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I have a solar system containing multiple planets, most having an intelligent species inhabiting it. To keep a common peace, they created a large government system to help stop territorial wars between planets for uninhabited planets as well as trade between each planet. But to do this I need to come up with a common time system but have no idea how to do it since I won't be able to go based on any one planet's rotation, or if I did it would inconsistent for most other planets.

Is there any way for an entire solar system to share a common time zone so they can consistently communicate with each other?

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    $\begingroup$ The time system only needs to start from a common point. After that, you just need a unit of time. Vingean kiloseconds (megaseconds, gigaseconds) are the way to go. The starting point should of course be 1999, when Will Smith starred in the universal classic Wild Wild West. $\endgroup$ – John O Mar 9 at 19:51
  • $\begingroup$ A long-solved problem in the United Federation of Planets: Stardates $\endgroup$ – user535733 Mar 9 at 21:17
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    $\begingroup$ Your problems may run deeper than you think. The people of one planet, with five fingers and two thumbs per hand, and three arms, want a number system based on 3*7. Another people, whose tendrils can split any number of times, prefers powers of two. The silly humans prefer 2*5. And the religious order ruling one planet declared prime numbers holy and uses a different prime multiplier for each digit. So how do you divide up or even count the time units once you have them? (Computers may help $\endgroup$ – David G. Mar 10 at 0:43
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    $\begingroup$ @DavidG. Do we? I mean - our clocks use 12 hours, split in 60 minutes. We group our workdays in packs of 7, in blocks of more or less 30 days. And that's just for one calendar system! Dates are weird. $\endgroup$ – T. Sar Mar 10 at 15:59
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    $\begingroup$ Who has the most money in your solar system? Their superior economic position will carry a lot of weight in negotiations during the formation of your government. $\endgroup$ – Muuski Mar 10 at 16:41
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Central Orbit Time (COT). The system can use the planet closest to the star, as it's unlikely to be inhabited and therefore not showing bias to any one planet or people. This orbit is then divided into reasonable intervals, such as taking 1/100th of its orbit, so on and so forth. A "Solar year" Can be a number of these orbits that fits closest to most planets' calendars.

Example: Mercury.

  • COT Orbit: 87.97 days
  • COT Day: 0.88 days (1/100th of orbit)
  • COT Hour: 2.11 hours (1/10th of COT day)
  • COT Minute: 1.27 minutes (1/100th of a COT hour)
  • COT Second: 0.76 seconds (1/100th of a COT minute)
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    $\begingroup$ I’d avoid redefining a second. We’ve got definitions for that based on fundamental properties of the universe. $\endgroup$ – Joe Bloggs Mar 9 at 19:35
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    $\begingroup$ @JoeBloggs We do, but this is a question about some fictional solar system. Each planet/intelligent species might have their own definitions of any unit of time, second included. Those were just an example based on Human measurements. $\endgroup$ – Skyler Mar 9 at 19:45
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    $\begingroup$ @Skyler I see what you’re getting at. Eventually they’ll redefine the COT second in terms of fundamental constants, but it will still be based on some fraction of the COT orbit (which is, of course, a fictional orbit) $\endgroup$ – Joe Bloggs Mar 9 at 20:42
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    $\begingroup$ @JoeBloggs While the duration of a caesium vibration is pretty fundamental, the value of 9,192,631,770 is fairly arbitrary. Planck lengths, atomic mass units, and electronvolts are what I'd call "fundamental" units in that they're based on observed facts of the universe; things that alien life anywhere in the universe could plausibly have derived independently. The duration of a second may be expressed in fundamental units, but it's still tied to the rotational period of a specific rocky planet orbiting a certain star in the unfashionable end of the western spiral arm of a particular galaxy. $\endgroup$ – anaximander Mar 10 at 16:04
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    $\begingroup$ Clearly proposed by someone favoring the Inners. $\endgroup$ – jeffronicus Mar 10 at 16:55
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I need to come up with a common time system but have no idea how to do it since I won't be able to go based on any one planet's rotation, or if I did it would inconsistent for most other planets.

You have no problem at all.

Greenwich Mean Time

As a point of reference, it's perfectly OK. There's no reason why everyone cannot constant reference the time from that point. OK, your species may not have Greenwich itself, but you can basically pick any arbitrary location as a reference and everyone else uses it.

