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Brad wrote:
"Units are arbitrary for the most part.  Established by convention, not by nature."

Some other examples for the thread:
-In the US, carbonated soft drinks are sold in metric bottles. Yes, even 
Coca-Cola is sold in one and two liter bottles. Milk and fruit juices are 
sold by the quart and gallon. It makes no difference to anyone since, within 
the normal tolerances of an average persons ability to estimate the volume of 
a container, the ratios are close to integers (one quart is 5% less than one 
liter). Back in the 1970s when this change occurred, there were perhaps two 
months of grumbling since a container change may also be a cheap excuse to 
raise prices (a liter is 5% more than a quart so why not charge 7% more per 
bottle! They'll never know!!). Indeed, that's the best argument against 
changing units in an active market. It creates price confusion and introduces 
market inefficiency (much the same thing happened for example when euro coins 
and notes were first introduced in parts of Europe), but it's a one-time 
inefficiency. 
-In good old celestial navigation, we use nautical miles. Does anyone imagine 
that we will ever shift over to kilometers for celestial fixes? It's been 
tried, but it never sticks (how many sextants have you seen with angles 
measured in grads? Nicolas has one... anyone else?). There have been some 
cases where metric versions of certain equations have replaced the English 
versions in traditional navigation. It is common to see approximations for 
dip of the horizon using observer height in meters as well as feet. 
-Throughout astronomy, distances are quoted in AUs (astronomical units), 
light-years, and parsecs. Only in journal articles of a certain style will 
you find these replaced by purely metric equivalents. There is no advantage 
in doing so. One might argue that a light-year is "actually" a metric unit 
since it is, by current definition, equal to an exact integral number of 
meters (1ly = 9,460,730,472,580,800 m). But if that's the standard, then 
English distances qualify as metric units since one statute mile is an exact 
integral number of millimeters (one statute mile = 1,609,344 mm).

Finally, there's an irony about the advantages of the metric system. Some of 
its best features as far as ease of calculation arise not from the rigorous 
modern definitions, but from the early definitions which are no longer 
strictly valid. For example, I find it extremely useful in many 
back-of-the-envelope calculations to know that a liter of water has a mass of 
one kilogram (one cubic meter of water has a mass of one metric ton). The 
connection between the meter and the circumference of the Earth (about 40 
million meters) turns out to be less useful in practical calculations, mostly 
because "grads" never caught on (one grad of latitude would have been 100km), 
but it does come up once in a while.

-FER




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