3 ms·

The issue is that time runs faster on the moon, so clocks will get out of sync, or you need to define a lunar second which will be different from an earth secon

The issue is that time runs faster on the moon, so clocks will get out of sync, or you need to define a lunar second which will be different from an earth second. So pure UTC is not going to make it. General relativity strikes again.

vlovich123 4y ago
A quick search yields 59 microseconds per day, right? That doesn’t seem significant at all for people to people interactions (21ms per year). For automated systems that’s of course a different challenge and I think rather than clocks it may be useful to think about causality signatures if you’re comparing across the moon and earth. But even for automated systems, the time delay between earth and the moon is going to swamp 59 microseconds except for a minority of use cases that need ultra precise clocks that work across vastly different inertial frames. Now for local operations on the moon? You probably don’t want UTC but that’s probably better established by sending scientists and engineers to the moon and letting them figure out what would make sense. Until then UTC is fine I think
- treve 4y ago
  GPS satellites comes to mind.
  - PeterisP 4y ago
    Yes, GPS satellites are a good example. We don't keep a "GPS time zone"[1] or define "GPS second" to account for that - we calculate the dilation the satellite gets due to relativity (e.g. https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Relativity https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...), treat that dilation as a predictable "error" from the correct (Earth) time, and adjust for that. [1] There actually is a time zone called "GPS time", however, that explicitly doesn't include the relativistic effects, it has the same second length as UTC, not what an atomic clock on a GPS satellite would measure, it differs from UTC only in how it counts days - in "GPS time" there's always 86400 seconds in a day and leap seconds are never added to adjust for Earth's rotation speed.
    - zokier 4y ago
      The same way it makes sense for GPS to use Earth-based timescale instead of something like TCG[1] or TCB[2], it makes sense for lunar positioning to use lunar based timescale. > There actually is a time zone called "GPS time", however, that explicitly doesn't include the relativistic effects It does include the relativistic effects of Earth [1] https://en.wikipedia.org/wiki/Geocentric_Coordinate_Time https://en.wikipedia.org/wiki/Geocentric_Coordinate_Time [2] https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time
- Xylakant 4y ago
  Obviously no Astronaut is going to miss their teams daily standup call over this. But it’s already an issue for gps satellite precision and if you want to go into precise scientific experiments it’s probably also an issue, especially once we get into long-term experiments. Not to speak off a manned base. So it makes sense to consider what can be done - and a defined lunar timezone with a known (tough changing) offset would not be the worst of all options.
amysox 4y ago
Definitely do not redefine the second. So many of our scientific units are based on the definition of a "second" as we see it on Earth, you'd throw everything into chaos. I believe one of the characters in The Moon is a Harsh Mistress pointed this out, as the newly-empowered Lunar government was considering redefining time to match the lunar day/night cycle. (In that book, they certainly recognized "semi-lunars" reflecting the lunar day and night cycle, but, since Loonies, by and large, lived in pressure domes or underground, those didn't matter unless you were actually going to the surface. It definitely affected when people went out on the surface, why they went, and--because of the solar radiation hazard during bright semi-lunar--how long they stayed out.)
- deleted 4y ago
  [deleted]
- dekhn 4y ago
  Seconds are now defined on atomic transitions, which can be measured at the moon and used to form a base clock that other clocks sync to, although I'm pretty certain loonies would instead use an earth seconds-based clock with corrections, periodically ensuring that they stayed in sync.
Consultant32452 4y ago
Remember that time we accidentally crashed a rover or something into Mars because someone forgot to convert their units? That was as hilarious as it was unfortunate.
- HWR_14 4y ago
  It was because there were two modules, one that used lbs-seconds for impulse and one that used newton-seconds for impulse. The spec dictated newton-seconds.
PeterisP 4y ago
There is literally zero need to have long-term tracking of "Moon time". For all long-term (day/week/year scale) you can work in whatever Earth time zone suits you best, and when the clocks will drift - no matter if that drift is due to relativity or plain old inaccuracy - just sync them to Earth time. Yes, an isolated non-adjusted clock on the moon would have slowly accumulated some drift from the correct time, but who cares about that? For use cases which need extreme accuracy, do explicit adjustments for the time dilation in these specific conditions, just as we do for GPS satellites, instead of trying to define a single moon-wide lunar second or some offset from UTC.