Whatever time, day and date they say, you use as a base for your local system.

Your local system can apply it's own offsets ("timezones") any way it likes. It may have "local Greenwich Mean Time" which it's own planetary timezones reference, but which stays calibrated in some well defined way to the Main GMT.

You can pick any orbiting body or an artificial satellite for the Main GMT - this might be politically better, but it makes no difference to the system. I'd probably go with a system of reference satellites that can be tracked accurately and which can supply redundancy and self correction - orbital mechanics and perturbations from a variety of sources will need something like this.

Days and Dates are kept in broad sync (allowing for local, but well defined, offsets) by everyone following the Main GMT reference for date.

Atomic Clocks

Precision timing will be done using atomic clocks. These can be used to keep a strict scientific watch on time as measured in each reference frame (relativity applies). But there's no need to use these for date and time directly. We'll have our Main GMT (and it's own clocks) and the Local GMTs which reference off the Main GMT so that relativistic effects don't get lost.

Now this isn't exactly trivial. There are lots of complex effects due to relativity, but there are scientists and computers for handling the details and they're pretty good at this, so leave them to it and don't argue.

How long is a Year, etc.

This is a political thing. There's no reason why local systems cannot operate (as custom) their own day, month and year stuff. But in practical terms for business, military and (really) staying sane, people will either work with the standard date or translate to it (aren't computers wonderful - no need to actually do the calculations yourself !).

If experience tells us anything it's that politically you solve the problem of different systems largely by working around it. Trying to ram a time system down one planet's throat that they don't want might well start a war, so diplomats just rename things and define abstract standards that people can "adopt". Computers will do the dirty work. You'll find suddenly that companies work to e.g. ISO-19288645 time and only some poor software developer has to actually understand the darn thing - everyone else just looks at their smart phone !

it would inconsistent for most other planets

Just to emphasize : you do not need consistency in terms of using the same system. You need systematically well defined local time systems that can be translated into each other (by computers) and kept track of using well defined reference points.

Politics

Because politics is involved (different races, different planets), they'll not actually agree an existing standard. To save face they'll invent a complete new one that is different from all the existing systems and no one likes for that reason. It will, over time, become an unavoidable standard everyone kind of knows or has to occasionally reference (e.g. in a business memo), but locally we'll all still use the system defined for local convenience.

So The Year of The Twelve Tailed Finch may be starting on Planet Mongo, but here on Planet Mango we're in the middle of the Imperial Year 27816. The ISO standard may say it's Dec 7th 1942, but no one cares about that.

Computers - stopping wars is our business.

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We have sent probes to all the most important planets in our own system now. The most interesting ones, in regarding to this question, were the ones sent to Mars. There is an article in Wikipedia about timekeeping on Mars:

A convention used by spacecraft lander projects to date has been to enumerate local solar time using a 24-hour "Mars clock" on which the hours, minutes and seconds are 2.7% longer than their standard (Earth) durations. For the Mars Pathfinder, Mars Exploration Rover (MER), Phoenix, and Mars Science Laboratory missions, the operations teams have worked on "Mars time", with a work schedule synchronized to the local time at the landing site on Mars, rather than the Earth day. This results in the crew's schedule sliding approximately 40 minutes later in Earth time each day. Wristwatches calibrated in Martian time, rather than Earth time, were used by many of the MER team members.

Local solar time has a significant impact on planning the daily activities of Mars landers. Daylight is needed for the solar panels of landed spacecraft. Its temperature rises and falls rapidly at sunrise and sunset because Mars does not have the Earth's thick atmosphere and oceans that soften such fluctuations. Consensus as has recently been gained in the scientific community studying Mars to similarly define martian local hours as 1/24th of a Mars day.

Each planet in your system may have its own timekeeping, convertible to the timekeeping of the central government the same way we convert currencies. In our own world, the value of most currencies is counted as relative to either Euros or US dollars. You could do like the martian timekeeping, which has a martian second valued at 1.027 Earth seconds. The difference here is that time conversions would not fluctuate following economics.

