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I’m a game developer making a space-based game. I want to implement a system where the time is different depending on the planet. I think this would work like time zones, but to be honest, I’m not sure how this works in real life, so that’s why I ask here instead of in stack overflow or other forum.

I'd like to know the basics first. Is there any way of knowing what time is on other planet? Does it exist a “universal” time that could be used to measure the time in the Solar System (like UTC but for planets)?.

Sorry if this might sound ignorant, but as you may have already noticed, I’m not a lot into these subjects.

EDIT:
I have read a lot of your answers, and I came up with the question: Wouldn't it be possible to have a time system relative to the Sun (In case that one does not exist already)?

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All of the answers above are great and they are all based on precise science. Yeah, recalculating the time from "here" to "there" might be tedious and, well, unfortunately boring.

However, you are developing a game and so a precise science does not really have to apply as long as there are still some "rules" of how does the time flow. So far the coolest idea that I ever saw about that was a science fiction story where the time was flowing differently in different parts of the planet. E.g. if you go North from the Equator then the time would speed up and if you go South, then it would slow down.

If I were developing a game where time is an essence, then I would explore the situations like that rather than bothered myself (and the players) with boring questions of how to recalculate Mars daytime to Earth daytime.

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    $\begingroup$ I think there's a happy medium though, like the black holes in Interstellar -- yes it's very much fiction, but based on real science. For example what would time be like on a planet with a 90-degree tilted orbit? and what if it were tidally locked to its star? I don't know if that's even possible cause I'm not scientist, but it's an idea. $\endgroup$ – wjandrea May 22 at 1:52
  • $\begingroup$ Answers to such follow-up questions might be suited for the Worldbuilding stack exchange. I love that place :) $\endgroup$ – Anton Hengst May 22 at 22:11
  • $\begingroup$ "boring questions of how to recalculate Mars daytime to Earth daytime" You and I have a very different idea of what is boring! $\endgroup$ – JiK May 25 at 8:55
  • $\begingroup$ @JiK Yeah, you are right. I do find the stuff like that: arxiv.org/abs/1105.3735 , arxiv.org/pdf/1401.4173.pdf , ... fascinating and, yes, I do understand most of what's written there, and ... it is not actually my job to know that. However, here we are talking about game development, and the game must be interesting to many people (mostly kids?), who do not (yet) have a PhD in some science. So the game rules, imho, must be simple and cool, so that the people who play it, will be able to understand, like, and use them in the game. $\endgroup$ – Konstantin Konstantinov May 26 at 0:24
  • $\begingroup$ this is not an answer. $\endgroup$ – jumpjack Aug 30 at 14:04
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Some basics:

Every planet, moon or asteroid have their own rotation period along some specific axis. In very good approximation the period is constant (but not exactly). So every celestial body has own daylength.

All planets and asteroids have different periods of rotation around the Sun. So different yearlengths.

All Most known regular moons of planets are tidally locked. So daylength of a moon is equal to its rotation period around parent planet.

(Here and below I assume all orbits circular. For most planets and regular moons it's good approximation, but even in these cases there are some subtle effects of ellipticity I will not duscuss in this post.)

So, from this - the basic time units for planets, moons and asteroids are day length (sun day, or synodic day - between two noons) and year length. Other units - hours, minutes - are artifical, they are our invention.

The day length and phase of day define current conditions at given surface point on a planet. Noon is bright and warm, night is dark and cold, to simplify.

Year phase (analog of summer, autumn, winter) can be important if the planet has orbital tilt (angle of rotation is not perpendicular to the rlane of orbit around the Sun). Most of planets have some tilt, so they have seasons (cold winter for upper hemisphere is warm summer for lower hemisphere and wise versa, change every half-year). Also because of a tilt the polar regions will have polar night and polar day periods. Uranus (with moons) is an ultimate example - because of its tilt and year length it has decades-long polar nights.

At Earth we define meridians. Historically Greenwich meridian "won", so we now use GMT noon as reference point for time measurement (simplification, actually it's some more complex). For other celestial body astronomers do the same. Some prominent (easily identifiable) topografic point is chosen as zero meridian. Well, for gaseous planets it can't be done, but these beasts also don't rotate as a whole, their rotation is differential...

About time sinchronisation between two celestial bodies - it's conducted by two technicues. First you can launch spacecraft from celestual body A to body B wich carries precise clock. Currently atomic clocks are used. The second technique is two-way radio (or laser) communication. The signal with current time mark is sent from A to B and then immediately retranslated back with timestamp at B. The speed of light in vacuum is known with high precision. So clocks at B at A can be sychronized with B.