vlovich123 4y ago

A quick search yields 59 microseconds per day, right? That doesn’t seem significant at all for people to people interactions (21ms per year). For automated systems that’s of course a different challenge and I think rather than clocks it may be useful to think about causality signatures if you’re comparing across the moon and earth. But even for automated systems, the time delay between earth and the moon is going to swamp 59 microseconds except for a minority of use cases that need ultra precise clocks that work across vastly different inertial frames. Now for local operations on the moon? You probably don’t want UTC but that’s probably better established by sending scientists and engineers to the moon and letting them figure out what would make sense. Until then UTC is fine I think

treve 4y ago
GPS satellites comes to mind.
- PeterisP 4y ago
  Yes, GPS satellites are a good example. We don't keep a "GPS time zone"[1] or define "GPS second" to account for that - we calculate the dilation the satellite gets due to relativity (e.g. https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Relativity https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...), treat that dilation as a predictable "error" from the correct (Earth) time, and adjust for that. [1] There actually is a time zone called "GPS time", however, that explicitly doesn't include the relativistic effects, it has the same second length as UTC, not what an atomic clock on a GPS satellite would measure, it differs from UTC only in how it counts days - in "GPS time" there's always 86400 seconds in a day and leap seconds are never added to adjust for Earth's rotation speed.
  - zokier 4y ago
    The same way it makes sense for GPS to use Earth-based timescale instead of something like TCG[1] or TCB[2], it makes sense for lunar positioning to use lunar based timescale. > There actually is a time zone called "GPS time", however, that explicitly doesn't include the relativistic effects It does include the relativistic effects of Earth [1] https://en.wikipedia.org/wiki/Geocentric_Coordinate_Time https://en.wikipedia.org/wiki/Geocentric_Coordinate_Time [2] https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time
Xylakant 4y ago
Obviously no Astronaut is going to miss their teams daily standup call over this. But it’s already an issue for gps satellite precision and if you want to go into precise scientific experiments it’s probably also an issue, especially once we get into long-term experiments. Not to speak off a manned base. So it makes sense to consider what can be done - and a defined lunar timezone with a known (tough changing) offset would not be the worst of all options.

amysox 4y ago

Definitely do not redefine the second. So many of our scientific units are based on the definition of a "second" as we see it on Earth, you'd throw everything into chaos. I believe one of the characters in The Moon is a Harsh Mistress pointed this out, as the newly-empowered Lunar government was considering redefining time to match the lunar day/night cycle. (In that book, they certainly recognized "semi-lunars" reflecting the lunar day and night cycle, but, since Loonies, by and large, lived in pressure domes or underground, those didn't matter unless you were actually going to the surface. It definitely affected when people went out on the surface, why they went, and--because of the solar radiation hazard during bright semi-lunar--how long they stayed out.)

deleted 4y ago
[deleted]
dekhn 4y ago
Seconds are now defined on atomic transitions, which can be measured at the moon and used to form a base clock that other clocks sync to, although I'm pretty certain loonies would instead use an earth seconds-based clock with corrections, periodically ensuring that they stayed in sync.

Consultant32452 4y ago

Remember that time we accidentally crashed a rover or something into Mars because someone forgot to convert their units? That was as hilarious as it was unfortunate.

HWR_14 4y ago
It was because there were two modules, one that used lbs-seconds for impulse and one that used newton-seconds for impulse. The spec dictated newton-seconds.

PeterisP 4y ago

There is literally zero need to have long-term tracking of "Moon time". For all long-term (day/week/year scale) you can work in whatever Earth time zone suits you best, and when the clocks will drift - no matter if that drift is due to relativity or plain old inaccuracy - just sync them to Earth time. Yes, an isolated non-adjusted clock on the moon would have slowly accumulated some drift from the correct time, but who cares about that? For use cases which need extreme accuracy, do explicit adjustments for the time dilation in these specific conditions, just as we do for GPS satellites, instead of trying to define a single moon-wide lunar second or some offset from UTC.