Subject: SI Units (was Re: AAS meeting, Atlanta)

From: [email-masked] (Don Wells)
Date: Thu, 23 Jan 1992 14:31:07 GMT

In article <[email-masked]> [email-masked] (Tom Van
Flandern) gives a glossary of certain astronomical units and
terminology. Some corrections are needed:

TVF> Angstrom: A unit of length equal to 10^-10 meters = 10^-8 cm.
     ^^^^^^^^                               ^^^                ^^

TVF> Used primarily to measure the wavelengths of photons of visible
TVF> light.  

The IAU Style Manual (1989, "The Preparation of Astronomical Papers
and Reports", prepared by G.A. Wilkins, President of Commission 5, on
behalf of the Executive Committee of the International Astronomical
Union) recommends use of SI (MKS) units, which in this context means
the *nanometer*, 10^-9 meters. The Angstrom is listed in Table 7
("Non-SI units and symbols whose continued use is deprecated", p.S24).

TVF> Electron Volt: A unit of energy equal to 1.60 x 10^-12 ergs...  
                                              ^^^^^^^^^^^^^^^^^^
                                              0.1602177 aJ (attoJoule)
                                                     ("atto" = 10^-18)

The eV is listed in Table 6 ("Non-SI units that are recognised for use
in astronomy", p.S23), and "atto" is listed in Table 5 ("SI prefixes
and symbols for multiples and submultiples").

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."

--

Donald C. Wells             Associate Scientist        [email-masked]
National Radio Astronomy Observatory                   +1-804-296-0277
520 Edgemont Road                                 Fax= +1-804-296-0278
Charlottesville, Virginia 22903-2475 USA            78:31.1W, 38:02.2N

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From: [email-masked] (Don Wells)
Date: Fri, 24 Jan 1992 18:08:43 GMT

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

"...  The IAU Style Manual (1989, "The Preparation of Astronomical
Papers and Reports", prepared by G.A. Wilkins, President of Commission
5, on behalf of the Executive Committee of the International
Astronomical Union) recommends use of SI (MKS) units..."

I have received several Email messages in response to my posting on SI
units, and after reading them (and answering some) I offer the
following additional remarks:

1. Undergraduate Training vs. Graduate Training

I was an undergraduate physics student in the early 60s, 30 years ago,
and was taught in MKS (SI) units. Later in that decade, in graduate
school, I encountered the CGS approach. I thought at that time that
both systems were rational, each in its own way. I assumed that SI was
destined to be the accepted standard, but that in that era one still
needed to be bilingual. As a result of my posting on this subject I
have received Email from recent graduate students who tell me that the
situation I experienced 20-30 years ago still persists!  I.e., physics
undergraduates are still taught MKS and must still learn CGS in
astronomy graduate school. The persistence of this anomaly over such a
timespan is telling us something.

2. SI Units in the IAU Style Manual

It is, of course, true that IAU standards are only recommendations,
and that adherance to them is voluntary. However --- if the
recommendation to use SI units is opposed by a large fraction (a
majority?)  of our community, why isn't that fraction represented in
Commission 5 and/or the IAU Executive Committee? Is the persistence of
usage of both MKS and CGS units purely a phenomenon of (North)
American graduate departments, or is it present in astronomy
departments elsewhere in the world? The IAU is *the* international
organization of professional astronomers; in the end the practices it
recommends should really represent the consensus of astronomers
everywhere, or else those recommendations should be changed. 

3. Importance of Standard Units

Several decades of experience demonstrate that astronomers are able to
operate with both systems of units, and that few problems are caused
by the potential ambiguities. I myself don't particularly prefer
either system over the other. What I *do* care about is that
inconsistencies and ambiguities should be minimized in astronomical
archives (both observational data and published text) as much as
possible, in order to facilitate robust automated search and retrieval
technology in the years to come. My worries about this problem are the
real motivation for my posting on this subject.

For example, if the TUNITn keywords of FITS tables do not use a
consistent system of units we will surely find that it will be harder
to write our data analysis software. In March 1979 Eric Greisen and I
decides that the Basic FITS Agreement would specify that SI units
should be used in FITS files, and that decision was recently endorsed
by members of the IAU FITS Working Group and the NASA Technical Panel
during the NASA-supported codification of the FITS agreements. These
decisions were made precisely because of the worry that software will
prove to be less robust than humans in interpreting ambiguous or
inconsistent situations in our future archives.

4. Brief Bibliography

* "A Physicist's Desk Reference" (The Second Edition of Physics Vade
Mecum, H.L. Anderson, Editor in Chief, Am. Inst. Physics 1989) gives a
detailed tabulation of SI and units conversions in section 1.02 ("The
International System of Units", pp.5-10). It also contains a
convenient summary of the real MKS-vs-CGS (Gaussian units) issues in
section 1.05 (Physics Formulary), subsection C (Electricity and
Magnetism), where the notations are compared for E&M case by case. It
is clear that both systems are rational, and attractive, each in its
own way.

* Chicago Manual of Style says: "[section] 14.41 The International
System of Units (..abbreviated "SI") is used generally by scientists
around the world...". 

* Astrophysical Journal Instructions to Authors (1992 December 1, p.i)
says: "... 3. The standard abbreviations (without periods) for SI units
and some cgs units may be used..."

* Monthly Notices RAS Notice to Authors (recent issue) says: "... 7.
UNITS. Authors are strongly encouraged to use SI units."

* Document NOST-100-0.3b (6-November-1991, "Implementation of the
Flexible Image Transport System [FITS]", section 8.1.2, p.34, "TUNITn
Keywords") says: "...Use of the units defined in the IAU Style Manual
is recommended."
--

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From: [email-masked] (Joe Wang)
Date: 30 Jan 92 21:07:48 GMT

Speaking of which, my undergraduate training (only two years ago) was
completely in cgs units, which meant that I was in for a shock when I
took the GRE.

OUCH!!!