> Indeed. I guess they'd have "re-calibrated" the cart daily whenever 
> possible. Sunrise or sunset would have been a good opportunity. So the 
> device would have had to work for typical distances of, say, some 100 km:

FWIW modern cars today use the same principal.  That's why if you have a car 
with GPS (a built-in one, not an aftermarket one) it still knows which way 
you are facing when driving around underground (eg parking garage or 
tunnel).

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