P.S. Of course if the game's universe has different civilisations/cultures at different planets - they can have their own time units. Astronomicaly-derived time units will be based on local daylength an yearlength, I think, because it's practical.

Period of a moon rotation can be involved too, but in our current Gregorian calendar months are not equal to the Moon rotation period, now months are set to have integer number of months in year.

If a planet has more than one moon - it's interesting theme. If the moons have proportional periods, these proportionalities can be time units too. Most prominent example - three moons of Jupiter (Io, Europa and Ganimede). Their periods are proportional as 1:2:4 respectfully. So every least common multiple of the periods their positions observed from the planet repeat exactly. And it's not the only example in our Solar System.

In the case the periods of moons are not proportional - there will be happening "moon parades", like "planet parades" we observe from Earth, but not in regular basis.

Also - not all time units are astronomy-based. For example the week is just a regulation of work-rest cycle. (Although maybe 7-day week was influenced by 28-day lunar month).

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    $\begingroup$ Note that the statement "all known regular moons of planets are tidally locked" is only true for a specific formal definition of "regular moon" that excludes e.g. Hyperion and Phoebe. $\endgroup$ – Ilmari Karonen May 21 at 14:18
  • $\begingroup$ And in our Moon the tidal lock is not quite total. Our Moon's orbital period is locked with its rotational period, but the orbit has not been forced to (near)-circular. So the Moon actually wobbles around its ostensible locked orientation relative to Earth. $\endgroup$ – Oscar Lanzi May 21 at 14:39
  • $\begingroup$ Gaseous planets do rotate as a whole, giving well-defined days. $\endgroup$ – Mark May 21 at 22:43
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Currently, UTC is the basis for time that is used across the Solar System (edit: not in science, however). You not only could, but would have to have a universal time in a future universe. It's a good assumption that any realistic future will have computers, and these rely on a universal time to store server-side events.

There is no strong reason for the official date-time to correspond to any solar radiation or climate patterns on the side of the end user. Across the planet, they already don't - knowing it's 6 PM in the middle of May has completely different implications in Stockholm, Singapore, Cairo, Adelaide and McMurdo Station.

It depends on how interconnected your world is, of course. The distances within the reasonably-usable zone of the Solar System are quite short - Earth to Mars emails would take 3 to 22 minutes - so the most realistic outcome will be for all non-Earth planets to set their official clocks by some Earth timezone. It will most likely be UTC, but alt-futures could have it be the dominant timezone of the most culturally and economically important country.

Like with timezones not quite matching insolation patterns, this is partially the case already. When dividing timezones, countries consider not only geography, but also population density. Some low-populated areas get merged into common timezones, while areas close to the capital timezone often set their clock to it for convenience.

Within a one-star future, Earth will dominate in population and economic importance, so knowing whether the New York Stock Exchange is open or if it's time for The Tonight Show will likely matter more than whether it's daylight on Mars... possibly even to more people on Mars itself.

Other planets will not have the same seasonal and day-and-night patterns that Earth does, so trying to find Titan's equivalents of January and June is going to be a fool's errand. So, most likely, time will be Earth's time, and local illumination and seasonal cycles will be tracked separately, the way we treat weather broadcasts. These cycles can be your local times.

The real definition challenge begins once you enter star systems that are moving at relativistic speeds relative to one another, which means that clocks cannot be kept in sync without using different time units. Local time and Earth time will change differently in the same amount of local time. GPS satellites already have to correct for it, but they only have one simple job, not a whole infrastructure to run.

There is currently no agreed-upon framework for managing relativistic time across the universe (edit: but there is in the Solar System). The hardware second has to be the local second, to keep clock generators compatible, but managing it not being a second on the homeworld will take a lot of mental adjustment. It gets seriously complicated when two colonies in different reference frames try to agree between themselves and Earth. In all likelihood, the concept of linear time will end up inapplicable for interstellar communication; timestamps will have to get 4-dimensional.

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Regarding "Is there any way of knowing what time is on other planet?", you could of course keep track of local time at a specific place on another planet. Similar to how a news room might have clocks for New York, London, and Tokyo, you could add another clock for Mars City. This clock would need to run slightly slower than Earth clocks, since a Martian day is 24h 37m, so it should take longer for the hands to complete one revolution.

