11-11-2018, 11:43 AM | #21 | |
Join Date: Jan 2006
Location: Central Europe
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Re: How do time-travelers calculate the date?
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L. Sprague de Camp played with this in Lest Darkness Fall, GURPS Infinite Worlds has a section too.
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11-11-2018, 12:20 PM | #22 | |||
Join Date: Jul 2007
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Re: How do time-travelers calculate the date?
From There Will Be Time by Poul Anderson:
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11-12-2018, 07:38 AM | #23 |
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Join Date: Mar 2012
Location: far from the ocean
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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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11-12-2018, 07:57 AM | #24 |
Join Date: Aug 2004
Location: traveller
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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). |
11-12-2018, 08:26 AM | #25 | |
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Join Date: Sep 2004
Location: Southeast NC
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Re: How do time-travelers calculate the date?
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11-12-2018, 08:49 AM | #26 |
Join Date: Dec 2006
Location: Meifumado
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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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11-12-2018, 09:15 AM | #27 |
Join Date: Jul 2009
Location: UK
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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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11-12-2018, 09:55 AM | #28 |
Join Date: Aug 2007
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Re: How do time-travelers calculate the date?
The exact distance to the Moon will do the same.
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11-13-2018, 07:18 AM | #29 | |
Join Date: Jun 2006
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Re: How do time-travelers calculate the date?
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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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11-13-2018, 07:22 AM | #30 |
Join Date: Jun 2006
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Re: How do time-travelers calculate the date?
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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