[LEAPSECS] LEAPSECS Digest, Vol 45, Issue 4

Tony Finch dot at dotat.at
Thu Sep 2 22:25:55 EDT 2010

On 2 Sep 2010, at 22:57, Ian Batten <igb at batten.eu.org> wrote:


> I think that there would be a perfectly reasonable debate as to if it meant UTC as defined at the point of legislation or UTC as currently defined. This is why US common measure and UK imperial measure diverged: the UK passed legislation in 1824 to systematise it, while the US system is based on earlier practice. Not merely do we have a different number of fluid ounces in a pint, our fluid ounce is different too. So in that case, at one point US law incorporated an external standard, but then when that external standard changed, the US law continued with what it had incorporated.

The history of measures of volume isn't so easily comparable with the current debate over UTC. In the late 1700s it was normal to have different versions of a measure for different purposes. There are still remnants of this in the pound avoirdupois vs the Troy pound, and nautical vs statute miles. At that time there were a number of different gallons, most prominently the ale and wine gallons. The wine gallon as standardized to 231 cu. in. in Queen Anne's reign is the one that Americans still use for all purposes.

In the 1820s the Brits made some reforms of measures roughly following French metric principles. One was the two shilling piece (24 pence or 1/10th pound sterling) which survived decimalization in 1971 when it was redefined to be the 10 pence piece and did not change physically until the 1990s when it got shrunk. Another was the imperial gallon.

This was defined in metric style as the volume of 10lb water. The pint remained 1/8th gallon as usual; however it was divided into 20 fl.oz. instead of the usual 16. The reason for that change was to make the fluid ounce the volume of one ounce of water. Hence the rhyme "a pint of water weighs a pound and a quarter".

(The Americans have a rhyme "a pint weighs a pound the world around" which is wrong in every respect. A US pint weighs several percent more than a pound.)

The imperial volume measures were a unified standard with a new name that replaced multiple predecessor measures used for different goods. So (getting back to the point) they are comparable to the original introduction of UTC, which is a unified timescale that replaced multiple predecessor timescales used by the various broadcast time signals.

The proposed redefinition of UTC is perhaps more like the redefinition of the yard from an independent standard of length to a multiple of the metre, or the redefinition of the metre from the length of a metal rod to a multiple of the wavelength of a particular colour of light then to the distance light travels in a particular fraction of a second, or (even more so) the redefinition of the second from a fraction of the length of the day to a fraction of the length of a year then to a multiple of a particular microwave frequency.

f.anthony.n.finch <dot at dotat.at> http://dotat.at/

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