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John, you wrote:
"Exactly!!!  What amazes me is the long shelf-life of these."

Not terribly reliable, no, but such observations are, in fact, one of the last 
remaining practical uses of celestial navigation... in "fish tags".

The idea is to package a light sensor, a pressure gauge, a clock set to UT, 
and a data recorder into a small tag that can be attached to a large fish or 
a marine mammal. As the animal swims about for some months or even years, 
travelling probably thousands of miles, the tag records the light intensity 
continuously, recording every five minutes if available memory permits. When 
the tag's data is recovered, there will be daily cycles in the light 
modulated by large changes as the animal moves up and down in the water 
column which can be backed out to some extent using the pressure data. The 
light levels can also be corrected for clouds after the fact (from satellite 
imagery). The width and overall shape of the light curve yields a very rough 
estimate of latitude. Note that there's more data here than just the time 
interval between sunrise and sunset. The symmetry axis of the light curve 
yields a rather good estimate of longitude. Clearly this is not precision 
navigation, but it's pretty clever and it works well enough to be 
scientifically useful. Also note that this is largely a commercial technology 
so there may well be big differences in the way things are done depending on 
the vendor.

-FER
PS: Times flies. I posted a message about these things six years ago: 
http://www.fer3.com/arc/m2.aspx?y=200312&i=012506


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