Note that because the day lengths of Earth and Mars aren't simple integer multiples, the relationship between New York and Mars City time will continually drift over time and have a very long cycle before repeating. One day, New York and Mars City local noon might coincide, but a week later, Mars City local noon will occur at 4:31pm New York time. So, the conversion between New York and Mars City time isn't as simple as converting between time zones - on Earth if I know what time it is in New York, I can tell you what time it is in London, since that relationship is fixed. That is not the case for Mars City, since that relationship will change every day - given only New York time, I cannot tell you what time it is in Mars City. The same is true for the calendar instead of the clock. A Mars year is 1.88 Earth years, so the relationships between the calendars will drift over time as well.

It would be possible to have a universal clock and calendar, but it would have to be decoupled from the normal celestial patterns we associate with daily and yearly cycles in most places. We could force Mars City to use a 24-hour clock like New York, which would allow for simpler synchronization of times across planets, but the time wouldn't be as meaningful for the daily routines of Martian citizens. One day 12pm would be the time the sun is directly overhead, while a few weeks later, 12pm would occur in the dead of night. A clock like this would not be very useful for humans who are sensitive to circadian rhythms, but it might have applications in communications to avoid having to convert times, which will require knowing both the time and date.

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Everyone else has given very insightful answers. Let me give a more direct one,

I think this would work like time zones

[...] like UTC but for planets

On Earth we express moments in a local time in relation to UTC as offsets. For example, I am in Eastern Daylight Time right now which has an offset of -04:00. So my timestamp is 2020-05-21 14:11:00-04:00. To convert to UTC you just do the opposite of the offset, so add 4 hours. 2020-05-21 18:11:00Z. This works everywhere on Earth because we are all on one planet.

Mars's days are 1 earth day and 37 minutes. Because of that there's not really a way with a simple offset like our timezones to express the difference between Martian local time and UTC. Same thing for years, they aren't the same length, they're 687 days. (I'm rounding and ignoring leap years.) To convert from a Martian local time to UTC you'd need to know how long Martian days are (1477 minutes), how long a Martian year is (687 days), but that's not enough. You'd need some moment in time when the local datetime of all planets was known. An epoch. Let's go with UTC 0000-01-01 00:00:00Z. Let's just make up that the Martian date is June 1st and the time is noon. So we'd need to know something like +06-01 12:00:00. And you'd need to account for the difference in years/days so some sort of multiplier.

So a Martian timestamp could look something like 2020-11-22 00:11:00 +06-01 12:00:00 -- but this is only converting a sort of "GMT" for Mars to our time. You could throw in some other offset if you want. But it gets complicated. But this is wrong because I didn't include the multiplier. I have no idea how to specify that tersely.

All in all, you need to think of what times will be used for. In our world synchronized time wasn't really a major concern until railways were constructed. Similarly, maybe only space ports and interplanetary communications need to be concerned with this.

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It may help to read up on the history of human timekeeping, and the various different problems it was designed to solve.

The first and most obvious unit of time is the day. Evolution has invented the circadian rhythm to alter our biological behaviors around the day/night cycle, including a period of nighttime sleep. Melatonin is the human sleep hormone and its production is supressed by the visible presence of 400nm sky blue light, this is hardwired into our biology by evolution.

An exoplanet might also have a very short day of say under an hour, or it might be a moon with month long day/night cycle. If your planet is tidally locked, then you may not have a day-night cycle, or you may have a polar region that experiences seasonal "days" where the sun doesn't rise or set for 6 months at a time.

There was an experiment where humans were kept in a cave without access to clocks or sunlight. The humans eventually adjusted to a 36-hour sleep/wake schedule.

In a prison environment, you would simply start counting the day-night cycles from the most significant event in the history of your environment: such as upon first arriving, or a major change in social structure. Most calendars count from the birth of their founding prophet/leader.

Not all languages have the ability to count, and the linguistic framework may be limited to: one, two, three, many

The next evolution in time is recognizing other seasonal patterns, such as the moon, seasons and the year. The lunar calendar doesn't require any technology to keep track off, but it doesn't synchronize exactly with the year cycle (12.37 moons per year).

It took quite a few attempts to get the measurement of a year correct. The Roman Julian Calendar has at times been 304, 355, 365, 377 days long, with between 10 and 12 months. The usual historical design pattern is that the calendar got noticeably out of sync with the seasons, then the calendar was monkeypatched with leap months, inventing new months, or occasional redesigns to tweak the number of days in the year. Measuring a year requires statisticians to measure the seasons (such as the first blossom of spring). We are still adding leap days and leap seconds.

The year is important mostly to measure the seasons, upon which our survival through agriculture is dependant upon. It is important to know when to plant the crops and when to harvest.

Festivals are often held to mark the transition between seasons, and the changing of cultural rules/expectations. In ancient Greece, the Olympics was held every four years and marked a period during which war could not be conducted. Other festivals mark days remember historical events or times where exceptions to the usual social rules can be practice (Venice masked ball, or 29th February when a woman can propose to a man)

For a tidally locked planet without seasons, the sun is always at midday and the year might be a fairly meaningless concept.

