Subject: Re: SI Units vs cgs units

From: [email-masked] (T. Joseph Lazio, Cornell University)
Date: 24 Jan 92 09:38:33 EST

Keywords:  SI vs cgs
Summary:  Oh, no, not another change!

In article <[email-masked]>,
[email-masked] (Don Wells) writes: 
> In article <[email-masked]> [email-masked] (Tom Van
> Flandern) gives a glossary of certain astronomical units and
> terminology. Some corrections are in order [...]
> 
> TVF> Electromagnetic Spectrum: ... range of possible energies or
> TVF> wavelengths of photons... Visible light..	wavelengths.. 3,000
> TVF> and 10,000 Angstroms..	infrared.. wavelength.. 1 micron and
>      ^^^^^^^^^^^^^^^^^^^^
>      300 and 1000 nm
> 
> TVF> 1000 microns (= 1 millimeter).  Microwave.. 1 mm and 10 cm.
>      ^^^^^^^^^^^^      ^^^^^^^^^^                  ^^        ^^
>      1-1000 {\mu}m      mm                (both are legitimate SI units)
>      
> TVF> Radio waves..  10 cm and 10 meters... ultraviolet.. 100 Angstroms
>                                  ^^^^^^
>                                   m
> 
> TVF> and 3000 Angstroms... 
>      ^^^^^^^^^^^^^^^^^^
>        10-300 nm
> 
> Section 5.11 of the Manual ("SI Units", p.S20) says: "... The units of
> the centimetre-gram-second (CGS) system and other non-SI units, which
> will be unfamiliar to most young scientists, should not be used even
> though they may be considered to have some advantages over SI units by
> some astronomers. The IAU Executive Committee has recommended (see
> Appendix B) that astronomers should complete the change-over quickly,
> and so, for example, reports and papers for the IAU General Assembly
> in 1991 should all use SI units or other recognised units."

 Sigh.  After spending all my undergraduate years learning SI units, 
 I have spent my years in grad. school changing over and becoming 
 quite comfortable with cgs units.  Am I going to have to change, 
 again?
 
-- 
 T. Joseph Lazio                | Why relativity? and Why  
 514 Space Sciences             | turbulence?  I really believe   
 Ithaca, NY  14853              | [God] will have an answer for the
 (607) 255-6420                 | first [question].
 [email-masked]  |   -- W. Heisenberg, on his death bed
ICBM:                           |
 42 deg. 20' 08" N              |              STOP RAPE
 76 deg. 28' 48" W              |

---

From: [email-masked] (Joe Cain)
Date: 26 Jan 92 15:40:35 GMT

> quite comfortable with cgs units.  Am I going to have to change, 

	As one who indeed encountered MKS at Caltech in the 50's but
worked under Sydney Chapman at the U. of Alaska for graduate school
and found him a diehard cgs person, I can indeed empathize with this
uneeded confusion. However, I chose for my planetary geology text this
year what I thought was the latest published (Wagner's "Introduction
to the Solar System," 1991) which has a prominent table in cgs only. (He
also makes the mistake of equating to old English units throught the
text). Thus the emergence of SI seems slow and will require continued
pressure to keep stomping on the multiplicity of systems. It would
help a lot to also push hard in the US to get a complete conversion
>from the old English so students see it as useful and not just more
learning they should forget.

	The actual units can be SI compatible and still understandable
if they are kept fairly simple. For my course I use c = 300,000 km/sec
instead of 3x10**8 m/sec simply because it is easier to understand
compared with the 150,000 km AU. However, I doubt that the SI units of
density (kg/m**3) will ever supplant gm/cc simply because 1, 3, 5, are
simple compared to 1000, 3000, 5000, etc. For radiation also the units
can be more or less SI and still useful. I have had to convert my
thinking that 5577A was apple green and 6300A red to 500, 600 nM, but
this is not so bad. Microns are hard to integrate into SI thinking,
but you are going to have a hard time getting radio astronomers to use
units they can't relate to waveguide and antenna sizes.

