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Old 11-15-2018, 10:17 AM   #51
khorboth
 
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Default Re: How do time-travelers calculate the date?

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Originally Posted by Fred Brackin View Post
With only a little thought given to how reflective your satelite is i.e. avoiding the "Iridium flare" you can put satelites into Earth orbit until at least 1900.
A few satellites broadcasting the current time could be kept in orbit with minimal effort starting very early. They could then be de-orbited during the worldwide red-sky phenomenon following the Krakatoa disruption in 1883.

As an alternative, one could be, with minimal additional effort, placed at Earth's trailing and leading Lagrange points. This means a signal would reach everywhere on earth except perhaps the places currently experiencing midnight and noon. Chance of discovery would be minuscule, but maintenance would be more costly. I would assume, though, that space travel gets better and cheaper at some point.
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Old 11-15-2018, 01:13 PM   #52
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A
As an alternative, one could be, with minimal additional effort, placed at Earth's trailing and leading Lagrange points. This means a signal would reach everywhere on earth except perhaps the places currently experiencing midnight and noon. Chance of discovery would be minuscule, but maintenance would be more costly. I would assume, though, that space travel gets better and cheaper at some point.
I was mostly being conservative about the natives picking up the satelite signals. No radio means no signal interception. Historically though it wasn't until the late 30s that radio signals from celestial bodies were recognized.
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Old 11-15-2018, 02:40 PM   #53
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Default Re: How do time-travelers calculate the date?

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As an alternative, one could be, with minimal additional effort, placed at Earth's trailing and leading Lagrange points. This means a signal would reach everywhere on earth except perhaps the places currently experiencing midnight and noon.
As the Lagrange points are 60° from the sun in the sky, they're always on the day side of the Earth, and they're not in line-of-sight from 8pm to 4am.

It's quite a good idea. With a decently encrypted digital radio broadcast, they won't get detected until radio astronomy advances a fair bit, although they're far enough from Earth that you'd need a fair-sized antenna to pick them up.
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Old 11-15-2018, 02:56 PM   #54
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Default Re: How do time-travelers calculate the date?

I wouldn't mess around with Lagrange points. If people could possibly pick up your signal at all, you're in a time period with newspapers, clocks, and globe-trotting transportation who are actually likely to speak your language, so the satellite is less useful.



On the other hand, you'll want to clean up your satellite mess, which is actually fairly difficult. I do not regard creating a cloud of metallic space-junk at a Lagrange point to be "keeping the secret" or "preserving the timeline". You'll want to burn the satellite up in an atmosphere, so you may as well keep it reasonably close to earth.
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Old 11-15-2018, 03:14 PM   #55
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On the other hand, you'll want to clean up your satellite mess, which is actually fairly difficult.
Don't LEO satellites decay naturally in a matter of decades at best? Given that you're trying to cover possibly billions of years of Earth history, that's not a very high percentage loss of coverage time. And it is all on the well-documented, easier-to-discover-otherwise end, if you time your satellites to be destroyed by around 1850 or 1900.

The other nice thing about reentry instead of Lagrange is that it gives you a free adventure to maintain the secret when the tempori-locals discover the debris and realize it's not just a space rock.

But I actually wouldn't think any time-travelling organization with the means to deploy and keep satellites in orbit over a billion-year span of history (or even thousands, just to cover human civilization) would find picking them up again before discovery very difficult at all.
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Old 11-15-2018, 09:46 PM   #56
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Default Re: How do time-travelers calculate the date?

And, of course, there's dendrochronology. Some trees are known to be several thousand years old. So, if you are in a world with some form of civilization, it should be possible to make an expedition to one of these Methuselah trees, take a core sample, and count the rings.

This means that if time travel is popular and lots of time travelers need to figure out when they are, the old trees will eventually end up with more holes in them than Swiss cheese. Maybe time travel also comes with some sort of superscience non-destructive deep scanner that can count growth rings without damaging the tree.

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Old 11-15-2018, 10:16 PM   #57
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Default Re: How do time-travelers calculate the date?

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And, of course, there's dendrochronology. Some trees are known to be several thousand years old. So, if you are in a world with some form of civilization, it should be possible to make an expedition to one of these Methuselah trees, take a core sample, and count the rings.

This means that if time travel is popular and lots of time travelers need to figure out when they are, the old trees will eventually end up with more holes in them than Swiss cheese. Maybe time travel also comes with some sort of superscience non-destructive deep scanner that can count growth rings without damaging the tree.
You could probably get a pretty good estimate (maybe not a precise year, but the decade) by just measuring the trunk circumference. Where it would work is also where celestial methods would be fairly easy, though. Planetary orbits aren't completely stable, but over the lifespan of individual trees, it is effectively clockwork.
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Old 11-15-2018, 11:14 PM   #58
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Default Re: How do time-travelers calculate the date?

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You could probably get a pretty good estimate (maybe not a precise year, but the decade) by just measuring the trunk circumference. Where it would work is also where celestial methods would be fairly easy, though. Planetary orbits aren't completely stable, but over the lifespan of individual trees, it is effectively clockwork.
But not nearly so exciting as commandeering a ship made only for punting up and down the Nile to make a trans-Atlantic crossing; using only a makeshift compass, astrolabe, and sunstone for navigation; followed by a cross-continental trek through wild terrain and exotic cultures to a far-away land that will one day be called California. All so you can take a sample of a sapling bristlecone pine. I mean, these are supposed to be adventurers, right? Geez, taking the easy way out. What are PC's becoming these days? <Grumble. Grumble.>

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Old 11-16-2018, 06:47 AM   #59
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But not nearly so exciting as commandeering a ship made only for punting up and down the Nile to make a trans-Atlantic crossing; using only a makeshift compass, astrolabe, and sunstone for navigation; followed by a cross-continental trek through wild terrain and exotic cultures to a far-away land that will one day be called California.
Although if you are going to need a calendar repeatedly somewhere that isn't so remote, planting a somewhat unusual tree for around here with some sort of marker (say a stone ring around it with a date on one of them that could pass for a grave marker) is a lot easier to deploy than a satellite and pretty much immune to changing history through accidental discovery. Of course there's the risk somebody will cut it down or something, but if you use a worthwhile fruit or nut tree they probably won't.

You could even do an orchard long term. Come back every 10 years and plant another tree in some fixed spacing and direction and somebody who knows that can find the decade just from finding the marker and the youngest tree that's some integer multiple of 10 meters north/south and east/west of it.
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Old 11-16-2018, 09:05 AM   #60
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Default Re: How do time-travelers calculate the date?

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On the other hand, you'll want to clean up your satellite mess, which is actually fairly difficult. I do not regard creating a cloud of metallic space-junk at a Lagrange point to be "keeping the secret" or "preserving the timeline". You'll want to burn the satellite up in an atmosphere, so you may as well keep it reasonably close to earth.
I think a large enough explosion would sufficiently scatter your satellite bits to the spacewinds that it'd be nigh impossible to find any debris later.

One major problem is that L4/5 are each 1AU away. You'd need significant spaceflight capability to get anything there - especially launching from a prehistoric Earth - which means you'd probably have enough tech to put a self-sustaining stealth satellite in Earth geosynch orbit instead.

I guess the Earth-Moon L-points might be more viable though.
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