Re: Metric Time (was Re: Why not 13 months? (Was La Systeme Metrique))

Fred Read (
Tue, 10 Oct 95 21:00:33 GMT

In article <45ea7c$ip1@cnn.Princeton.EDU> writes:

> In article <45e44t$>,
> (Wouter den Otter) writes:
> >What do you mean, 'the meter is known to a very high precision'?
> >It is DEFINED with a very high precision.
> I meant exactly what I said. The meter is DEFINED in terms of
> two other constants- the speed of light and the second. The
> speed of light is defined exactly in meters per second. The
> second is defined exactly as how long it takes a certain atom
> (Cesium, I think) to make a certain number of oscillations.
> Then, of course, any measurement we make of the speed of light in
> vacuum will help us FIND a value for the meter. The value for a
> meter is known to a very high precision, as I said. This is the
> current convention (since 1983). For an explanation or
> confirmation, pick up any introductory physics text published
> since then. The metal bars kept in Paris and elsewhere are
> relics of the past.

Sorry Jason, but you are wrong and Wouter is right.

It is correct to say that the [circumference of the earth, the
speed of light, the atomic weight of Uranium] is KNOWN very
precisely because these are measurable quantities.

The metre, however, is a contrived unit of length - an abstract
concept, which is DEFININED very, very precisely. You may 'know
the metre to a very high precision', but that knowledge is the

Fred Read
How many people do *you* know with a C function named after them ?