	In summary, I think you have to keep pushing SI, be kind to those
whose thinking was frozen decades ago, and try to use the smallest
number of units to promote the most interdisciplinary understanding. 

Joseph Cain		[email-masked]
[email-masked]		scri::cain

---

From: [email-masked] (Arnold Gill visiting astrophys phd std)
Date: Mon, 27 Jan 1992 17:35:12 GMT

>number of units to promote the most interdisciplinary understanding. 

     And everyone also has to stay `bilingual' so that one can understand papers
     written in the `Dark Ages'.
--
Arnold Gill --- astrophysician trainee in exile             [email-masked]

---

From: [email-masked] (Terry Gaetz)
Date: Mon, 27 Jan 1992 22:35:05 GMT

>     written in the `Dark Ages'.

'Bilingualism' will stay important, too; too many standard reference 
tables have used (and new ones continue to use) non-SI units.

It is not only North American astronomers who persist in using non-SI 
units, either - see many physics papers and reference tables:

<Rooting through the shelves...>
  . "Atomic Transition Probabilities: Iron through Nickel" (1988) 
      - wavelengths in angstroms, energies in cm^-1
	(This is an NBS/NSRDS publication to boot!)
  . National Institute for Fusion Science (Nagoya, Japan) 
    "Total and Partial Cross Sections of Electron Transfer 
     Processes for Be^q+...", preprint, (Jun, 1991)
             - energy in eV, cross sections in cm^2
    "Observation of Parallel Viscosity in the CHS 
     Heliotron/Torsatron", preprint, (Jan, 1991)
	     - temperature in eV, density in 10^13 cm^-3, velocity in
	       km s^-1, viscosity in m^2 s^-1

SI may be the mandated standard, but I don't see cgs and non-SI units
going away anytime soon.
--
Terry Gaetz  --  [email-masked]

---

From: [email-masked] (Doug McDonald)
Date: Mon, 27 Jan 1992 14:46:48 GMT

>>[email-masked] (Don Wells) writes: 

[SI flame wars] [Note: Im not an astronomer, I'm a physicist and chemist]

I have a very strong opinion on this, one shared my many (most?) of my
colleagues, including several Nobel Prize winners:

A plague on those who try to force pure SI units on everybody.

Point one:

The metric system was originally designed to have convenient multiples
of ten for every unit - grams, kilograms, gigagrams, meters, centimeters,
micrometers, seconds, milliseconds, picoseconds. Why not use that which is
convenient? I can remember the basic numbers (Planck's constant, mass
of proton, etc.) in both MKS and cgs units. I use whichever is easier.
It is trivial to convert. 

Point two:

In the actual world it is stupid to express everythings in say, meters and
seconds. Everybody I know expresses times in either picoseconds or femtoseconds
when relating to actual events, because that is the times that things actually
occur in our lab, for example, a laser pulse lasts roughly 1 picosecond,
always between .01 and 100 picoseconds. 

Consider lengths - teh wavelengths of light are often expressed in microns.
Now the micron happen to be one micrometer. Why worry? People express
energies in that unphysical unit the "wavenumber" which indeed is
not SI or cgs or anything - its not an energy unit at all, just a reciprocal
wavelength. Yet it is very convenient indeed. It is just silly to
try to talk about energies of molecular events induced by light in units
unreleated to the commonly used measure of light, the wavelength.

Then there are  pressure units used in vacuum work. You will note that
pressure gauges are calibrated in Torr. Everybody uses those units in the
lab. Really. Except for pressures greater than one atmosphere, where
they use atmospheres or even, heaven forbid, psi. I do admit to
removing psi from papers and putting in atmosphere.

I think that worrying about units is silly. I'd rather worry about the
science. In fact, using SI units in fields where they are not normally used,
such as mine, is counterproductive. IF somebody actually tries to give a talk
in SI units, they will invariably be asked to give, for example, the
pressures of their beam sources in either Torr or atmospheres. That is what
people think in. 

