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Let's assume I can travel in a space-time on Earth. Every time I jump I get to almost the same place, but in different past time relative to the first jump: it solves the problem of planet rotation, orbital moving, etc. Also if there is an obstacle in that place in another time, then I get softly pushed a little bit away, like if I appear in a collapsing invisible Zorb ball: it solves problem with survival in 99.9%.

There is one problem, I know the exact location before the first jump, but I don't know the exact time I jump in. To do my tasks I need to find out at least the Earth year I'm in - as fast as possible!

So what I can't:

  • Most of my locations are uninhabited almost always (like desert islands), so I can't rely on a local population or artifacts all the time.
  • I'm an average human, so I can't learn everything about every specific place I will travel - I can only memorize some common rules or heuristics, some limited simple or specific information.
  • I can't take or drastically change anything in past.

What I can:

  • I can get a backpack and take everything I need and can bear.

(I will update "can"s and "can't"s to clarify the question if required).

Some leading questions:

  • What should I learn and take with me to determine the year almost everywhere?
  • What are the locations, where I can detect the year in the fastest way?
  • What is the most remote year in the past I can travel, so that I'm able to detect the year by any means at all?

Update #1:

Please, add some meaningful limitations to your answers: like, the average time for detection, the approximate area where your method works within declared detection-time, etc. Different answers may cover different areas or different historical periods.

Update #2: Time-traveler worker schedule example.

The time-traveler is a freelancer. He signs a contract to do something specific in the past with some specific people, events, items, etc. - in exact location. Different companies work with different locations. He has a limited series of time jumps - the last jump always brings him back. So, when he makes another jump he has to decide - can he do anything here and now, or may be he should wait a couple of years, or jump again for a better luck. That is why the known methods of detection of the year is a very valuable information for such freelancers.

Update #3: Time machine technology.

The technological level of some companies allows to program their time machines to visit some limited number of time periods in the past. The best machine has precision of ±15 years from a specified point. Also a single series of jumps (for all machines) is very-very costly - the more jumps machine can do in one series the better it is. But all machines unfortunately jumps to specified time periods in a random order. The list of time points is known to applicants beforehand, so they can decide if they possibly can distinguish at least epochs - not talking about the exact years.

P.S. Of course, the year detection is probabilistic - i.e. methods should work with at least 85% probability.

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    $\begingroup$ Get a camera and a computer, check star positions. This works anywhere at night when it's not cloudy. You know the date down to the day (moon phases) with 100% accuracy and works basically forever back. $\endgroup$
    – zzz
    Nov 27, 2016 at 17:33
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    $\begingroup$ What time frame and desired accuracy do you need? The way to pin down the year within 1 year between 1800 and 2016 is very different from what you'd use tp in down the year within 1,000,000 years between 3.6 and 3.9 billion years ago. $\endgroup$
    – Cort Ammon
    Nov 27, 2016 at 18:18
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    $\begingroup$ @CortAmmon there is no "desired" time frame, but there is a part of a question about the limitation of proposed methods. So, if it can be proved, that your method doesn't work for 3.6 billions years ago to find the year with 85% probability, but 3.5 billions years ago it works - it's the answer too. I guess no one knows all possible methods for all ages. $\endgroup$
    – abyss.7
    Nov 27, 2016 at 18:26
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    $\begingroup$ @MichaelKjörling No, it's not - my question is about travel in the past not future. It's about a physical (real scientific) methods of the year-detection: even in times when there was no mankind or any technology. Also I don't know how to get rid of the banner about duplicate in the header of my question. $\endgroup$
    – abyss.7
    Nov 28, 2016 at 10:44
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    $\begingroup$ @Zxu Get a camera and a computer (...) So... basically, get your phone? $\endgroup$
    – xDaizu
    Nov 29, 2016 at 8:53

