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Timekeeping on the Moon

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August 26, 2024

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For decades, the Moon’s subtle gravitational pull has posed a vexing challenge — atomic clocks on its surface would tick faster than those on Earth by about 56 microseconds per day. This extremely small difference doesn’t seem like much, but it could disrupt the precise timing needed for important activities like spacecraft landings and communicating with Earth.

Now, researchers at the National Institute of Standards and Technology (NIST) have developed a plan for precise timekeeping on the Moon, paving the way for a GPS-like navigation system for lunar exploration. The research, published in The Astronomical Journal, focuses on defining a theoretical framework and mathematical models necessary for creating a lunar coordinate time system.

This innovation is crucial for NASA’s ambitious Artemis program, which aims to establish a sustained human presence on the Moon and may be an important steppingstone for exploration of the cosmos.

GPS on Earth relies heavily on precise timekeeping. Each satellite in the GPS constellation carries atomic clocks that are synchronized to a common time reference. By measuring the time it takes for signals from multiple satellites to reach a receiver, GPS can determine the receiver’s position and time. However, implementing a similar system on the Moon, and relating it accurately to Earth’s system, presents unique challenges due to the effects of relativity.

Einstein’s theory of relativity states that gravity affects the passage of time. Time doesn’t flow uniformly for everyone. For instance, on the Moon, where gravity is weaker than on Earth, clocks tick slightly faster. In addition, an observer on Earth measures time slightly differently than an observer on the Moon due to a number of gravity-related effects, including the Moon’s orbit around Earth and Earth’s orbit around the Sun. These effects can significantly impact precise navigation and communication over time.

To address this issue, NIST researchers have created a system to establish and implement lunar time that accounts for the Moon’s unique gravitational environment. This system establishes a new master “Moon time” that serves as the timekeeping reference specifically for the entire lunar surface, similar to how Coordinated Universal Time (UTC) functions on Earth.

“It’s like having the entire Moon synchronized to one ‘time zone’ adjusted for the Moon’s gravity, rather than having clocks gradually drift out of sync with Earth’s time,” said NIST physicist Bijunath Patla.

“This work lays the foundation for adopting a navigation and timing system similar to GPS, which would serve near-Earth and Earth-bound users, for lunar exploration,” said NIST physicist Neil Ashby.

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