Subject: Re: NASA and metric

From: [email-masked] (James Ashton)
Date: 6 Sep 1993 02:53:28 GMT

In article <[email-masked]>, [email-masked] (Phil Karn) writes:
...
>                                                     So for all
> practical purposes, you can think of 1 bar as being 1 atmosphere.
> This has far more physical meaning to me than 1 PSI. I don't know of
> anyone in AMSAT (American or otherwise) who feels the need to convert
> to PSI. I sure don't.
> 
> You may have noticed that the US weather service has for some time
> been using millibars as units for barometric pressure.

A couple of years ago the ABC (The Australian Broadcasting Corporation)
changed their weather terminology.  Instead of millibars we now have
hectapascals with a conversion factor of 1.  I guess this is the
ideologically sound metric way.
--
James Ashton						  System Administrator
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From: [email-masked] (Jean-Francois Gonzalez)
Date: 6 Sep 1993 07:25:17 GMT

In article [email-masked], [email-masked] (James Ashton) writes:
> ....

The same thing happened in France. The numbers are the same, but hectopascal is the
correct SI unit.

Jean-Francois.

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From: [email-masked] (Dick Edgar)
Date: 9 Sep 1993 13:06:30 GMT

In all this discussion of NASA and metric units (i.e. SI) there's the
related question of doing astronomy in SI units.  At the moment, most
astronomers use a mixture of metric units (mostly cgs, but velocities
are in km/s) and "natural" astronomical units, like years, AU, parsecs
and the like.  Also solar masses and solar luminosities.

What I'd like to ask is, if we want to do astronomy in SI units, what
are the proper prefixes for, say, 10^33?  A solar mass is roughly 2x10^33
grams.  My understanding is that the SI prefix list ends with "Y" which
represents 10^24.  A solar luminosity is roughly the same order (in
ergs/sec), so we'd also need some large prefix for watts to represent
this natural power unit (for astronomy).

Any ideas?
---------
Richard J. Edgar  ([email-masked])
University of Wisconsin--Madison, Department of Astronomy
"An astrophysicist is someone who sees something working in practice,
and wonders whether it will work in principle." -- Harvey Liszt

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From: [email-masked] (Bernd Lehle)
Date: 9 Sep 1993 15:55:26 GMT

Dick Edgar ([email-masked]) wrote:

: What I'd like to ask is, if we want to do astronomy in SI units, what
: are the proper prefixes for, say, 10^33?  A solar mass is roughly 2x10^33
: grams.  My understanding is that the SI prefix list ends with "Y" which
: represents 10^24.  A solar luminosity is roughly the same order (in
: ergs/sec), so we'd also need some large prefix for watts to represent
: this natural power unit (for astronomy).

: Any ideas?

There is absolutely no use for that, since the orders of magnitude
vary so strongly, that You will need dozens of Megas, Gigas, Teras
and so on that nobody will understand in the end.

In publications and discussions it is better to use natural units
like lightyear, parsec and solar properties to avoid senseless
conversions.
In calculations it is better to use SI to avoid senseless conversions
during the process of computation and working out equations.

SI base units are designed for earth- or human-scaled measures.
A human is roughly 1m tall, weighs hundred kilograms, has a heart-
beat frequency of one second and his electric household machines
operate at 1 Ampere. 

One should always use the advantages of a system. SI has the advan-
tage of simple conversion factors in all physical laws. Natural
units have the advantage that one can easily picture them. The SI
base units are natural units for humans nowadays. The Imperial units 
were the natural units for middle-ages people, because that is the 
scientific environment that created them.

I hope that I will not trigger another followup-avalanche by that...

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