And to the astronomers: if you want to use SI units only please remember to
NEVER use AU or parsecs!!!

Doug McDonald

---

From: [email-masked] (David Mark)
Date: 27 Jan 92 22:44:15 GMT

>It is trivial to convert. 

I'll ask the naive question; I hope I'm not the only sci.geo.geology 
subscriber who doesn't know this!

I thought I knew what the 'metric system' was, and also that SI was the
abbreviation for the scholarly or 'official' term for the metric system.

So, what are 'cgs' and 'MKS' and how do the differ from 'SI' and 'the metric
system'?

David Mark
[email-masked]

---

From: [email-masked] (Nick Fitzpatrick)
Date: Tue, 28 Jan 1992 00:56:35 GMT

>So, what are 'cgs' and 'MKS' and how do the differ from 'SI' and 'the metric

cgs, is a length-mass-time system, with cm, g, s as the base units, it is not
widely used anymore, except perhaps in the US which I here actually even
uses Imperial units on occasion  (I mean can you imagine!! (Flame me!)

MKS (if I remember correctly) is a length-force-time system, where metre, (that
is meter for our american cousins who can't spell) kg and s are the base units
Nobody uses this.  I even recall hearing that it is banned in some European 
countries!  (Mind you Imperial was banned here in Canada at one point, until
someone pointed out that with freedom of speech, one could not ban Imperial)

SI, or Systeme Internationale, which is what we often mistakenly call the
metric system, uses a length-mass-time with metres, kilograms, and seconds
as the base units (+ Celcius, and a few other special things that I
forget)..  Derived units include Litres for volume, Newtons for force
Pascals for pressure, ares, for area, etc,etc

Nicholas Fitzpatrick,  Waterloo, Ontario, Canada

---

From: [email-masked] (Nick Fitzpatrick)
Date: Tue, 28 Jan 1992 03:56:26 GMT

>MKS (if I remember correctly) is a length-force-time system, where metre, (that

Okay, before anyone else has the chance to flame me, let be the first!  MKS
is a length-mass-time system.  (Boy I'd look to meet the fool who thought 
it was force ;-)   So the question arises, what is the difference between
MKS and SI.

And before anyone else writes me telling me how great cgs is for Gaussian
whatever, please note from the >> that I was following up a comment from
sci.geo.geology.  (sometimes I think the sci.space lot are from another
planet or something! :-)
>Nicholas Fitzpatrick,  Waterloo, Ontario, Canada
ditto

---

From: [email-masked] (Terry Gaetz)
Date: Tue, 28 Jan 1992 03:31:27 GMT

>So, what are 'cgs' and 'MKS' and how do the differ from 'SI' and 'the metric

metric system:  any decimal system of measures based on the metre
	        (e.g. SI, cgs, MKS, MKSA); often SI is meant.
cgs:  system of units based on centimetres (cm), grams (g), and 
      seconds (s).
      (also variants:  electrostatic, electromagnetic, Gaussian, and
		       Heaviside-Lorentz units, depending on the units
                       used for electromagnetic quantities)
MKS:  system of units based on metres (m), kilograms (kg), and 
      seconds (s).
      (variant:  rationalized MKSA adds the ampere)
SI:   Syst\`eme International d'Unit\'es:
         m     metre       (length)
         kg    kilogram    (mass)
         s     second      (time)
         K     kelvin      (temperature interval)
         A     ampere      (electric current)
         cd    candela     (luminous intensity)
         mol   mole        (amount of substance)

      Other units are formed as combinations of these basic units
      and selected powers of ten.  Permitted prefixes are
       E   exa     10^(18)            m   milli    10^(-3)
       P   peta    10^(15)          \mu   micro    10^(-6)
       T   tera    10^(12)            n   nano     10^(-9)
       G   giga    10^(9)             p   pico     10^(-12)
       M   mega    10^(6)             f   femto    10^(-15)
       k   kilo    10^(3)             a   atto     10^(-18)

   hecto (10^2), deca (10), deci (10^-1), and centi (10^-2) are allowed,
   but are to be used only where kilo- or milli- would be impractical.
   Thus, the centimetre (cm) is not a prefered SI unit.