The clock was first invented so monks could have an alarmclock for morning prayers. Before that, there was either the sundial (based on the relative position of the sun) or the hourglass (which is somewhat inaccurate). The quest for ever more accurate clocks turned out to be the solution for naval navigation and later GPS.

The modern physics method of the atomic clock is to measure the constant vibrational frequency of cesium atoms. The software complications of describing leap seconds is now driving a trend towards absolute atomic time, with a greater acceptance of allowing the solar day to become a few seconds out of synchronization with atomic time.

Timezones were only really required after the invention of the telegraph and the telephone. This beaks the sundial that assumes the shared position of the midday sun. The requirement is to synchronize clocks, even at the abuse of the solar days. Some extreme timezones can (like Western China), can be 2+ hours out of sync from the solar day. Here the timezone has almost been imposed by conquest.

An alien civilization could be forced to live under an imposed clock or timezone. Else for a colony on Mars, they would keep two different clocks and calendars: Mars time and Earth time. Different tasks and events would be aligned with each calendar, so some Mars individuals may still choose to keep an Earth-clock sleep schedule.

The Martian day (referred to as “sol”) is approximately 40 minutes longer than a day on Earth. This leads to interesting synchronization issues, equivalent to the jet lag caused on a merchant ship traveling one timezone every day.

The human body can also be trained into alterative sleep cycles such as the Uberman, which uses 6 x 20 minute naps during the day, without a 6h+ deep sleep. This is sometimes used by solo yachters who need to keep constant watch.

For an alien planet, ask the following questions:

  • What are the easily observable repeatable natural events in the environment?
  • Are there any important seasonal trends in the environment?
  • How does behavior change during different time periods?
  • What technology is available for measuring time?
  • What are the major historical events in your civilization?
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This is a longshot. But hear is my less than two cents from a non-scientific viewpoint.

Except for days, solar/lunar years, and seasons, time is a man-made construct. Societies have standardized our expressions of time. The dominant society has coerced, convinced, coordinated, compromised with, or forced the other remaining societies to adopt universal conventions of time. The length of a second has no scientific basis besides the ones we have retroactively assigned to it. Therefore, your problem is a sociological one instead of a scientific one.

Case in point. There have been many different times that mark the beginning of an Earth year based on each society. Now, we generally except a universal New Year Day with a standardized year numbering system. Even the start of the day is arbitrarily based on the time on a line on the globe going through a (relatively) arbitrary city on Earth. Pilots (especially international ones) have to deal with this. Aviation clocks are based on the 24 hour UTC convention. Not necessarily when the sun comes up.

So, getting back to your problem. Whatever person in whatever society on whatever planet on which your game or book is based will have to comply to the time conventions of the dominant society. Even if the dominant society is not based on that particular planet. If the society belonged to a cooperating alliance, they may share a coordinated time scheme for the interests of the alliance while maintaining the internal use of their own planetary time standards.

To coordinate times across planets, you have to measure the increments of time with something that does not change regardless of where it is being measured. The vibrations of a certain atom or element comes to mind. The trick will be to coordinate the simultaneous start of when the time convention begins. When is the zero hour (Year 0, day 0, 00:00:00.00). It could even be retroactive to an arbitrary point in time prior to the formation of the alliance.

Time distillation/compression/whatever due to the relativistic effects of things like gravity will be a different ball of wax. You will have to have a measurement tool that is not affected by the effects. Or, you will have to have a way of adjusting time measurement to keep all clocks coordinated. A clock outside the range of the effects, or a communications signal that broadcasts the time to all citizens and time pieces comes to mind.

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"True" (or "apparent") time is given by local solar altitude (=elevation) and azimuth (=bearing); reference points are sunrise, noon/transit and sunset w.r.t local meridian. But Sun alt/az of these 3 moments change from one day to another, so "mean time" is also defined.

How alt/az path of the Sun changes in months:

Sun paths

The difference between the true and mean times gives birth to analemma curve (alt vs az or declination vs time) and Equation of Time (time vs time).

Analemma as declination vs time (planetocentric coordinates): equation of time and analemma

Analemma as altitude vs azimuth (topocentric coordinates):

Analemma as altitude vs azimuth

Analemma evolution during day:

Multiple alt/az analemmas

To know local planet time you need to know ecliptic longitude of Sun (Ls) and rotation status of planet prime meridian: when they match, it is what we could call "Planet Reference Noon".

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