I was wondering if you could possibly make a form of time measurement that would apply to a planet in a galaxy and then another planet that is ~20,000 light years away. I want a unit of time that, for both planets, will be a whole number. Also here's useful info about the first planet (the planet I've been working on)...

  1. 1 year on the planet is 539 earth days.

  2. Plants and trees are thriving.

  3. There is a star that provides light about 150 million km away from the planet.

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    $\begingroup$ Are you asking for: a)a time system that can coordinate timestamps across arbitrary astronomical distances, or b) a time system for use on each planet that will give a whole number of units per day, in either planet? The second is doable, the first needs faster than light communication to be accurate or useful $\endgroup$
    – nzaman
    Commented Mar 2, 2018 at 6:44
  • $\begingroup$ nzaman - useful is correct. If there's FTL magic simultaneous communication they can easily work out an arrangement for "call you in an hour" with a basic calculator. If not, then what's the point? This question comes up a lot and haven't heard the point of it yet. $\endgroup$
    – Hebekiah
    Commented Mar 2, 2018 at 8:36
  • $\begingroup$ @Hebekiah: I know. People just don't get the scale of time and distance involved. 20,000 light years away means the sender would be dead for millennia before the message even got there. $\endgroup$
    – nzaman
    Commented Mar 2, 2018 at 9:32
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    $\begingroup$ Asking about FTL, communication, etc. is irrelevant. (a) Because it is outside the scope of the OP's question (we worry too often about the "why") and (b) because "call you in an hour" may be a whole lot less valuable than "news of the emporer's death" with an accurate timestamp. When it arrives, even millenia later, is irrelevant so long as it has an accurate correspondance to local time. $\endgroup$
    – JBH
    Commented May 7, 2018 at 16:34
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    $\begingroup$ @JBH I was referring to talking about the notion of the Emperor's death $\endgroup$
    – DKNguyen
    Commented Sep 19, 2020 at 15:12

6 Answers 6


Simple an atomic clock

An atomic clock is a clock device that uses an electron transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services,

In a bit more detail

The actual time-reference of an atomic clock consists of an electronic oscillator operating at microwave frequency. The oscillator is arranged so that its frequency-determining components include an element that can be controlled by a feedback signal. The feedback signal keeps the oscillator tuned in resonance with the frequency of the electronic transition of caesium or rubidium.

The core of the atomic clock is a tunable microwave cavity containing a gas. In a hydrogen maser clock the gas emits microwaves (the gas mases) on a hyperfine transition, the field in the cavity oscillates, and the cavity is tuned for maximum microwave amplitude. Alternatively, in a caesium or rubidium clock, the beam or gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate. For both types the atoms in the gas are prepared in one electronic state prior to filling them into the cavity. For the second type the number of atoms which change electronic state is detected and the cavity is tuned for a maximum of detected state changes.

You can also extend the idea into the realm of Quantum mechanics

Quantum clocks Further information: Quantum clock

In March 2008, physicists at NIST described a quantum logic clock based on individual ions of beryllium and aluminium. This clock was compared to NIST's mercury ion clock. These were the most accurate clocks that had been constructed, with neither clock gaining nor losing time at a rate that would exceed a second in over a billion years.[40] In February 2010, NIST physicists described a second, enhanced version of the quantum logic clock based on individual ions of magnesium and aluminium. Considered the world's most precise clock in 2010 with a fractional frequency inaccuracy of 8.6 × 10−18, it offers more than twice the precision of the original.[41] [42]

Optical lattice clocks The accuracy of experimental quantum clocks has since been superseded by experimental optical lattice clocks based on strontium-87 and ytterbium-171.

Essentially you need a super stable timer..

For relativistic effects, you could still caculate the offset of "normal" time as long as you know you speed. Just as the GPS system does.

Einstein's general relativity theory says that gravity curves space and time, resulting in a tendency for the orbiting clocks to tick slightly faster, by about 45 microseconds per day. The net result is that time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds per day.

  • $\begingroup$ Seriously though, you could tell by the frequency shift in the EM spectrum. AS we well know light moves at a constant speed, so if you accelerate and then emit em, it must still move at the speed of light, but the frequency will change. I don't know the exact mechanism but it could even be calculated using relativity, assuming you know your speed that is. And, if you can attain such speeds a little math shouldn't be an issue. $\endgroup$ Commented Mar 2, 2018 at 2:59
  • $\begingroup$ In fact the GPS system, corrects for relativity because of it's speed and distance from earth.... So yea if we can do it for that I am pretty sure that is a trivial issue once you have a stable timer. $\endgroup$ Commented Mar 2, 2018 at 3:08
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    $\begingroup$ You also have to take into account the effects of gravity at the ground based clocks location, The current atomic time standard works by regularly (I think it's probably monthly) combining results from multiple atomic clocks, and working out the Official time - and offsets from that for the individual clocks. The precision of the clocks is such that they have to apply a relativistic correction factor for the differences caused by the different gravity strength at the different altitudes of the various clocks, since that makes them "tick" at slightly different rates. $\endgroup$
    – JerryTheC
    Commented May 6, 2018 at 18:32
  • $\begingroup$ As comments say this is way more difficult. If you sync A's and B's, at point A, by the time you bring the B clock 20k light years it will be completely out of sync. Even if it was, the moment the clock is on a different planet with even marginally different gravity, it will go out of sync $\endgroup$
    – Andrey
    Commented May 7, 2018 at 14:47
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    $\begingroup$ @CortAmmon I think that you are underestimating the additional difficulty from an everchanging set of relativistic effects at a distance. Earth approximations only work because we are mostly in the same inertial frame and because we are close enough (inside Earth orbit is a trivial distance by relativistic standards, only a fraction of a light second). An error of .1% is less than .1% of a second on Earth. A similar error at ten light years is more than three days. And that ignores that we have a much better understanding of objects in the solar system than we do in the rest of the galaxy $\endgroup$
    – Brythan
    Commented May 8, 2018 at 17:55