[the above is based on: appendix to Chambers English Dictionary,
                        Jackson: "Classical Electrodynamics"]

SI is the adopted international standard, but (Gaussian) cgs units and 
others (e.g. various forms of 'natural' units) are also widely used in 
astronomy and physics.
--
Terry Gaetz  --  [email-masked]

"Without Units, dimensionless analysis itself would be impossible."
                   (somebody's .sig; sorry, I don't have the attribution)

---

From: [email-masked] (Henry Spencer)
Date: Tue, 28 Jan 1992 17:17:08 GMT

>So, what are 'cgs' and 'MKS' and how do the differ from 'SI' and 'the metric

They're all metric systems.  The question is, what is the unit of (say)
energy?  Is it kg-m^2/s^2, or g-cm^2/s^2, or what?  And what is its name?
In this case the units differ only by factors of ten, but the situation
can be worse -- there are some electromagnetic units which might or might
not have a factor of pi in them, for example, depending on what convention
you choose.  Over the years various groups chose various different sets of
conventions, and the result was difficult and troublesome conversion
problems when, say, a thermodynamicist wanted to talk to a magnet expert.
You had to be careful even when the names seemed similar; for example,
there were three or four subtly-different units named "calorie".

SI, Systeme International, is an attempt to pull all this together and
have *one* metric system, with *one* unit for expressing, say, magnetic
flux density, and a consistent set of conversions with as few magic
conversion factors as possible.

Although SI is essentially universally accepted as desirable, the switchover
was always expected to be a very long process, and indeed it is proving
to be so.  It takes an awfully long time to convert things like reference
books, not to mention all the problems of equipment.
-- 
"Breakthrough ideas are not             | Henry Spencer @ U of Toronto Zoology
>from teams."  -- Hans von Ohain         |  [email-masked]  utzoo!henry

---

From: [email-masked] (W. D. Rolph)
Date: Tue, 28 Jan 1992 21:51:29 GMT

>books, not to mention all the problems of equipment.

The wonders of the "standard" metric system.  Now for grins is force in
kilograms, or newtons or what?
-- 

Regards.
 
Don Rolph  [email-masked] WD3 MS10-13 (508)-699-1263

---

From: [email-masked] (Lee E. Brotzman)
Date: 29 Jan 92 02:32:11 GMT

>>books, not to mention all the problems of equipment.

I think that this thread has rapidly diverged from its original intent.  This
is sci.astro, and the discussion was about standardized units for use in
astronomy.  The International Astronomical Union has firmly come down on the
side of SI units.

I have spent my entire nine-year professional career providing data to the 
astronomical community under the auspices of the Astronomical Data Center at 
NASA Goddard Space Flight Center.  In the early 80's, individual requesters 
were provided with one or two datasets on magnetic tape and wrote their own 
analysis routines to reduce the data.  Now they can request large volumes of 
data over the networks or on CD-ROM and can access all of the data 
simultaneously.  Intercomparison of the data is essential to modern data
analysis, and that implies that the data should be in a common system of units.

As a data archive, the ADC must be responsive to the requirements of our
contributors and maintain the data in its original form as given.  On the other
hand, we must be responsive to our requesters and provide the data in the form
they require to complete their work.  The path that we chose was to select a 
standard nomenclature for the field names and for the units that those fields
appeared in.  The work that went into our CD-ROM of astronomical catalogs
was mostly directed to standardizing the nomenclature of fields names and
units.

With a common set of field names (visual magnitude always V, hours of right
acsension always RAH, etc.) and a common set of units (mag always magnitude,
h always hours of time, etc.) it is possible to write software that can 
determine the correct operations to perform on a given datum.  The promulgation 
of these standards is an important function of the international astronomical 
data centers and of the North American and European FITS committees.

