What time is it on the moon? NIST proposes an answer

On Earth, it’s generally pretty easy to find the exact date and time regardless of where you happen to be located. Everything is neatly divided up by calendars and time zones and kept in careful precision so that it will always be sunny at noon and dark at midnight. Summers are warm while winters are cold, with corrections like leap years made over time to prevent any drift. 

It’s easy to know the exact time down to the second, because there are always watches, clocks and even smartphones within easy reach almost everywhere. Not knowing the time isn’t even a good excuse for being late to a meeting anymore.

But on the moon, it’s a much different story. Not only is there a serious lack of time pieces, but nobody has really established how moon times sync with Earth. If it’s noon in New York, what time is it on the moon? And then there is the not so tiny problem of Einstein’s theory of relativity, which states that gravity affects time. Because there is less gravity on the moon, time passes just a bit more quickly up there, with atomic clocks advancing about 56 microseconds per day faster than those on Earth.

The question of having a reliable moon time zone may seem almost trivial or academic at first glance. After all, it’s not like there is a coffee shop up there taking pickup orders. But it becomes a much more serious problem as NASA ramps up exploratory programs to the moon, like the Artemis missions. That’s because accurate positioning on the moon requires an Earth-like GPS system, which in turn requires accurate time measurements.

On Earth, each satellite in the GPS system carries an atomic clock which is synchronized to a common time reference. Someone holding a GPS device queries several satellites to find out where they are in the world. Their position is determined by measuring the time it takes for each of those signals to reach back to the receiver, which gives a user’s accurate position down to just a few feet. 

On the moon, that can’t happen right now because there is no universal moon time, and a network of atomic clocks positioned there would be affected by gravity that is different from Earth. They would require a standard moon time relative to Earth in order to compensate for the changes in the way that time flows faster there due to the differences in gravity.

That is exactly what NIST is proposing, with researchers developing a precise timekeeping system for the moon that could eventually be used to foster a lunar GPS system to support future missions. It could also be used as a blueprint for a similar system on Mars, just with unique calculations to compensate for yet another gravitational difference.

The NIST proposal was published in The Astronomical Journal and goes into great depth about both the theoretical frameworks and mathematical models needed to create such a system. The blueprint calls for all of the moon to be folded into a single time zone called Lunar Coordinate Time, while the GPS system would be supported by atomic clocks on both the surface of the moon and in orbit around it.

“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, who co-authored the proposal. 

According to the proposal, how that new time zone would work is that atomic clock rates on the moon would be compared with clocks on Earth using a metric “appropriate for a locally, freely falling frame such as the center of mass of the Earth–moon system in the Sun's gravitational field.” In other words, the GPS system on the moon could compensate for time moving at a slightly faster rate compared with Earth in order to keep everything from drifting out of sync.

Everything would be designed with lunar exploration in mind. “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 and proposal co-author Neil Ashby. 

“The goal is to ensure that spacecraft can land within a few meters of their intended destination,” Patla added.

Once physicists have figured out how to account for relativistic time differences and perfected a GPS for the moon, the sky will no longer be the limit moving forward. After we have mastered that very complex skill, it will be a lot easier to set up similar systems anywhere else humanity wants to travel and explore.

“The proposed framework underpinning lunar coordinate time could eventually enable exploration beyond the moon and even beyond our solar system,” Patla said. “Once humans develop the capability for such ambitious missions, of course.”

John Breeden II is an award-winning journalist and reviewer with over 20 years of experience covering technology. He is the CEO of the Tech Writers Bureau, a group that creates technological thought leadership content for organizations of all sizes. Twitter: @LabGuys