14 Answers 14

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  • If you travel in the recent past (say, not before 1930): a shortwave radio. Listen for news bulletins. Works everywhere. Time to determine what year you are in: one or two hours, depending on what broadcasts you receive.
  • If you travel the just a little bit less recent past: a longwave radio. May or may not work; for example, it may not work in the middle of the ocean. If it works, you can determine the year in a few hours, depending on what broadcasts you receive. If it doesn't work, see next point.
  • If you travel in the not so recent past, but still within history, say up to 10000 years BC: a small astronomical telescope and a laptop with a practical astronomy program. Observe the planets and use the program to figure out the date. You may need to wait for two or three nights in order to identify the planets. "Practical" astronomy refers to observations made with the naked eye or with portable instruments, with practical applications. There are computer programs which can show the position of stars and planets at any given time within the last and next few millenia.
  • If you travel in the distant past: same as above, but observe the stars. The starts have proper motions, and their positions change, albeit slowly. You may also want to observe the position of the north and south poles; for this you may need a camera. The poles move on the sky in a 26000 years cycle, called the precession of the equinoxes (This is dawning of the Age of Aquarius ...). You may need to make observations during two or three nights.
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    $\begingroup$ Listen for news bulletins, as I wrote. Many countries broadcast news bulletins regulary. $\endgroup$
    – AlexP
    Nov 27, 2016 at 20:25
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    $\begingroup$ If close enough to the present, a GPS receiver may give you all required information in seconds, anywhere. Also a list of the locations and frequencies of all radio time broadcast signals (like DCF77) should be on your laptop, for when you are in the range of one of them, it will give you the time information in a few minutes (at one bit per second). $\endgroup$
    – celtschk
    Nov 27, 2016 at 23:12
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    $\begingroup$ @celtschk "in seconds" may be a little optimistic, given that the receiver needs to perform a cold fix. But, a minute isn't bad either, and a solar powered charger should be more than plausible. I'm also wondering if there's such a thing as a GPS-synchronized watch, that could be even easier to get powered. $\endgroup$
    – John Dvorak
    Nov 28, 2016 at 0:51
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    $\begingroup$ I can't remember the last time the news anchor mentioned the current date with the year, though, so you may have to rely on in-passing mentions of "2016 world cup" and suchlike. $\endgroup$
    – KlaymenDK
    Nov 28, 2016 at 11:37
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    $\begingroup$ @Jasper -- Bulletin hydrologique. I don't know about other countries, but Romania broadcasts a dayly hydrological bulletin giving the levels of the Danube at several points in three languages (Romanian, Russian and French). It's a very uniform text, and always gives the full date. (It's an ancient international treaty obligation.) $\endgroup$
    – AlexP
    Nov 28, 2016 at 15:45
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This was going to be a comment on @SRM 's answer, but it ended up going longer and more elaborate.

Infrastructure

There's something that happens any time the use of a tool becomes routine - the development of infrastructure to support it. Lots of ship traffic drives the development of ports. Colonies on a new continent build roads, trade with them builds new ports on the new continent. Access to small hand-portable radio devices drives the construction of repeater towers, and turns into a cell phone network.

In this case, there would be a driving motivation to establish some kind of infrastructure to determine the date and time. It seems likely that the organisations paying people to travel in time would be willing to pay towards a system that would improve the accuracy of their mission. With that in mind,

Hide a clock and transmitter on the moon

It could encrypt its transmissions such that they look like random background noise to astronomers who aren't in the know, but be clear time signals to time travelers who have the encryption key. Maintenance isn't a problem, either - remember, you have a time machine. Make however many jumps you need through time to change the batteries/add uranium/clean the solar panels/what have you - as soon as you place it in the distant past, you can tell what the outcome was in your present and fix any problems.

There's an odd phenomenon that could be tied into this as well - the issue of Transient Lunar Phenomena, lights on the moon that occasionally appear and disappear. Nobody today has any idea what they are; for our purposes, they could simply be the time jumps needed to service the clock.

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  • $\begingroup$ I wonder if the LLR has anything to do with TLP $\endgroup$
    – Wayne Werner
    Nov 29, 2016 at 16:19
  • $\begingroup$ Unlikely, as the TLP has been reported since well over 1,000 YBP. $\endgroup$
    – Werrf
    Nov 29, 2016 at 16:49
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Since you are prepared for this task it's going to be simple enough:

You need a small computer and a decent camera. Point the camera at the night sky (day does not work--while the stars are out they're swamped by the reflected light in the sky) and let the computer find planets. It can easily have been programmed to do this, find what piece of sky the view corresponds to, subtract the known stars and what's left are planets.

If the trip is long enough, look for the displacement of the stars instead.

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    $\begingroup$ That’s the same thing I posted a day earlier. $\endgroup$
    – JDługosz
    Dec 1, 2016 at 9:47
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For travelling millions of years into the past:

The Moon was closer in the far past.

