> Primarily you have a choice between a second defined in terms of a precise
> elapsed time period measured from a stable source - eg an atomic clock,
> and seconds defined as being a fixed fraction of the measured period for a
> day, week, year whatever...
Can't speak for POSIX....
I was always taught the definition of a second was the 'duration of
9,192,631,770 periods of radiation emitted from the transition between two
hyperfine levels of ground state of a cesium-133 atom'. (Someone might want
to check that number, its from memory and could be wrong).
For the curious, the speed of light (in a vacuum) is defined as 299,792,458
meters/second (also from memory, only I'm pretty sure thats correct).
This, indirectly, the meter is also defined.
Other less well defined (i.e. the definition isn't tied to some physical
constant which is independent of your culture and location in the universe)
units are:
the kilogram, which is the weight of a platinum iridium standard which is
kept in a cold air-tight container in France somewhere (Sevres?)
the unit of temperature (the unit kelvin) is defined to be 1/273.16 of the
temperature of water's triple point (where all three phases can coexist)
the newton (and hence the ampere) depend on the definition of the
kilogram, so have no exact definition.
I believe most imperial units are now defined as some fraction of an SI
unit, but, presumably they aren't used much anymore in the scientific
community. (All my recent US physics texts use SI units).
I think the inch is defined to be 25.4 mm _exactly_, which pretty much
infers the length of a foot, yard, mile, fathom.
But, I digress.....
-Chris
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