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.