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Old 11-11-2018, 11:43 AM   #21
Polydamas
 
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Default Re: How do time-travelers calculate the date?

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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?
Asking may not help given that even if you speak the local language, in most places and times the answer will be in a calendar that left no historical record, or reference to any of a hundred different kings with more or less that name. Before the 20th century, most people could not tell you the year in absolute time. Then there are issues like what day a year anno domini starts on ... astronomy might be easier than sending a PhD in the world history of chronological systems along ;)

L. Sprague de Camp played with this in Lest Darkness Fall, GURPS Infinite Worlds has a section too.
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Old 11-11-2018, 12:20 PM   #22
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Default Re: How do time-travelers calculate the date?

From There Will Be Time by Poul Anderson:
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Having stowed his modern clothes in the bag, he drew forth his last item of equipment. He called it a chronolog. It was designed and built to his specifications in 1980, to take advantage of the superb solid-state electronics then available. The engineers who made it had perhaps required less ingenuity than Havig had put into his cover story. I have seen the apparatus. It’s contained in a green crackle-finish box with a carrying handle, about 24 by 12 by 6 inches. When the lid is opened, you can fold out an optical instrument vaguely suggestive of a sextant, and you can set the controls and read the meters.
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On a cloudy night in the late nineteenth century he must reenter normal time for air. The chronolog could have given him the exact date, had he wanted to shoot the stars again; its detectors included sensitivities to those radiations which pierce an over-cast. But no point in that.
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A warning light blinked upon the chronolog. It could follow sun, moon, planets, and stars with a speed and precision denied to flesh. It could allow for precession, perturbation, proper motion, even continental drift; and when it identified an aspect of heaven corresponding to the destination, that could be nothing except the hour which was sought. A light flashed red, and Havig stopped.
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Old 11-12-2018, 07:38 AM   #23
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Default Re: How do time-travelers calculate the date?

To figure out which thousand year window you are in, there are constellations that slowly change shape. The big dipper is one of the fastest changers. careful measurements will give you a thousand year window. After that, you look at the planets, you'll find their locations, and you can cross reference that with an almanac.



Yes, an almanacs were the books originally created for this kind of reference. If you have a science-based time travel system, your inventor can generate tables for you.



All planets appear in a band of constellations known as the Zodiac (yes, that's what the Zodiac actually is: the stars along the solar system's ecliptic plane). The planets orbit at constant speed. You can't just use the angle they make to the sun though, you also have to account for earth's position.
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Old 11-12-2018, 07:57 AM   #24
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Default Re: How do time-travelers calculate the date?

Note that, due to the inherent chaos in planetary orbits, we can't accurately predict their exact motions more than maybe about 100 million years back in time; anything beyond a few tens of millions of years is questionable. That much is only possible because we've orbited probes around most of them, which gives the most accurate position and mass data.

Of course, as others have pointed out, when you are that far back there are other means to determine your general time frame. Just don't count on planetary ephemerides to give you the exact date that far in the past (or future, for that matter).
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Old 11-12-2018, 08:26 AM   #25
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Default Re: How do time-travelers calculate the date?

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To figure out which thousand year window you are in, there are constellations that slowly change shape. The big dipper is one of the fastest changers. careful measurements will give you a thousand year window. After that, you look at the planets, you'll find their locations, and you can cross reference that with an almanac.
I'm thinking 61 Cygni might make an excellent clock. It has high proper motion, so it's position is the "hour hand." It has two stars that orbit every 659 years, which is the "minute hand."
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Old 11-12-2018, 08:49 AM   #26
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Default Re: How do time-travelers calculate the date?

As another alternative to sidereal dating, and if you have the luxury to plan your trip, you could find a naturally occurring geological radioactive source, such as a uranium deposit, and use its emissions to calculate how far back you've travelled. This might be done to calibrate a time machine's onboard chrono-sensors.
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Old 11-12-2018, 09:15 AM   #27
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Default Re: How do time-travelers calculate the date?

Another method involves measuring the length of the day to the nearest microsecond, as the Earth's spin has been slowing down since it was formed.
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Old 11-12-2018, 09:55 AM   #28
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Default Re: How do time-travelers calculate the date?

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Another method involves measuring the length of the day to the nearest microsecond, as the Earth's spin has been slowing down since it was formed.
The exact distance to the Moon will do the same.
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Old 11-13-2018, 07:18 AM   #29
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Default Re: How do time-travelers calculate the date?

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Another method involves measuring the length of the day to the nearest microsecond, as the Earth's spin has been slowing down since it was formed.
That's another one that might get you the right century, but isn't too helpful beyond that. The Earth's spin fluctuates seasonally for example as the surface water redistributes with the weather. And even then I expect to need a lot of work to construct a calibration curve that will work through ice ages.

An advantage of the pulsar spin method is you have multiple spin down curves to work with. They won't all glitch the same way at the same time, which will allow you to detect those sorts of complications.
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Old 11-13-2018, 07:22 AM   #30
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Default Re: How do time-travelers calculate the date?

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The exact distance to the Moon will do the same.
Though you need to know your location relative to the center of the Earth and the Earth-Moon system barycenter equally exactly to pull that off. And without a reflector at a known fixed location on the Moon, your return signal probably isn't crisp enough to get "exact" either. Throwing a spaceship into the distant past to set up a good coordinate system really does look like it would be very helpful....
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