If you know your exact location and see the moon - a device which measure the distance to the moon would be able to calculate what time you are in (roughly). Measuring the distance to the moon can be done in many ways but one simple way is to simply measure its diameter in the sky (although since the orbit of the moon is not circular, you would have to measure for several nights in a row using just a visual measurement).

Atmospheric composition have changed over geologic timescales.

Measuring carbon dioxide, oxygen and trace elements and comparing with a detailed chart might give a hunch of where you are.

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    $\begingroup$ A good database of atmospheric composition values would be a simpler way of checking against a reading for the time when the OP's time machine has landed. $\endgroup$
    – a4android
    Nov 28, 2016 at 0:36
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    $\begingroup$ You might be able to make the Moon measurement method work super accurately if you instead jump way into the past and leave a retroreflector up there like the Apollo program did. Then, given current technology, if you can get a powerful enough laser you can get the distance of the moon to centimeter resolution, which will allow you to pinpoint your time to within a very small window. $\endgroup$
    – Cody
    Nov 28, 2016 at 21:46
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    $\begingroup$ If jumping far back and leaving something in space was an option, then all that is needed is to place a satellite into orbit which sends an encoded time-signal. If the satellite runs on some future energy source like a antimatter battery it might last almost forever and the signal could tell a receiver what time it is on the second. The signal could be designed to look like static unless one knows exactly what to look for. I get the feeling from the OP though that such solutions are off the table. $\endgroup$
    – Sesdun
    Nov 29, 2016 at 0:48
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You've posed an interesting problem; I like it. I, like many others on this board, also believe a contraption that can track the stars is ideal, but I suggest a bit of techno-fantasy - a camera that can take a picture of the sky at any time, day or night, and pick up the pattern of the stars, then spit out a year. The stars are always shining; we just don't see them during the day because the Sun overpowers them. This contraption would just eliminate the light from the Sun. Like I said, techno-fantasy.

I do have a question: If the traveler is jumping to uninhabited areas and is restricted from changing the past - why does the specific year make a difference?

I tend to think of it a little like theft in the night. The jumper slips in, gets what they need, then slips back out. Does it really matter if he entered at 2:54 am or if he entered at 2:55 am? Am I missing something?

Stated another way (for those who don't understand metaphor): If your jumper is targeting a window for between 1201 AD and 1216 AD, will not be seen, and can't change the past, why does it matter if he arrives in the year 1203 AD vs. 1205 AD?

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    $\begingroup$ "eliminate the light from the Sun" not really doable. The sun's light is scattered in the atmosphere, so it overrides everything, to the point that light from the other stars are not distinguishable from a glitch in the atmosphere composition. $\endgroup$
    – njzk2
    Nov 28, 2016 at 14:59
  • $\begingroup$ @njzk2, "Like I said, techno-fantasy". $\endgroup$
    – user26892
    Nov 28, 2016 at 15:02
  • $\begingroup$ People are being sent in the past. Presumably to do things they can't do in the present. Imagine you are being sent to prevent the assassination of the Archduke Franz Ferdinand. It matter to know if you are in 1915 or in 1913. $\endgroup$
    – njzk2
    Nov 28, 2016 at 15:05
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    $\begingroup$ "why does it matter if he arrives in the year 1203 AD vs. 1205 AD?" - that is very simple: you lose the years of your life and work-time. For example, do you want to come to your client to sign a contract and he tells: "Oh, sorry! I mixed up the time - wait here for 3 more years". $\endgroup$
    – abyss.7
    Nov 28, 2016 at 22:37
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    $\begingroup$ Except, if you already know your going to have a window of +/- 15 years from your target date, it would have to be generally accepted that there will be significant portions of time wasted. It might make sense for a jumper to bundle together a series of objectives within a certain era of history, which several employers would then contribute to the costs of covering. $\endgroup$
    – Cadence
    Nov 29, 2016 at 2:42
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The easiest thing to do, especially in the desert, is to make astronomical observations. The planets are a great clock face. To prevent ultra-high precision, make that work on a cycle of a few hundred years and then look at proper motion of stars and the exact pointing of the Earth’s axis to get a longer range ballpark.

If you arrive at night, which will happen 50% of the time, and the sky is clear, an automated instrument should make a determination as fast as it can point itself to different places. I’m thinking 5 minutes or so.

