In April 2024, the White House launched a challenge to scientists: establish a lunar time standard in view of a growing international presence on the Moon and potential human bases as part of NASA’s Artemis initiative. However, the real question is not simply “What time is it?”, but rather “How quickly does time pass?”.
The time displayed on a clock can be set by any timekeeper, but physics determines how quickly time flows. In the early 20th century, Albert Einstein demonstrated that two observers would not agree on the duration of an hour if they were not moving at the same speed and in the same direction. This disagreement is also present between a person on Earth and one on the Moon.
Relativity Effects on the Moon
“If we were on the Moon, clocks would tick differently compared to those on Earth,” explained Bijunath Patla, theoretical physicist at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. The Moon’s movement relative to Earth causes lunar clocks to slow down compared to terrestrial ones, but the lesser lunar gravity accelerates them. “These two opposing effects result in a net drift of 56 microseconds per day“, said Patla.
Patla and his colleague Neil Ashby used Einstein’s general relativity theory to calculate this difference, improving previous analyses. Their results were published in the Astronomical Journal.
Implications for Lunar Missions
Although a difference of 56 microseconds may seem negligible by human standards, it is crucial for missions requiring millimeter precision or communications between Earth and the Moon. “The safety of navigation in a future lunar ecosystem depends on precise clock synchronization“, stated Cheryl Gramling, system engineer at NASA’s Goddard Space Flight Center. A 56-microsecond error per day could cause navigation inaccuracies up to 17 kilometers per day, an unacceptable problem for Artemis missions, which will require knowing the exact position of every rover, lander, and astronaut within a 10-meter margin.
The Lunar Time Challenge
A fundamental principle of relativity theory is that there is no absolute time. A clock on the Earth’s surface ticks more slowly than one in orbit due to gravitational effects, which is why GPS satellites must account for relativity. Establishing time on the Moon is further complicated by its orbit around Earth and Earth’s rotation, which influence time perception.
Ashby and Patla recognized that the Earth-Moon system is in free fall under solar gravity’s influence, allowing them to consider complex contributions such as celestial body rotation, tidal forces, and deviations from perfect spheres. They also calculated gravitationally stable positions between Earth and the Moon, known as Lagrange points, useful for communication relay satellites.
Towards a Standard Lunar Time Future
Other scientists, such as Sergei Kopeikin from the University of Missouri and George Kaplan from the US Naval Observatory, have also calculated a 56-microsecond time shift between Earth and the Moon. They also considered small periodic fluctuations due to variations in tidal force caused by the Sun and Jupiter.
“The scientific community has done a great service by publishing this work,” said Gramling. “Now we can propose a standardized model for lunar time to the entire international timekeeping community.”
Although it will take many years or decades before the Moon hosts enough humans and robots to necessitate precise timekeeping, scientists and engineers recognize the importance of establishing a standard much earlier than expected. They have already taken the first, difficult step towards determining time on the Moon.