For further discussions of the standardized nomenclature proposed by the ADC,
we suggest that you stay in touch with the sci.astro and alt.sci.astro.fits
discussion groups.  The ADC CD-ROM of astronomical catalogs is currently
available from the National Space Science Data Center at NASA Goddard Space
Flight Center, and complete information on how to obtain the data will be
given in this forum as soon as the material is made available.

--
-- Lee E. Brotzman                    Internet:  [email-masked]
-- Hughes STX                         SPAN:      NSSDCA::BROTZMAN
-- National Space Science Data Center BITNET:    ZMLEB@SCFVM
-- NASA Goddard Space Flight Center

---

From: [email-masked] (Phil OKunewick)
Date: Wed, 29 Jan 1992 18:07:46 GMT

This is an old debate.  And a stupid one.

   The "metric" versus "traditional" measuring debate has been going
on ever since the metric system was first introduced.  Since the
metric system is usually easier to use, it's gradually taking over.
But so what??

   If you can't handle all measurements currently used in your field
(including those in still-useful old texts), then you probably should
consider another line of work.  The information is what's important,
not the units it's expressed in.

   If you want to really improve things, then drop this stupid
base-ten system.  The factors are five and two.  Two is okay, but what
kind of a nut would want to work with fives?  Oh sure - you say you're
comfortable with bast ten.  That's because it's been drummed into your
head since you were a toddler - "Now junior, count to ten on your
eight fingers and two thumbs..."  But did you ever try to cut a pie
into five slices?  Damn near impossible.

   It's time to dump this nonsense, and start working with something
clean.  What's wrong with dividing by two's?  Dividing a pie into four
or eight slices is easy.

   Our entire civilization should start using Base-16 - it's a power
of two that people can easily deal with.  Measuring becomes infinitely
simpler - start with a standard that's one unit long, and keep
doubling it or dividing it in half.  Heck, some cultures already do
this - 1/8", 1/2", 1/32", pints, quarts, you get the idea.  Each time
you divide or multiply four times (see that power of two there?), you
shift a decimal place.  Simple as that.

   Still not convinced?  Then consider the alternative - someday in the
distant future we may encounter intelligent life in another part of the
universe.  And their first assessment of us will be, "Base-ten??  What
kind of a numbering system is that?  No wonder it took you earthlings so
long to get into space - you spend all your time measuring and no time
thinking.  What a bunch of nimrods."  And they'll tell all their buddies,
and earthlings will become the laughing stock of the universe.

   Now we don't really want that that to happen, do we?  I think not.

---

From: [email-masked] (Arnold Gill visiting astrophys phd std)
Date: 29 Jan 92 21:43:49 GMT

>not the units it's expressed in.

     Quite agreed - but it makes things easier if future generations
     didn't have to worry about what units a specific quantity is
     measured in.

     Of course, if you want really strange units, then look at some
     GR or QM papers where all of the standard constants are equal to 1.

>kind of a nut would want to work with fives?  

[...and...]

>of two that people can easily deal with.  

     I've read novels which were of the opinion that the most sensible
     base is actually base 12 -- a small number tht has lots of small 
     factors (2,3,4,6).  Potentially more useful.  However, its not going
     to happen, and base 16 is only useful for computers and people who
     can only say "Yes" and "No".  :-)

---

From: [email-masked] (david michelson)
Date: Thu, 30 Jan 1992 00:59:48 GMT

Well, I for one don't like cgs *at all*.  I mean, how can anyone like
a system where capacitance is measured in units of *length*! :-)
 
Dave Michelson             "[email-masked]"
UBC Electrical Engineering
Applied Electromagnetics Group

---

From: [email-masked] (John H. Kim)
Date: 30 Jan 92 03:26:23 GMT

>      and selected powers of ten.  Permitted prefixes are
       ....
>       k   kilo    10^(3)             a   atto     10^(-18)

