NavList:

Örjan Sandström, you wrote:
"My bet would be making a nocturnal [...] that should get you to ±20min of actual time, not GPS but built from nothing it is about only way I can think of to get longitude this side of full blown lunar."

But that won't do! A nocturnal gives local apparent time. You need some absolute time in addition to local time to find a longitude. You need some globally observable event that occurs at the same absolute time everywhere, like an ordinary lunar eclipse (accurate to +/-3 minutes or so), an eclipse of one of Jupiter's moons (accurate to +/-30 seconds or so on land), an eclipse in a binary star system like Algol (maybe accurate to +/- 15 minutes), an explosion in space (very accurate if you have a convenient supply of nukes), or yes, the Moon reaching some specific angular distance from a known star (accurate to +/-30 seconds on single trials). Any two observers on the globe, or at least a good part of one hemisphere, can observe some event like this, and then turn it into longitude by noting what stars are on the meridian, maybe with a nocturnal, or in the zenith, with a plumb bob, or at some specific altitude for given declination and latitude as in the navigator's time sight. And naturally we convert the difference between absolute time and local time in hours to degrees of longitude with the usual rule of 15 degrees per hour difference in time.

For miraculous inventions, you suggested:
"making a decent running pendulum clock."

Alex has made the excellent point for similar suggestions that they would have no reason to be impressed. How could they see such a clock as anything but a strange mechanical toy, or perhaps a peculiar work of art? What applications did they have that required accurate measurement of time intervals?

You also suggested:
"making a spark transmitter (powered by a salt-water or vinegar battery) and crystal receiver and use the combo to send a message few miles across a lake or field"

This is a good one. It's possible with medieval materials, though clearly difficult, and it would have immediate applications in warfare and espionage that would have been as obvious a thousand or two thousand years ago as they are today. Plus you could really zap a broad frequency signal without getting in trouble with the FCC (the US electromagnetic spectrum police). And for my next trick, I will invent the FCC. :)

My votes in the this game would go for 1) CPR --reliably raising the recently deceased by pounding on their chests and breathing into their mouths is sure to impress, or 2) hot air balloons.
Performing CPR might get you burned as a witch or declared to be the Almighty "Kirok" with all the problems that involves, while hot air ballooning would have the same espionage and warfare value as simple radio, and it's easier to do.

-FER

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