow a 24-hour cycle as they do on the International Space Station,” Mr. Ventura-Traveset told Wired. However, he added that it will be out of sync with the moon’s light and dark periods (due to its slow rotation, the moon faces away or towards the Sun for long periods of time), and that it wouldn’t be sensible to have a weeks-long “day” followed by a weeks-long “night.”
The White House’s Celestial Time Standardization policy seeks to assign a time standard to each celestial body and its surrounding space environment, first focusing on the lunar surface and missions operating in cislunar space. It outlines the four features such a time standard must possess:
- Traceability to UTC: Lunar Time is analogous to Terrestrial Time on Earth (TAI+ 32.184 seconds). Similar to Terrestrial Time, Lunar Time may be set through an ensemble of clocks on the moon. This time standard, i.e., LTC may directly employ or distribute the UTC offsets required to maintain both local time and UTC time within tolerance limits.
- Scalability beyond the Earth-Moon system: Conversion of LTC to UTC for operations involving interactions with Earth will be possible by using the above approach to set the LTC. This approach is also extensible to space environments beyond the Earth-Moon system (for example, for Mars).
- Accuracy for precision navigation and science: The LTC will give users in cislunar space a reference time standard near the gravitational environment in which they operate. Space assets can synchronise with each other with precision for navigation.
- Resilience to loss of contact with Earth: The reference time – LTC – must survive independently when contact to Earth is lost.
Unlike Earth, the moon will have only one time zone and daylight saving will be unnecessary, the Smithsonian magazine estimates.
Why is LTC needed?
Previous moon missions involved astronauts visiting the lunar surface, completing their work and flying home. However, with space agencies across the world aiming to establish a permanent human presence on the moon, LTC is required, Mr. Ventura-Traveset said. “Up to now, when you have a mission on the moon, you would always synchronise with a time zone on Earth. But now we will have many missions in the future, and having a common reference time is really needed,” he said.
According The Scientific American, the pressing need for LTC is due to the plan to create a dedicated global satellite navigation system (GNSS) for the moon by 2030. This system will function similar to how the Global positioning system (GPS) and other navigation networks work on Earth.
Moon missions of various agencies will need an official lunar time to communicate with Earth-based stations and each other. “All this has to trace to one kind of a time reference, otherwise you have chaos and things do not work together,” ESA engineer Jörg Hahn said to The Scientific American.
Commercial operations on lunar surface involving transactions and logistics will be more reliable with the LTC, the OSTP says.
Issues in defining and implementing LTC
The process of defining lunar time is complicated by the effect of the moon’s gravitational pull. As per special relativity theory, due to the weaker gravitational pull of the moon, a clock on the moon would run faster than one on Earth, explains Patrizia Tavella, Head of Time department at the International Bureau of Weights and Measures in Sèvres, France, in an interview with The Scientific American. “A clock’s speed would also change depending on its position on the lunar surface, because of the moon’s rotation,” she adds.
NASA aerospace engineer Cheryl Gramling estimates that any clock on the moon would gain 56 microseconds over 24 hours. Per an estimate by The Scientific American, at least three master clocks which tick at the moon’s natural pace must be installed. The output of these three clocks coupled with an algorithm is expected to generate a more accurate time standard.
“Most importantly, lunar time will have to be practical for astronauts there (ISS),” ESA’s strategic planning head Bernhard Hufenbach said to the Smithsonian magazine. Each lunar day lasts as long as 29.5 Earth days. With the Artemis Programme aiming for a lunar landing as early as 2026, it needs to consider how to adapt to this challenge for a long-duration stay.
“But having established a working time system for the moon, we can go on to do the same for other planetary destinations,” said Mr. Hufenbach, alluding to setting time standards for other nearby celestial bodies like Mars.
In November 2022, the need for a unified lunar time was voiced globally by space agencies and academic organizations at an ESA meeting in Noordwijk, the Netherlands. Most participants agreed on “a common lunar reference time,” but debated if a single organisation should set and maintain time on the moon.
Apart from the US, several countries have lunar ambitions. China has stated that it will put its astronauts on the moon by 2030, while India plans to land in 2040. In January, Japan became the fifth country to land a spacecraft on the moon, after US, Russia, India and China. However, India is the only one to land a spacecraft near the lunar south pole.
“U.S. leadership in defining a suitable standard will benefit all spacefaring nations,” the OSTP stated. Getting consensus on LCT should be easier for the U.S due to the involvement of the Artemis Accord nations. However, two of its biggest space rivals – China and Russia – have not signed the accords, posing hurdles.