So it could go like:

-Hey this task me is going to take me foo's to accomplish.
-Yeah but here in Kerbin we measure time in bars. How much are three foo's in bars?
-Let's see... one foo is like one hour and three minutes in gov time, rounded down. A bar is like forty seven gov minutes and one second. So carry the three... It will take about four bars, give or take.

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Timezones work on Earth because they don't change very often. California is always 3 hours behind NY. This is not true between planets. Even calendars don't work.

Saying it's noon on March 9 in New York tells you what time of day it is and what season it is. You know it's light out, and you know the weather is likely to be warming up. But what does that tell you about the conditions at Olympus Mons on Mars? Nothing. Mars's year is longer than Earth's, so Martian and Earthling calendars will quickly desync. Mars's days are longer too, so even their clocks can't sync up. This situation just gets worse the more planets you try to include.

But I don't think it really matters honestly. We need timezones here on Earth because you can easily call California from NY, so it's good to know what time it is there so you can be sure someone will answer. But due to the fact that light takes about 40 minutes to get to Mars, that kind of thing just isn't possible. Imagine dialing someone on Olympus Mons and listening to the phone ring for over an hour; remember it takes 40 minutes for your call to reach them and 40 minutes for their reply to reach you. This just gets worse if you start adding colonies outside the asteroid belt.

You don't need a unified clock system because people can't communicate in real time over these distances anyways.

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    $\begingroup$ "Timezones work on Earth" - Citation needed. Timezones are a total pain to deal with. Heck, just look at how silly timezones are in Australia! $\endgroup$ – T. Sar Mar 10 at 19:58
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    $\begingroup$ @T.Sar I'm a software engineer, I know all about how annoying timezones can be here on Earth. They're even worse if there's more than one planet. I wonder how much Microsoft will pay out in workman's comp claims when all their programmers suffer nervous breakdowns from trying to come up with software to handle interplanetary timezones. $\endgroup$ – Ryan_L Mar 10 at 21:11
  • $\begingroup$ This is the first thing I thought when reading the question - but I'm not really an answerer in this community. Remote communication will be mostly asynchronous, so the time isn't that relevant as it seems - unless physics works differently and they find a way to, say, do instant calls between planets. $\endgroup$ – igorsantos07 Mar 11 at 3:06
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Use the rotation of the sun. 1 solar rotation would be the equivalent of a day. So many days is a week, month, year and so on. That way you are not favoring any one planet.

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    $\begingroup$ Except that our star, the Sun, takes about a month to rotate. Actually it takes longer to rotate in higher latitudes. So a day equivalent would be a fraction of a stellar rotation, a stellar rotation would probably make a good month equivalent, and several stellar rotations could make a year equivalent. $\endgroup$ – M. A. Golding Mar 9 at 21:38
  • $\begingroup$ There's an eleven-year cycle of dimming and brightening, but I'm not sure how much variation there is, or how much tech you need to sync up to it once you know it's there. $\endgroup$ – John Dvorak Mar 10 at 4:32
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    $\begingroup$ I was thinking in the same direction: Using the sun as a reference would mean they refer to something they all have in common. Be it practical or not, it has great diplomatic benefit. $\endgroup$ – Burki Mar 10 at 11:56
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The individual units don't really matter.

They would have multi calendars and calculators that just worked out the relationships of whatever the local times were given their tech.

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  • $\begingroup$ This, plus an agreement to use one of the system's uninhabited planets as a central reference. $\endgroup$ – Jontia Mar 10 at 21:25
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They evolved separately, but at some point settled on someone's language as a common means of communication.

You'd use that language's host culture's time system and calendar.

This would be true by default prior to the invention of any universal standard, because it'd be the time system that would make the most sense in the common language. If someone did invent a universal standard, nobody would bother using it, because it'd be really awkward to talk about for everyone, whereas by that time anyone who knew the common tongue would've already gotten used to using the common tongue's calendar.

