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This is a hard one to sum up.

In the most basic sense: I want to set up a monument that communicates when it was made to people "Reading" it. But, I don't know who will read it, what language they will have, or even if/how they measure time. I also want it to be "Read" and not "Measured" - So this rules out isotopes. (Clarification: Direct measurements. We can carve in references to cyclical events, but not make it out of Unobtanium) I want it to be part of a story of events, so it'd be "At this point we did X. At this point before that, we did Y." and so on.

My current thoughts are star charts, but I'm not sure how precise these are. A year, a decade, a century? What other ways are there to state an "absolute" time?

Updates and Clarifications

Humans are building the monolith, and it is meant to be ready by humans. The purpose of the monument is to communicate a general pre-history (Relative to the monolith), and anchor "new" history to this pre-history. Why? Well, we've fled a cataclysm and found a new planet to settle - but we don't have the industry, technology, or other support to set up with proper, or much of any, technology. This means we don't really have the time or energy to spare in hunting down special elements - We've used the travel time to plan and design this monolith, so now it's a matter of carving it out. That can be done with hand tools, without using up valuable, diminishing resources.

We can't guarantee that language will stay the same, or that we'll even be able to keep writing around consistently - We come from a pretty technologically advanced place, so even something as "basic" as paper will take a significant amount of time to re-create. Especially using alien plants. And even when we figure out how to make it, we'll have to figure out how to harvest materials, process them, build the machinery and tools to do so... It's very time consuming even if you know exactly how to do it.

We can't guarantee that timekeeping will stay the same. Honestly, it's more of a guarantee that it won't. Calendars are useful for tracking seasons, so it's almost certain to be adjusted, tweaked, fiddled with, and probably completely replaced with something more accurate to the new world.

Relying on some form of oral history on how to translate the monolith will help, but it's primarily designed to communicate history for when it becomes relevant - When you're trying to find out how to build a civilization a few centuries prior to yours, without any preparation, your primary concern is surviving. When humanity has established itself, it can go back to the monolith and figure out what happened before.

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  • $\begingroup$ Cepheid variable stars are a good "key" to measuring astronomical distances; their brightness and period are related, and the difference between apparent brightness and intrinsic brightness can be used to determine distance from the observer. Similarly, pulsars are regular enough that you can use them as a clock; their pulse rate decays predictably and gives a measure of time. By identifying several Cepheid variables and pulsars, and giving their brightnesses and periods, you give your readers enough info to calculate when that reading must have occurred... (continued next comment) $\endgroup$ – Jeff Zeitlin May 4 '17 at 18:48
  • $\begingroup$ ...and to give a time base for any event, you give the same information, or, alternatively, use measurable constants to define a time base, and give your "calendar of events" in terms of your time base relative to your base reading (from the previous comment). The assumption, of course, is that your readers have developed the science needed to make relevant observations and carry out the relevant calculations. $\endgroup$ – Jeff Zeitlin May 4 '17 at 18:51
  • $\begingroup$ Honestly? It's not really clear what the querent means by "absolute time"; in a world where Special and General Relativity hold, it's technically meaningless. The best one can do is some sort of relative time measurement, which it wasn't clear would be suitable. There is also the question of whether the discussion above would violate the "no measurement" rule; the readers would have to be capable of measuring the brightness of the stars, and the pulse intervals of the pulsars. $\endgroup$ – Jeff Zeitlin May 4 '17 at 19:38
  • $\begingroup$ By "Absolute" - 1800 was 217 years ago. In another century, it'll be 317 years ago. 1800 is, however, still 1800. I want the "Reader" to be able to "read" at whatever time, and know when the thing happened. Discussion/answer above doesn't violate no measurements, as I intended on no measurements of the monument directly. I'll clarify the question. $\endgroup$ – Andon May 4 '17 at 19:41
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    $\begingroup$ Are you launching this thing into space, or building it on a planet? If the former, you can define some unit of time as "the duration it takes light to travel the length of this object" and multiply from there. For something that's emplaced on the surface of a body with sufficient gravity, you can define a unit of time as "the duration it takes an object to fall from the top of this obelisk to the bottom." $\endgroup$ – Draco18s May 5 '17 at 3:23
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Astrological readings. Astrology describe a moment of time with planet positions and the relation between them. For example, a moment of time can be described as follow: Mars in Aries, Mercury in Libra... conjuntion between Jupiter and Mars and Saturn in opposition to the Moon. Each combination might be unique.
Neptune for example needs 165 years to make one orbit, so this kind of calendar can be used for a long period of time.

