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Old 11-11-2018, 06:20 AM   #1
SilvercatMoonpaw
 
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Default How do time-travelers calculate the date?

So when you travel to an unknown date you might get lucky and there's a convenient human to tell you when it is (or, at least, give your historical expert a rough era).

But what if you don't have a convenient source? I usually hear vague use of star charts. But does the night sky change enough to tell other than a vague couple-thousand-year window?

I guess the question is mostly "With how much precision can you calculate when you are without Newspaper Dating?"


EDIT: *sigh* This is what happens when I overthink a question.

Really all I care about is "Hey, I heard a character in a cartoon once calculate the date based on 'star charts'. Does that actually work and how accurate is it?"

Last edited by SilvercatMoonpaw; 11-11-2018 at 09:33 AM.
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Old 11-11-2018, 06:51 AM   #2
Anaraxes
 
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Default Re: How do time-travelers calculate the date?

How do time-travelers travel through time? The easiest answer is that their time machine tells them, because time travel involves measuring your movement through time.

Assuming we have some unfortunately displaced victims without their TARDIS, available technology will influence how easily and how precisely you can determine the date. Available knowledge also matters.

For example, you can determine the date by observing the positions of planets in the sky. It's not that the stars change position so much (probably what you're thinking of for that thousand-year window), but the planets do. This is where "available knowledge" appears. If you know the positions of the planets at some date on Earth, then if you're on Earth, observing the planets will help. If the time portal sent you through naked, all you have are (literal) stone knives and bearskins, and you know nothing of astronomy, spotting planets in the skit won't help. Similarly, if you get teleported to a different planet, and you know nothing about that system, you have no reference point from which to measure changes. You could also get some date information from other Solar System objects like comets, asteroids, the Moon, or Jupiter's moons (observation of which had its uses in ocean navigation before people could build good chronometers).

You can determine the day of the year by measuring how far the sun is from an equinox. That assumes you've been around long enough to know when that last equinox was -- or know enough about your location and the system to work it out from the sun's position in the sky. With precise measurements, you can determine where that equinox was pointing; the precession of the equinox will give you the year. (At least within that precession's 26,000 year cycle -- but then, that's long enough that you could use other methods to decide which cycle you were in.)

More high-tech approaches might include observation the pulse rates of pulsars (their rotation rate, which slows down over time). Radiodating could give you estimates for really long changes (up to billion-year scales); shorter-term isotopes would let you date things down to a few centuries. Once again, you'll need to have some reference points with which to compare the elapsed time you observe.

Historical events are a popular source for dating in fiction, at least. If your time travelers are showing up in recorded human history, then learning the name of a king, or seeing whether or not there's a star where the Crab Nebula should be (SN 1054), or an fortuitous appearance of Halley's Comet can give your time travelers a date. If you happen to be in Italy and people are growing tomatoes, then it's after 1550. Other such details can provide some fun and color to the "when are we" part of the adventure.

Last edited by Anaraxes; 11-11-2018 at 06:57 AM.
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Old 11-11-2018, 06:57 AM   #3
malloyd
 
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Default Re: How do time-travelers calculate the date?

Quote:
Originally Posted by SilvercatMoonpaw View Post
But what if you don't have a convenient source? I usually hear vague use of star charts. But does the night sky change enough to tell other than a vague couple-thousand-year window?
Yes, because you can locate the planets (and their moons) and use their positions relative to each other to nail down where you are within a couple thousand year window easily enough. If you brought along a serious amateur astronomy set up and an accurate set of tables (or equivalent sky visualization software) and know how to use it in this situation (which may be slightly different than knowing how to use it at home, given that you are going to need to do a bit of work to establish a coordinate system first), you should be able to nail it to within a few days easily enough with a night or two of observation. Sometimes the gas giant moon systems will be too close to the sun to nail it down to fractional hours, so if you don't know your longitude you might not be closer than that yet. And of course some places are short of clear nights. You could perhaps do this faster, without worrying about clouds with a radio telescope and pulsar spin down rates, but radio telescopes aren't so portable, and if you can't see the stars to get approximately the right directions to aim it, you might not be able to find the pulsars in the first place.

This assumes of course that there isn't a dial on the time machine that tells you. Which there usually would be given that you'd have to regulate the energy requirements for how far you traveled, or fiddle with a lot of variables to set a destination coordinate that didn't land you in interstellar space, or otherwise do *something* different depending on when you were going to.
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Old 11-11-2018, 07:11 AM   #4
johndallman
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Default Re: How do time-travelers calculate the date?