Sounds an awful lot like ZULU time (also known as Greenwich Mean Time) used by the military and various large internationally coordinated projects (like spacecraft positioning or aircraft direction). To avoid conflicts with time zones ZULU time is simply the time it is currently at the zero meridian on our planet. This way if you are say, ordering an artillery missile strike from one time zone into another on a target that will then be bombed by aircraft and assaulted by infantry being fed positional data from a satellite. These events and assets all NEED to be in the proper position at precise timing. If the bomber pilots taking off from an airbase 1000 miles away forget that "0600 sharp" is really noon where they are dropping the bombs white the satellite team is back home in Arizona where 0600 was 4 hours ago then it all goes awry. You got missiles hitting a target with troops standing by waiting 4 hours for the bombs to drop while the satellite is looking at the wrong part of the planet. If everybody synchronizes their watches on ZULU time then "0600" is going to be "0600" no matter what time zone the various assets involved are in.

Maybe both planets could measure time by the rotational bursts from a pulsar. So each rotational outburst from the pulsar is equivalent to one unit of time and is then broken down into smaller portions for purposes of measurement (say 1 rotation equals a "stellar month" 1/10th of that time span is a "stellar week" and 1/100th of that rotational period is a "stellar day" and 10 rotations of the pulsar are a "year") Both planets keep their own local time measurements for unofficial purposes, but for purposes of timing communications with each other or sending ships back and forth or just any inter-stellar action that requires a high degree of coordination they could use the interstellar clock and date system to ensure they were both on the same page.

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    $\begingroup$ As long as the pulsar moves with the same speed and in the same direction relative to both planets? $\endgroup$ Commented Mar 2, 2018 at 10:56
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    $\begingroup$ @RealSubtle the beauty of this solution is that even if the pulsar seems to burst at different rates for each planet, the amount of bursts is still a countable integer through which they can synchronize. It also does not require communication with the other side to know their time proper. $\endgroup$ Commented May 7, 2018 at 16:41

Here's my answer, even though I realize that it may not be exactly what you want.

What is time? It is a way to define when events happen. Time has no real meaning in the universe; it is a unit of measurement made up entirely by and for humanity. Indeed, the idea of time has changed often from our first conception of it. Time used to be tied to the Sun. When the sun was directly overhead, that was noon. Time across the Earth was uneven and not constant. So noon in one town was 11:20 in another. But everything was fine, because the reach of the town's time-setter's influence was contained to that area. Then, when trains started coming though, they needed an accurate way to note time. Then when we needed incredible precision, they changed time to be counted not by our orbit, but by atomic particles (see other answers).

I think for this you need to think out of the box. I don't think its as easy as defining a new minute or second, or even the transportation and syncing of it. I think you need to rework what time is, and what it means. Maybe go back to the old system of basing it off nature, like a pulsar. But in any case, like the kilogram, you should expect your definition to be fluid, and not exact. Time is relative, and its only function is to denote the passage of it.


You can measure time reasonably accurately, and at the same rate, at the individual locations, atomic clocks based on the same elemental oscillations being more-or-less equally accurate. Coordinating between locations tens of thousands of light years apart is more complex, the "easiest" answer is to use the signals from a repeating extra-galactic source, any strong radio source will do as long as it repeats regularly, an extra-galactic source is used to minimise issues relating to interstellar distance and signal arrival time agreement, 20,000 light years isn't all that much on an intergalactic scale, as long as you know the relative position of both points compared to the signal origin you can sync the clock time or at least get close enough that everyone thinks it's synced.

The thing is that even having perfectly synced clock time at the two locations does nothing for you unless you have the means to communicate between the two sites in real time. If you have that kind of instantaneous communication it provides, almost by definition, a self-synced and internally consistent "comms time". All the users would need to do would be to agree on a universal definition of a "cycle time", probably again culled from atomic clock time, but they need that to effectively coordinate communication in real time anyway. As soon as that cycle time is agreed it constitutes a universal clock independent of the clock and calendar of any planet that uses the communications system.

The point is that without instantaneous FTL communications "universal time" has little use and once you have communications at those speeds you have a universal clock whether you wanted one or not.

For interest see also Barycentric Dynamical Time, the current standard for measuring time across relativistic divides based on gravitational positions and high relative velocities.


This answer assumes non-causality busting star drive is in play but....

You could use Pulsars as Clocks? By mutual agreement two or more widely separated planets can use one or several (results averaged) highly regular pulsars as a clock. They then have to agree on is a scale for counting them e.g decimal. So 100 pulses equals X unit of time, then there are 1000 units of X in the next unit up and so on.

Once they have a scale worked however they still need an agreed start date for any official galactic 'calendar'. Perhaps the day light from a particular supernova first reached one of the planets would do. If not some other mutually recognizable event independent of both planets that occurred at a specific location is agreed upon. (Each planet can then calculate backwards to figure out how long ago that was in local time.

This might give you a Galactic Standard Time but hey everyone ignores it and uses local time until its needed for some specific purpose e.g a time/date stamp at the top of a message being sent from one planet for another.


The other answers here focus on a more "realistic" way to answer the question, but here's some inspiration for a more creative solution.

In the webcomic Spacetrawler a galactic society has their time system synched with an interdimensional revolving door that no one can figure out how to use. Each revolution is a week and other units of time are p


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