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  • $\begingroup$ So long as you know the planets are there - Which this monument could show, as well. $\endgroup$ – Andon May 4 '17 at 19:43
  • $\begingroup$ This seems to require measurements of the monument to compare to models of the solar system to make use of it. It might be hard to tell even which was before or after without good understanding of astronomy. $\endgroup$ – user25818 May 4 '17 at 19:56
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There is no such thing as 'absolute time' in the sense that you seem to mean from you contributions to the comments; all 'clock times' and 'calendar dates' are with reference to a specified but arbitrary point, and all time bases are equally arbitrary. The best you can do is attempt to establish a reference point and time base that can be calculted from observation at any given time - the other answer gets the principal, but since planetary orbits are cyclical, there will be some uncertainty, as there are arbitrarily many intervals where the planets could line up as described. An entropic reference - one that doesn't yield multiple possible intervals - is better; establishing your time base and reference point using Cepheid variable stars, pulsars, and measurable universal constants such as the speed of light, Planck Length, and Planck Time, provides a better set of references, albeit one that requires a greater level of scientific/technological sophistication on the part of your audience.

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Time is an illusion. Lunchtime doubly so.

What is time, what does a specific date mean, what does it mean to people in 1000 years time, who is going to read this monument, what is their level of technological advancement, what is their calendar based on.

It's very hard to communicate a specific date in one calendar to someone using an entirely different calendar. I could tell you that my date of birth is 15-1-5740 (it isn't) but that date would only mean something to a very small number of you. You're trying to say 1800, but 1800 on that calendar is somewhat before the rise of the Roman Empire.

You need to work out how to communicate a specific point in time using techniques accessible to someone using an arbitrary calendar at an arbitrary point in the future with no possible reliance on cultural common ground or technological basis.

Frankly, not possible. You could potentially communicate with their scientists, but not an ordinary person.

Astronomical observations have been mentioned, these give a long calendar. Just by position Pluto gives a 248 year cycle, in combination with the other planets it could be specifically defined, but that would still require some serious calculations to work out when the planets were in those particular positions. You might as well let them use Carbon14.

Speaking of C14 and natural materials, Dendrochonology is probably one of the easiest systems to read over human periods. Buildings and other structures can be dated precisely by the measurement of the tree rings in the beams they're built of. Make your monument out of a slice of a vast tree and key each event in your history to the ring matching the year in which it happened. Currently continuous anchored chronology is available for a little over 11,000 years.

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  • $\begingroup$ I'm not necessarily trying to communicate with ordinary persons. The scientists would be the targets - They can "Interpret" the monolith for the normal people. $\endgroup$ – Andon May 5 '17 at 21:35
  • $\begingroup$ @Andon, in that case as long as you have an analogue to a tree, something that gives rings, dendrochronology is the way to go. $\endgroup$ – Separatrix May 7 '17 at 20:20
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As people pointed before. Astronomy. You carve exactly all stars and planets you see from the place of the monument when you make it. Then some time later a person came, read this an say "oh, this planet wasn't close to this planet since 1300". The stars chart can be exact to the point of 1/4 of a day. Because you can write the position of the sun, moon, and visible planets. Of course if you only have your eyes at your disposal you can cut that to one day.

Example: did you know that your astrological sing is off a few weeks because the stars were in the different distance from each other when people made that stupid sing thing up?

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