OT: Alien Media (was Disasters and Recovery)

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Thu Jan 21 10:27:55 1999

> It was thus said that the Great Philip.Belben_at_pgen.com once stated:
>>
>> I'd like to define it in terms of SI units, but the Kilogram is not too
>> easy (yet).
>
> Really? The meter is defined (certain frequency of light from a
certain
> element for so many waves yada yada). Sea level is defined (don't know
the
> SI unit, but 780 millibars of pressure). Celcius is defined (0 is
freezing
> point of pure water at sea level, 100 boiling point of pure water at sea
> level) and that's all you need to define the gram: one cubic centimeter
of
> water at 4C at sea level. That also gets you volume (liters).

Well, sort of. The definition of a metre is actually 1/299792458 of the
distance travelled by light in vacuo in one second. Wavelengths of the
orange-red line in the spectrum of Krypton-86 went out long enough ago to
qualify for this list at least!

The definition of the kilogram is _still_ afaik "the mass of the
international prototype kilogram" (or kilogramme, I suppose).

And FWIW sea level pressure is 1013 millibars. 760 millimetres of mercury.
But kilogram = mass of 1 cubic decimetre of pure water is probably accurate
enough for most purposes as long as you don't have too much deuterium or
oxygen-18.

BTW Celsius isn't an SI unit. SI unit of temperature is the [degree]
Kelvin (K, written without the degree sign), defined as 1/273.16 of the
absolute temperature of the triple point of, oh no! It's water again!
Again, specify hydrogen-1 and oxygen-16 and it's probably accurate
enough...

See. Not as easy as it sounds, but still possible.

If you allow non-SI definitions, start with:

Second = 1/86400 mean solar day (that changes only by a couple of percent
every million years - enough that dinosaurs had a 400 day year, and ancient
Babylonians observed eclipses a few tens of miles from where modern
astronomers predicted them or at the wrong time of day)

Metre = 1/40000000 circumference of Earth. Originally 1E-7 distance from N
Pole to Equator through (I think) Paris. (Longer than SI metre by 0.02%)


Then proceed as above with Celsius and volumes of water...



Philip.
Received on Thu Jan 21 1999 - 10:27:55 GMT