How do time-travelers calculate the date?

[malloyd]

Originally Posted by Daigoro

I guess they could take a reading of the Cosmic Background Radiation (CBR).

Assuming you were aboard that time traveling spaceship. You can sort of measure it with ground based radio telescopes, but accuracy isn't very good - I suspect if the difference were big enough to reliably detect from the surface of the Earth there wouldn't *be* a surface of the Earth yet.

[lwcamp]

Originally Posted by Daigoro

I guess they could take a reading of the Cosmic Background Radiation (CBR).

There would also be measuring the Hubble "constant" (which isn't). But this will be imprecise, will require large observatories, and without a good idea of whatever is responsible for dark energy you won't be able to turn a measurement into a date without a bunch of calibration measurements at different times (which scientists would probably want to do, so they can understand dark energy better).

Luke

[Daigoro]

Originally Posted by malloyd

You can sort of measure it with ground based radio telescopes, but accuracy isn't very good

I thought so too, but there seem to be plenty of ground-based detectors. A TL 10-12 version of this might do the trick. Or failing that, there are balloon-carried detectors too.

The CMBR at least is something that exists though, unlike as-yet-undiscovered speed of light decay.

[lwcamp]

For journeys into the very deep past, you could also observe the positions and orientations (and general shapes) of the galaxies in our local cluster. Again, you will probably require calibration observations at various times in the deep past, and again it will only give approximate dates. Combine this with observations of starts and planets (using ephemera and star charts made with calibrated observations across a wide range of times) to home in on more precise dates.

Luke

[SilvercatMoonpaw]

Originally Posted by lwcamp

Again, you will probably require calibration observations at various times in the deep past, and again it will only give approximate dates.

Beyond a certain point in human history it probably doesn't matter what the exact date is because it's not like there's a specific event to watch out for.

[Flyndaran]

There would be specific events like volcanic eruptions and meteor strikes such as the dino-killer. But it's not like we know that the Chicxulub crater formed on a Tuesday at 9:51 A.M, for example.

[khorboth]

Originally Posted by Flyndaran

There would be specific events like volcanic eruptions and meteor strikes such as the dino-killer. But it's not like we know that the Chicxulub crater formed on a Tuesday at 9:51 A.M, for example.

That depends upon how thoroughly the past has been "mapped" at this point. This could totally be in an almanac after some historian spent a bit of effort tracking it down specifically. If our lost traveler is one of the first, then not so much.

[RyanW]

Originally Posted by khorboth

That depends upon how thoroughly the past has been "mapped" at this point. This could totally be in an almanac after some historian spent a bit of effort tracking it down specifically. If our lost traveler is one of the first, then not so much.

It occurs to me that a time travel agency might put in place beacons - maybe in an orbit that will decay before anyone is around to notice it, or in a location soon to be under lava or in an impact crater - featuring a warning and a countdown to something you probably don't want to be around to see.

[Fred Brackin]

Originally Posted by RyanW

It occurs to me that a time travel agency might put in place beacons - maybe in an orbit that will decay before anyone is around to notice it,

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.

[Boomerang]

Originally Posted by SilvercatMoonpaw

Beyond a certain point in human history it probably doesn't matter what the exact date is because it's not like there's a specific event to watch out for.

Unless you need an accurate start date to calculate the parameters for your return journey to your original time.

For my own campaigns time travellers need to be aware of local conditions such as: gravity, time, location, entropy, fundamental constants (because they aren’t constant) and acceleration. If they don’t know these for their initial position and destination they risk arriving way off course.