# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**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 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 --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To unsubscribe, email NavList+unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---