In general, units of time can be divided into two categories.
One category contains the units that measure something objective
and observable, typically the movements of astrological objects.
A day, for example, is the length of time it takes our planet to
complete one rotation with respect to the sun. The second
category is much more fun: totally random, basically meaningless
divisions of time that were created out of a combination of
superstition, incorrect science, and the need for greater
precision in timing.
The seven-day week is in the latter category. There's no good
reason for it, and yet, it's constant to almost every single
culture.


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