Everyone knows that the unit of measurement, the metre, is defined as the distance travelled by light (in a vacuum) in 1/299,792,458 of a second. So says the International Bureau of Weights and Measures (see their web site for more information.
They go on to tell you that “the second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom”.
From these “base units” there are a number of “derived units” that can be, er derived. For example, velocity, which is measured in metres per second. Or, if you are the International Bureau of Weights and Measures, you could presumably measure it in the distance travelled by light whilst a caesium 133 atom transits between its two hyperfine levels around 30.5 times. Or something.
Now, we didn't used to measure metres and seconds in terms of caesium decay and the speed of light. No, we used to do things based upon natural occurrences in the real world. For distance, we measured things in “feet”, which corresponded to the size of your feet. Or “hands” which corresponded to the size of your hands. Or “pouce” which was something to do with your fingers. Or “yards” (not sure about this one - perhaps people had very small gardens in those days?). For time, everything is measured in terms of the period of rotation of the earth (in other words, a day), or the duration of one full orbit of the sun (in other words, a year). The year was neatly divided up into 12 “months”, which were each (approximately) linked into the phases of the moon. And the number 12 sort of cropped up again when dividing the day into useful bitesized chunks (i.e. hours).
And it isn't just time and distance - it turns out that all sorts of units were originally based in everyday objects and observances made by our ancestors. For example, take degrees - a 90 degree angle (a “right angle”) was originally defined by the shape of a cow's posterior (interesting factoid : a cow's arse is the only naturally occurring right angle in nature - consequently our ancestors were fascinated by cow's arses, and there is widespread evidence that many cultures joined all of the stars in the northern hemisphere together to make an enormouse cow's arse, which they then worshipped).
The Pound (as in weight, denoted by the symbol 'lb', originally meaning Libra - a unit of Roman weight), was originally defined by a large blunt stone called a Pound, used for killing elderly relatives who had become a burden to society. It was found that the weight of the Pound was actually quite critical - too light, and you'd just knock them out, too heavy and the authorities may begin to suspect foul play. Note that in European countries, they had a similar device made of metal, called the kill-a-gran.
More next time….
Posted by nikn