I dug into this a fair bit for my Infinite Cabal campaign. There, the characters always know they're on Earth, and their location, and can bring equipment. A look at the night sky will tell them the season, even if it isn't clear from the weather.

They have pre-computed tables for using the positions of the planets as a clock. Because Jupiter and Saturn's orbital periods aren't in a neat ratio, their positions give you the year in a cycle of about 384 years, and adding Uranus to that gives you a cycle of about 84 years to check that against.

Seeing Uranus requires a small telescope, and that also lets you check the position of some stars with high proper motions. This is especially pleasing for their superior in the Cabal, Edmund Halley, because he was the first to demonstrate proper motion.

Their favourite star for this is Groombridge 1830, because they're usually in Great Britain, where it can be seen on any clear night. It moves by several seconds of arc per year, which means it serves very well to tell which of the 384-year Jupiter-Saturn cycles they're in.
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Old 11-11-2018, 07:43 AM   #5
malloyd
 
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Default Re: How do time-travelers calculate the date?

An unspoken issue here is if there aren't any people around to ask, and your time machine didn't need to know to get here (and hence wouldn't need it to get back either), why do you care what the date is? It's not like you know the exact date anything happened in prehistoric time to try to go see (or avoid) it. For that matter there are plenty of things in historical time we are uncertain about exactly which year they happened in. Does it really matter if you are in 10,000 BC or 10,003 BC?
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Old 11-11-2018, 07:46 AM   #6
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Default Re: How do time-travelers calculate the date?

Quote:
Originally Posted by malloyd View Post
An unspoken issue here is if there aren't any people around to ask, and your time machine didn't need to know to get here (and hence wouldn't need it to get back either), why do you care what the date is? It's not like you know the exact date anything happened in prehistoric time to try to go see (or avoid) it. For that matter there are plenty of things in historical time we are uncertain about exactly which year they happened in. Does it really matter if you are in 10,000 BC or 10,003 BC?
Once someone else has traveled in time, you may want to meet them. Or get then first. There's a Poul Anderson novel about that. . . .
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Old 11-11-2018, 07:59 AM   #7
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Default Re: How do time-travelers calculate the date?

Quote:
Originally Posted by malloyd View Post
An unspoken issue here is if there aren't any people around to ask, and your time machine didn't need to know to get here (and hence wouldn't need it to get back either), why do you care what the date is?
Maybe you're chasing other time travellers, or you want to be in the right time and place for a particular event that you know is going to happen. This was important for plot of Andre Norton's Time Traders (only picking that as an example because I've just read it).

Accurate atmospheric measurements might also give you hints, although I'm not sure how feasible that would be. Carbon dioxide and oxygen ratios vary over time, and events such as eruptions and nuclear testing leave different trace signatures. Actually, these might be better as an indicator of whether you're in the correct timeline by matching those to astronomical calculations.
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Old 11-11-2018, 09:06 AM   #8
Fred Brackin
 
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Default Re: How do time-travelers calculate the date?

Quote:
Originally Posted by SilvercatMoonpaw View Post
But what if you don't have a convenient source? I usually hear vague use of star charts. "
If you wonder exactly what the equipment package for doing the "read the sky as a calendar" thing is see "sidereal chronolog" in the equipemnt section for Infinite Worlds.
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Old 11-11-2018, 09:11 AM   #9
Fred Brackin
 
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Default Re: How do time-travelers calculate the date?

Quote:
Originally Posted by SilvercatMoonpaw View Post
But what if you don't have a convenient source? I usually hear vague use of star charts. "
If you wonder exactly what the equipment package for doing the "read the sky as a calendar" thing is see "sidereal chronolog" in the equipemnt section for Infinite Worlds.

Oh, and the "ask a local" system has issues too. For much of human history local dates were reckoned only on a "Year xx in the reign of King Whatsisname" basis if they were numbered at all.

How are you even manging to speak a language whatever locals you find know?
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Old 11-11-2018, 09:18 AM   #10
johndallman
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Default Re: How do time-travelers calculate the date?

Quote:
Originally Posted by Fred Brackin View Post
If you wonder exactly what the equipment package for doing the "read the sky as a calendar" thing is see "sidereal chronolog" in the equipemnt section for Infinite Worlds.
Sadly, it is not described, only named.
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