Except, because they made converting between units so much easier
than the english system, they decided to slip in KILO-grams just
to keep everyone on their toes.  Be careful of this if you've got
a whole mess of numbers and units and you try to simplify it by
canceling out prefixes like 'kilo' and 'Mega' when they're in both
the numerator and denominator.  (If you're careful, you'll notice
you have grams left over, not kg, but more often you'll end up
reading it as kg)
-- 
John H. Kim                 | (This space to be filled when I
[email-masked] | think of something very clever 
uunet!jarthur!jokim         | to use as a disclaimer)

---

From: [email-masked] (Henry Spencer)
Date: Thu, 30 Jan 1992 18:47:57 GMT

>     GR or QM papers where all of the standard constants are equal to 1.

An amusing side note on this:  in his JBIS paper on space warps (real ones
supported by current physics, not science-fictional ones), Robert Forward
complained loudly about this convention.  It makes the physics easier,
but translating the results back into normal units -- so you can decide
whether the engineering required is impossible or merely daunting -- is
tremendously difficult because it's hard to figure out where (and which)
conversion factors are needed.
-- 
"Breakthrough ideas are not             | Henry Spencer @ U of Toronto Zoology

---

From: [email-masked] (Paul Zander)
Date: 30 Jan 92 01:19:42 GMT

>kilograms, or newtons or what?

For the benefit of readers who don't know the answer:

Kilograms is a unit of mass.  A kilogram will be a kilogram on the
earth, the moon, or deep space.  If there is some gravitational field,
a balance scale and a set of calibrated standards (popularly but
incorrectly called "weights") might be used to determine mass.  This
works because in a scale is balanced, equal masses will have equal
weights.

One type of force is the gravitational attraction of the earth (moon,
etc) on an object.  Force can be measured by a spring scale.  An
object with a mass of 1 kilogram will have a weight of 1 newton on the
earth's surface.  The weight will be different in different places in
the universe, but the mass is constant (ignoring relativistic
effects).

The difference between mass and force seems subtle to many beginning
physics students, but is especially important in astronomy.  More
detailed coverage can be found in almost elementary physics text.

Paul Zander

---

From: [email-masked] (Stefano Casertano)
Date: 30 Jan 92 20:21:40 GMT

In <[email-masked]> [email-masked] (Paul Zander) writes:

>  (deleted about difference between mass and force)

>object with a mass of 1 kilogram will have a weight of 1 newton on the
                                                        ^^^^^^^^
>the universe, but the mass is constant (ignoring relativistic

No.  It will have a weight equal to the mass times the local gravitational
acceleration, typically about 9.81 m s^-2.  Since the newton (N) equals
1 kg m s^-2, the weigth of a mass of one kg will be around 9.81 N,
and it will be slightly different at different places on the Earth's 
surface.  

I commend your effort to clarify the difference between mass and weight,
better than I could do (and I have tried in the past...) but please
make sure of your facts next time...

Stefano Casertano         [email-masked]

---

From: [email-masked] (Paul Zander)
Date: 30 Jan 92 21:45:38 GMT

Correction:

In my previous comments, there was an error.  On the earth's surface, a
mass of 1 kilogram will have a weight of 9.8 Newtons.  That is because
the earths gravitaional field creates an acceleration of 9.8
meters/second.  

The key point remains, mass is unaffected by gravity (ignoring
relativistic effects) but weight is very much a function of gravity.

---

From: [email-masked] (David Mehringer)
Date: 1 Feb 92 00:56:01 GMT

>the earths gravitaional field creates an acceleration of 9.8
As long as we are being dimensionally correct, acceleration is 
measured in meters/(second)**2 in the mks system, not meters/second.

>relativistic effects) but weight is very much a function of gravity. 

The Newtonian relationship is weight = mass * gravity.

--
Dave Mehringer            | "There is a fine line between dignity and 
[email-masked] |  stupidity." 
National Radio Astronomy  |    -- Homey the Clown
Observatory, Socorro, NM  |