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Use a Geiger counter, and travel nearby a Isotope mineral field. You will know radiation intensity will become twice stronger each time you jump in past by a multiple of halftime.

Ideally you want to find depots that have halftimes around 200-300 years if you want do to time travels no longer than 1000-2000 years.

Note that on longer time travels the quickest way to know the average period is to become a Geologist so you know how much erosion actually "un-happened".

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    $\begingroup$ Would it be feasible to place a known amount of radioactive material at the location and measure that? $\endgroup$
    – aebabis
    Nov 30, 2016 at 17:42
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    $\begingroup$ If you can place something in the past.. sure! beware that the first time you travel in the past you will not know how far in the past you were so the "placing travel" (Epoch) will be at unknown time. So to know which time you were you have to return to present and then measure the radioactivity in starting day, then you will be sure of the date, in any time between present and epoch. You can also place multiple amounts of different materials at different places to increase accuracy of time measurement (that you will done through GeigerCounter+smallComputer) $\endgroup$
    – CoffeDeveloper
    Nov 30, 2016 at 18:27
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In the present, sync two atomic clocks. Jump into the deep past as far as you can go carrying one clock. Leave it. Return to the present (or future). Record the difference between the two clocks. After that, whenever you jump to the past or future you can always figure out when you are.

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    $\begingroup$ I don't think this would work very well. It assumes that your clock is easily accessible enough that you can get to it without special tools, but doesn't get stolen or damaged in the intervening years. $\endgroup$
    – DuckTapeAl
    Nov 28, 2016 at 8:02
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    $\begingroup$ Also power supply available for decades or centuries and no random errors in the electronics. $\endgroup$
    – The Vee
    Nov 28, 2016 at 8:32
  • $\begingroup$ Ok... how is this supposed to work at all? Is there anything in the question about an effect the time-travel is supposed to have on the atomic clocks? $\endgroup$
    – Michael Schumacher
    Nov 28, 2016 at 23:00
  • $\begingroup$ @michael_schumacher basic math. If I have two synced clocks and I offset on by N years, I know how far back in time I traveled. Thereafter, just by checking the time on that clock whenever I arrive, I know how far from the "present" I am, so long as I never go earlier than that clock (or later than the date it breaks). $\endgroup$
    – SRM
    Nov 29, 2016 at 1:54
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    $\begingroup$ It would work just as well if the "clocks" were two equal size piles of uranium going through atomic decay. Or a giagantic water clock slowly dropping into a bucket. $\endgroup$
    – SRM
    Nov 29, 2016 at 1:56
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Into your backpack go a sextant and a set of star charts, from which you can calculate not only when, but where, you landed at any time within at least the last several million years, maybe more.

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  • $\begingroup$ Intensity of variable stars might tell you the millennium. The positions of the planets should pinpoint the year and date more closely. When you bring a telescope, you can look for Uranus and Neptune which have 84 and 165 year periods. Doing the math by hand might get complicated, but a simple smartphone app might do the job. For more information ask on astronomy.stackexchange.com $\endgroup$
    – Philipp
    Nov 28, 2016 at 15:32
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Based on what you describe as the job of a time traveler, your best way to find out what year you are in would be given to you when you arrive.

If you're targeting a year in the past 300ish years, grabbing a newspaper would give you the exact date and year, making this easy. If you're targeting a year in the past 2000 years, and you were going to a culture which got near the Romans, you could ask for a Julian Calendar date.

Beyond that, you're going to want your employer to have picked specific times of interest. Perhaps they can send you back to a time right before a predicted solar eclipse.

Of course, we're going to need to understand the level of unpredictability in your time travel machine. Given that it sounds like the time machines are accurate enough to target actions within a decade, you'd quickly find that the tools at your disposal to identify a year are less precise than your time machine was in the first place. Many of the geologic timescales people recommend are great if you want to find out what epoch you are in, but totally ineffective in determining what millennia, much less decade.

Accordingly, I would expect your employer to have worked out a set of events worth looking at to determine what time you landed at. Perhaps there's a good sized meteor which is supposed to streak overhead on a certain day, and a solar eclipse a year later. These could be used to fine tune. Presumably when your employer gives you a task, they already know what they want to have done, so they'll know something about the time regions surrounding it.

