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Old 11-11-2018, 09:24 AM   #11
malloyd
 
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Default Re: How do time-travelers calculate the date?

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Originally Posted by whswhs View Post
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. . . .
Of course if there are bunches of other time travelers around, there is somebody to ask. If there really are enough time travelers for trying to find each other to be routine, they've probably established a coordinate system. Its not that hard - you could even use GPS satellites and nobody would know as long as you took them down before the invention of telescopes.

And if time travel exists, there *should* be bunches of other time travelers around even if this is the first trip anyone has ever made - all the ones from further in the future than you came from are already here right? Try your GPS unit, it might work.
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Old 11-11-2018, 09:33 AM   #12
SilvercatMoonpaw
 
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Default Re: How do time-travelers calculate the date?

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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?
*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?"
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Old 11-11-2018, 09:46 AM   #13
Fred Brackin
 
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Default Re: How do time-travelers calculate the date?

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Sadly, it is not described, only named.
I am bewildered. The sidereal chronolog has a long paragraph on p.24. I'd have thought "central processor and database" "3 8 inch opticak telescopes and 40 square yards of radio sensitive fabric" was fairly specific.
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Old 11-11-2018, 10:17 AM   #14
malloyd
 
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Default Re: How do time-travelers calculate the date?

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Originally Posted by SilvercatMoonpaw View Post
*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?"
In that case, the answer is almost certainly "planets". Comets can work too, and of course eclipse timing has a genre history dating back to Mark Twain, but mostly planets. In theory with really good angle measuring gear and star charts that include proper motions hitting the right year is possible from just the stars, but you're going to need a lot better gear. You can probably come within couple weeks with naked eye observations of the planets if you have a chart that marks the their positions for a particular instant in time and gives their periods to enough decimal places. Though if they are aligned wrong enough you might have to wait a month or two for them to be far enough from the Sun to get good measurements.

Honestly if you have a chart that marks the planets for a particular date, the local astrologer should be able to do the calculation for you and tell you how many years it will be until "the stars are right".
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Old 11-11-2018, 10:27 AM   #15
Anaraxes
 
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Default Re: How do time-travelers calculate the date?

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Sadly, it is not described, only named.
A thing of brass, ebony, ivory, and translucent glimmering quartz. Parts were of nickel, parts of ivory, parts had certainly been filed or sawn out of
rock crystal... One dial records days, and another thousands of days, another millions of days, and another thousands of millions.
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Old 11-11-2018, 10:30 AM   #16
Anaraxes
 
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Default Re: How do time-travelers calculate the date?

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*sigh* This is what happens when I overthink a question.
No, this is what happens when you ask a perfectly reasonable question on this forum. There's a whole army of us to do the overthinking for you!
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Old 11-11-2018, 10:31 AM   #17
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Default Re: How do time-travelers calculate the date?

Step 1- Figure out which star is the pole star.
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Old 11-11-2018, 11:09 AM   #18
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Default Re: How do time-travelers calculate the date?

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Originally Posted by Fred Brackin View Post
I am bewildered. The sidereal chronolog has a long paragraph on p.24. I'd have thought "central processor and database" "3 8 inch opticak telescopes and 40 square yards of radio sensitive fabric" was fairly specific.
Maybe johnd was looking at the list on p30.

But looking at the chronolog now, I'm not sure why it needs 3x25lb 8" optical telescopes. Wouldn't just one telescope be enough? It makes sightings of one celestial object, logs its co-ordinates to a spreadsheet, then sights the next object. Three scopes seems excessive (and cumbersome).
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Old 11-11-2018, 11:13 AM   #19
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Default Re: How do time-travelers calculate the date?

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Maybe johnd was looking at the list on p30.
I was - messed up my PDF searching.
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Old 11-11-2018, 11:28 AM   #20
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Default Re: How do time-travelers calculate the date?

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No, this is what happens when you ask a perfectly reasonable question on this forum. There's a whole army of us to do the overthinking for you!
I don't necessarily mind it, I just have trouble unpacking data from long answers.
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