[time-nuts] establishing your position w/o gps

Hal Murray hmurray at megapathdsl.net
Tue Jan 24 18:11:47 EST 2012

> But how do you untangle longitude and time? How do you know that you are
> looking exactly south (or north)? 

If I understand what you are asking, it's the same problem as navigating a 
ship without a clock.

Classic navigation with a sextant needs a clock and sightings on 3 objects in 
the sky.  Each sighting  gives you a circle on the globe, or a line if you 
know roughly where you are.  The lines form a triangle.  The size of the 
triangle is an indication of the accuracy.  You pick the objects so the 
triangle is roughly equilateral.
You can get time from the moon, so in theory at least, that's an answer to 
your question.

Years ago, when a friend was learning navigation, he was reading one of the 
old classic texts.  There was a good story about the guy off the coast of 
England/Ireland who didn't trust his clock, so he did the calculations again 
assuming his clock was a bit fast and again with it slow.  That gave him 3 
parallel lines for each sighting.  Anybody recognize that story?

Longitude by Dava Sobel is a good read, especially for time-nuts.  There is 
also a version with lots of good photographs.

One of the techniques they actually considered before Harrison built good 
enough clocks was to derive time from Jupiter's moons.  They knew enough to 
correct for the time shift due to speed of light delays as the Earth-Jupiter 
distance changed.  (I don't know if they knew if was due to speed of light.)

You can synchronize two clocks if both sites can see the same event in the 
sky,  Occultations are often used for this.

With modern technology, radio telescopes are very very good at this.   In 
order to do VLBI, you need to know where the telescopes are located.  With a 
big collection of data you can do least-squared fit type calculations to 
refine the location and clock calibration.

These are my opinions, not necessarily my employer's.  I hate spam.

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