[Icelander]

Originally Posted by johndallman

The main cause of that is imprecise historical descriptions of the positions of celestial bodies, plus things like accounts of total solar eclipses taking place at combinations of places and times when they definitely didn't.

The biblical accounts of darkness at the time of the crucifixion are often interpreted as a solar eclipse, but it was known at the time that this could not have been the case: solar eclipses can only happen at the new moon, and Passover can only happen at a full moon.

Indeed.

I take it that this means the ASNs would have pretty good calculations on when eclipses should occur on a planet matching Earth, but may have difficulty reconciling this with ancient systems for measuring time or the precise dating of historical events?

But is the discrepancy between calculated dates of historical eclipses from astronomy and possible historical events that might coincide with them likely to be a matter of days, months, years, decades or centuries, when trying to fix a specific era that corresponds with some time in the first millenium BCE?

Originally Posted by johndallman

As regards calibration, did the ASN think to bring high-quality chronometers with them? Or are they finding that Germania Hyperborea's rotation period is the same as Earth's to within the limited precision with which they can measure it, and assuming that it is actually the same?

Yes to both.

They have naval chronographs (certified as chronometers and possibly some equipment to perform tbeir own certifiactions) for navigation and to the precision they can measure, the rotational periods are identical.

Originally Posted by johndallman

If the latter applies, they'll use it to calibrate their clocks, and then they can try to locate themselves in time by finding the planets in the sky, getting their positions reasonably accurately and extrapolating their movements backwards from the orbital information for Earth. There are margins of error in that orbital information, but within a few months of observations, they'll be able to locate themselves to the day if they're within -10k to +10k years. Or know that they aren't in that range, or if the periods of any planets are significantly different, that this is not Earth.

It gets slowly vaguer outside that kind of range, as margins of error accumulate, and once you get to tens of millions of years, you can't date anything, because mutual perturbations make the positions of the planets along their orbits unpredictable on that timescale.

Yeah, this is one of the parts that doesn't work, as the position of the planets, while individually fairly plausible, is not in line with any Earth date when considered as a whole.

Which is why I thought of eclipses, as even if the stars and even planets are different (or even don't exist except as glamour), the moon seems to be real or at least close enough so that it might cover the sun at the same time in this place as on Earth.

So, even if Germania Hyperborea is not Earth, learning if it has any recorded eclipses before the ASNs arrived that might correspond with historical eclipses could narrow down the point in historical time which Germania Hyperborea is meant to represent... if any.

The ASNs have seen no eclipse in the fifty years they've been there, but if there are astrologers among the native cultures, it's plausible that they'd have some form of record of any that might have occurred within their collective cultural memory.