Tracking time is one of those things that seems easy, until you really start to get into the details of what time actually is. We define a second as 9,192,631,770 oscillations of a cesium atom. However, according to Einstein’s theory of general relativity, mass slows down these oscillations, making time appear to move more slowly for objects in large gravity wells. This distinction becomes critical as we start considering how to keep track of time between two separate gravity wells of varying strengths, such as on the Earth and the Moon. A new paper pre-print on arXiv by Pascale Defraigne at the Royal Observatory of Belgium and her co-authors discusses some potential frameworks for solving that problem and settles on using the new Lunar Coordinate Time (TCL) suggested by the International Astronomical Union (IAU).
So why is this a problem we should solve now? As humanity is preparing to go back to the moon, hopefully more permanently this time, we need some standardized way to navigate it. In support of their various crewed lunar missions, America, China, and the EU are working on programs that can provide Position, Navigation, and Timing (PNT) services to explorers, and importantly network infrastructure, on the Moon.
Each of these services hopes to provide meter-level accuracy for a network node’s position on the Moon, but to do so would require nanosecond-level precision in their synchronized clocks. Similarly, Earth-based satellites like GPS have to account for relativistic changes in time between the geosynchronous satellites barely in the planet’s gravity well and the users down on the surface. To help facilitate this process on the Moon, in 2024 the IAU came up with the LUnar Celestial Reference System (LCRS), and an associated coordinate time - TCL.
Fraser discusses time dilation - the phenomenon that, on a small scale, affects clocks on the lunar surface.But having a standard only takes engineers so far - they have to actually implement it, and that can be tricky when it comes to time-keeping standards. On the lunar surface, a clock would drift about 56 µsec a day from a baseline clock in orbit, such as one used for global positioning. That’s already far more than enough to break the meter-sized resolution accuracy of a few nanoseconds. Earth suffers from the same problem, though, so how do we correct it there?
One option is to create a “scaled time” framework. One of these on Earth is called “terrestrial time” (TT). It “scales” Earth’s coordinate time, as defined by tracking coordinates, with a factor to offset the known drift of a clock at sea level. Unfortunately, such a solution would be difficult on the Moon, as there is no accepted “sea level” on the small body that doesn’t have any seas. And small variations in heights from crater tops to rims can make a difference in the long run to how much drift two clocks experience.
Alternatively, timekeepers could take a step back and implement something equivalent to Barycentric Dynamic Time. On Earth, instead of scaling to sea level, this scales to a framework that describes the entire solar system’s coordinate time. The elegant part of this solution is that it eliminates any long-term drift from a time framework that was already scaled, like TT, to a standardized point on a world. Since it would be standardized, it would be easier to offset the known standard drift between the satellites and the surface clock.
Fraser and Pamela discuss Lunar Time.A third option is to simply use TCL directly, and add an additional “steering” component that resets an individual clock in the framework back to a value set by a “master” clock periodically. While this would allow for some drift, it would never allow any clock on the surface to get completely out of alignment causing navigational or communications errors. It also has the added advantage of being agnostic to the altitude of the surface clock in question, as the amount each is steered can vary from time to time.
According to the authors, this last option is the best. They argue that adding “scaling” to the timekeeping framework simply introduces added complexity for not much additional practical benefit for users on the Moon. They also point out that “steering”, though a slight inconvenience, is common practice amongst clocks on Earth, so the technology and understanding of how to do so is high.
So it looks like the IAU’s simple solution is the best in terms of implementation. But that still means it will have to be integrated into these various satellite and ground station frameworks before it starts being practical. Hopefully the various agencies looking to implement the satellites constellations can all agree to adopt the framework - otherwise there might be a complete jumble of times on the lunar surface as we begin to expand there.
Learn More:
P. Defraigne, F. Meynadier, & A. Bourgoin - Lunar Time
UT - What Time is it on the Moon? Lunar GPS Needs to Know
UT - If We Want to Live on Other Worlds, We're Going to Need New Clocks