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  • $\begingroup$ Sounds reasonable. But even if the time machine can be presetted to target some specific periods, the unpredictability is that the order of visiting those periods is random. So the freelancer looks at the contract and decides, can he detect the time-periods he has to get in or not. Also time machine possibly can bring him beyond the expected event and the time traveler then shouldn't wait for it. So the methods of time-detection with a certain precision are still required. $\endgroup$
    – abyss.7
    Nov 27, 2016 at 20:07
  • $\begingroup$ Is there any way for feedback in your system, so that the user can see what happened and make an adjustment? Having an accuracy of 1 decade a few million years out is an astonishingly demanding level of accuracy not seen in any real existing equipment without a feedback circuit to correct for errors. For perspective, the spheres from Project Avagadro are some of the most precise creations we have ever made, and they're barely on the accuracy scale you're looking for for any trip further back than 300 million years. $\endgroup$
    – Cort Ammon
    Nov 27, 2016 at 20:29
  • $\begingroup$ No feedback while time-travelling (excluding your own marks - since you travel in the same place all the time): only limited number of jumps to some time points with a certain precision - and the payment after return according to the number of complete tasks. If you suggest that 300M years is a current humankind limit - then it's ok, if there are some practical methods provided. $\endgroup$
    – abyss.7
    Nov 27, 2016 at 20:51
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    $\begingroup$ I'm saying that's the single most precise object we have ever created. Based on what I have heard of from your description of the device, I'd say the best we could do would be on the order of 10,000 years... of which much is recorded history so you could do measurements that way. Beyond that, you're entering the world of handwavium and technology so advanced it's indistinguishable from magic. At that point, it might be easier to just say the time machine can tell you when you landed. $\endgroup$
    – Cort Ammon
    Nov 27, 2016 at 21:31
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How technologically advanced do you want it to be? By Current technology the star is probably the best aproach.

The simplest science-fantasy way to do it would be a contraption; a kind of "Time-GPS", that could evaluate your position in time by sending a message back where you came from using the same technology as you yourself used to travel time.

Another way that could be by measuring the existence of certain particles (background radiation) that could only have been created during Big Bang that slowly decays.

Another Option is to measure the "color" or "heat" output of the nearest star (sun), knowing the star's life cycle and a very accurate measurement tool could potentially indetify your positions in time with a few years inaccuracy.

Based on human history (and the main method almost always used) is just a lot of information, a "Time Map" so to speak, future tech could potentially hold limitless amount of data and with a few decades of programming it could based on collected data simply pattern match your current position with changes in the enviroment trought out time. This would be more inaccurate on planets with no erogation system, and less accurate on planets with intelligent life.

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I think that some answers are based from our civilization perspective/progress. In my opinion it depends. For example of the context of how your civilization, culture or/and you have achieved the technology/science/magic of to travel in time. I think that depending of that, measure the time could be based from atoms, stars, galaxy, universe, magic, machines perspectives, and not only from an cultural perspective (like the b.c. years perspective) and how to measure from this perspective. This context can give you parameters to measure the time passed from differents points of time/space.

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Supplementary answer about the distant past: relative abundance of U235 and U238. U235 has a half-life of about one billion years, U238 a half-life of about 4 billion years, so if you go back hundreds of millions of years there will be relatively more U235. The relative abundance at any point in time is pretty much a global constant, unless you are unlucky enough to find yourself at the site of a fossil natural nuclear reactor (AFAIK only one is known).

The best way to measure it is a mass spectrometer, but they are not very portable. However, I think if you had a particle-energy sensitive alpha detector, you could detect U235 decay events separately from U238 decay events, and to a reasonable extent exclude cosmic ray events. Then just point it at an igneous rock (or seawater, if it's sensitive enough).

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Well, since you're already time-traveling, I figure you can have anti-grav technology.

Assuming you have this, then the easiest way to determine time would be to have a look at the position of the continents. Bring a small anti-grav drone with a transmitter, send it up, have it calculate the year, and then recall it. This would be much faster than waiting for stars, and wouldn't be subject to weather (good luck reading the stars from ground-level in monsoon season.)

And, just in case the drone can't see the continents due to arriving when the dinosaurs died, it should still be able to use star positions. It'll just take longer.

Finally, if all other options fail, measure the mass of the sun. It doesn't change perfectly regularly across its entire history, but assuming you're going back less than a billion years you should be able to find a model that works.

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