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  • $\begingroup$ This assumes, of course, that the "common language" isn't a creole formed by merging 2 or more of the languages (first as a simplified pidgin, and then evolving into a language in its own right) $\endgroup$ – Chronocidal Mar 25 at 8:35
  • $\begingroup$ @Chronocidal It'd be the time system used in the region that the pidgin language was formed. Unless we're talking about an artificially synthesized pidgin language, in which case, all bets are off, but I think that's several orders of magnitude more improbable than an artificially synthesized time system. The only scenario I'd buy that in is if two languages had fundamentally incompatible methods of communicating, like say verbal and completely non-verbal, in which case I may accept an artificially constructed middle ground as a probable solution. $\endgroup$ – Iron Gremlin Mar 25 at 23:32
  • $\begingroup$ A pidgin developed by traders will form on both planets, as the traders travel back and forth between them. Which of the two would you pick? $\endgroup$ – Chronocidal Mar 26 at 8:24
  • $\begingroup$ @Chronocidal I don't think there's enough social interaction in trading to organically synthesize a new language. You need a population to continue using the language 'at home,' as it were, because continuous social contact is necessary for the evolution of a language. You could have a fixed population that was nomadic, in which a case could be made for a calendar surrounding the phases of migration, should such a thing be practically measurable, but the lower order time units aren't practical to synthesize organically - it messes with too much in the way of shared assumptions. $\endgroup$ – Iron Gremlin Mar 26 at 13:30
  • $\begingroup$ Whyever not? That is how Chinese Pidgin English, Eskimo Trade Jargon and Tok Pisin all formed, so we have existing historical precedent.... $\endgroup$ – Chronocidal Mar 26 at 14:16
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Common periodic events that can be measured:

  • Solar Rotation
  • Galactic Revolution
  • Uninhabitable Body Revolution

A common time system can be built around this, and then this can be further subdivided down for each planet's common use. Since each planet has it's own system based on a common model, similar to how we have time zones, it would be simple to convert between different planetary times, and when travelling in interplanetary space, you have a mutual time system to use.

To be clear though, for a system of time measurement, you need both a source event, and a periodic measurement event to measure time by. Missing either and time is meaningless. For example, an atomic clock by itself is useless as a time piece. You need to know not only the decay rate of the atom, but also, how many atoms have decayed since a specific event. Whether that event is the big bang, all the planets in the solar system were in alignment for the first time in memory, the formation of your star, or planet, or what the event was, that is a choice, but you must have an event.

Remember this for your world, to make time have meaning, it must have a beginning and a direction.

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None of the inhabited planets are suitable, in part due to political ramifications; when a single planet of intelligent life has colonised other planets, you can just stick with the Home Planet time, but that's not what seems to have happened here. So, you need to use a timing mechanism that can be measured independently from any planet, anywhere in the solar system!

Something like basing it on certain Pulsars outside the solar system (the longest known pulsar period is just under 2 minutes - the shortest is about 1.4 milliseconds), or on the perihelion of an uninhabited (uninhabitable?) planet or significant short-period comet as the start of the new System Year. (There are several comets in our own solar system with a period of about 4 years - imagine syncing all of the Calendars on the 29th of February)

Of course, the problem then is that "standard time" won't sync up with "local time". This is much worse than time-zones on Earth: Sunrise in New York is always about the same distance behind Sunrise in London, and ever Daylight Saving will only shift this by about 2 hours. If Sunrise on Planet A is every 19 hours, and Sunrise on Planet B is every 31 hours, then the time-difference shifts by 12 hours every day - a 31-day month on Planet A is the same as a mere 19 days on Planet B.

People on-planet are likely to stick to their local time, because that's what syncs to the Sun, and what the animals will keep to. Commercial Pilots flying people back-and-forth between planets will stick to their own planet's local time (because that's what they'll sleep to!). Banks and other multi-planetary bodies / businesses will probably stick to Standard Time at a high-level, and for paying wages, but Local Time for branches.

Any long-term space habitation might stick to Local Time, if everyone is from one planet, or they might swap to Standard Time (for example, some research station in Antarctica stick to their home-nation's time zones, while others just operate on GMT instead.)

Watches and clocks will most likely have at least 2 dials - one for Local Time, one for Standard Time. People who travel or deal with other planets regularly will have more dials, or more watches.

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    $\begingroup$ +1 for multi-dial clocks and watches due to the time-difference shifting between locations on a regular basis. As much as a common system of time might be agreed upon, the local differences won't go away because of the biological cycles that are tied to the local time cycles. $\endgroup$ – LHM Mar 17 at 13:58

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