There are a lot of technical challenges humanity will have to tackle as we prepare a long-term presence on and around the Moon, and the European Space Agency just reminded us of one more: we don't have an agreed, coordinated method of telling time on our natural satellite.

That hasn't been a problem up until now, the ESA [1]explained , because the Moon hasn't ever had to deal with a crowd. Each mission to the Moon keeps its own time relative to the managing agency making the visit, which is synchronized with Earth using deep space antennas that relay communications and chronometric information between mission and control.

"As dozens of missions will be operating on and around the Moon and needing to communicate together and fix their positions independently of Earth, this new era will require its own time," the ESA warned.

You've gotta know when to know where

The international space community has been working on a computing network for the Moon dubbed " [2]LunaNet ," which Javier Ventura-Traveset, manager of the ESA's Moonlight lunar navigation program said will look a lot like how the Earth's international community built terrestrial networks and navigation systems.

"LunaNet is a framework of mutually agreed-upon standards, protocols and interface requirements allowing future lunar missions to work together, conceptually similar to what we did on Earth for joint use of GPS and Galileo," Ventura-Traveset explained. He added that the space community has a chance to agree on interoperability before navigation and networking satellite systems are deployed, this time around.

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On Earth, the various global navigation satellite systems (GNSS) were built by different countries – GPS by the US, Galileo from the EU, Glonass from Russia and BeiDou for China – all of which keep their own precise onboard time.

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Time being one of the [8]fundamental parts of determining position using satellites, interoperability standards had to be agreed upon after the launches of the various GNSS constellations.

That has meant making up for slight timing differences between GPS, Galileo and other GNSS systems by introducing fixed offsets – something that everyone involved would prefer to avoid in building out LunaNet and similar systems.

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ESA navigation system engineer Pietro Giordano said a joint international effort to create some form of Lunar standard time began after a November meeting of ESA's Space Research and Technology Centre.

"We agreed on the importance and urgency of defining a common lunar reference time," Giordano said, and it couldn't be more urgent as [11]NASA and the [12]ESA ; each have their own plans to deploy navigational satellites as part of LunaNet The US's Lunar Communications Relay and Navigation System, and the ESA's aforementioned Moonlight program.

Hey Moon - your time is, like, totally dilated

Deciding on a Universal Standard Lunar Time isn't going to be easy. In fact, it's probably going to be a good deal harder than it was on Earth, according to the ESA's chief Galileo engineer Jörg Hahn.

While the success of GNSS can be used as a basis for designing a Lunar navigation and timekeeping standard, "stable timekeeping on the Moon will throw up its own unique challenges – such as taking into account the fact that time passes at a different rate there due to the Moon's specific gravity and velocity effects," Hahn said.

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NASA engineers have known about the issue of time moving differently on the Moon for decades, as evidenced by a 1972 [14]technical note [PDF] NASA published on how to make relativistic time corrections for the Apollo 12 and 13 missions.

According to the ESA, a clock on the Moon gains around 56 millionths of a second per day compared to a terrestrial equivalent, and that rate changes based on whether the clock is in orbit or on the Moon's surface. It's not much, but with humans set to have a permanent presence in orbit once the [15]Lunar Gateway is built, those millionths of a second will add up.

Finally, time can't be measured without a "Selenocentric reference frame" that would focus like the [16]International Terrestrial Reference Frame – a three-dimensional point that sets a physical standard from which to measure universal time.

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Once they work out exactly how to measure it, the team is also trying to determine whether a single organization will be responsible for maintaining Lunar timekeeping, as is the case on Earth.

But these chronological triggers haven't upset Ventura-Traveset, who said exploration has long been a key driver of improving our understanding, and keeping, of time, and now we have an opportunity to extend that knowledge to the Moon.

Developing this standard now, Ventura-Traveset said, "will not only ensure interoperability between the different lunar navigation systems, but which will also foster a large number of research opportunities and applications in cislunar space." ®

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[1] https://www.esa.int/Applications/Navigation/Telling_time_on_the_Moon

[2] https://www.nasa.gov/feature/goddard/2021/lunanet-empowering-artemis-with-communications-and-navigation-interoperability

[3] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_offbeat/science&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2Y-8wVe6Q6Vfkb3ucgkW6lwAAAJY&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0

[4] https://www.theregister.com/2022/12/12/orion_returns/

[5] https://www.theregister.com/2023/02/06/field_trip_european_space_agency/

[6] https://www.theregister.com/2023/02/22/texas_meteorite_nasa/

[7] https://www.theregister.com/2022/11/25/esa_2022_astronauts/

[8] https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/gps/howitworks

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[11] https://www.theregister.com/2022/12/20/nasa_ai_moon_navigation/

[12] https://www.theregister.com/2021/05/21/esa_moonlight/

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[14] https://ntrs.nasa.gov/api/citations/19720022040/downloads/19720022040.pdf

[15] https://www.theregister.com/2022/11/15/nasa_capstone_cubesat_moon/

[16] https://www.iers.org/IERS/EN/DataProducts/ITRF/itrf.html

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[18] https://whitepapers.theregister.com/