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    Re: SI Units
    From: Frank Reed
    Date: 2009 Oct 22, 13:27 -0700

    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 
    -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.
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