Subject: SI units in astronomy

From: [email-masked] (BEN)
Date: Tue, 7 Sep 1993 01:37:56 GMT

People, 

In the August 1993 Newsletter of the American Astronomical Society, Hale
Bradt of Cambridge, MA calls for a complete transition to SI units for
Society journals, etc.  An e-mail address is given for responses to an
informal poll. I suppose it would be appropriate for Society members only to
respond to that poll. However my response is appended below, and discussion
in this forum would be welcomed. 

Reponse from Zellner:
--------------------
This is ridiculous.  It's fine to use SI units where they make sense, and
it's silly to measure intergalactic distances in cm as some do.  But if I
see the semimajor axis of an asteroid tabulated in AU, I know immediately
where it fits in the solar system.  It would be pointless to express that in
meters.  Are we going to tabulate the orbital periods of planets in seconds?
Are we going to date geological epochs and events like the Cretaceous-
Tertiary impact in exaseconds before the present?  Ridiculous! 

Likewise parsecs for the solar neighborhood, kpc for galactic distances, and
megaparsecs for intergalactic distances.  Actually for large distances it
would make better sense to use light-years, since that immediately gives the
look-back time.  And it's silly to point out that the length of the year
varies slowly with time.  Astronomers will never use years if precision at
that level is needed. 

Similarly if I see the pressure in a planetary atmosphere measured in bars,
I have a good idea of how  that compares to the pressure in this room.  SI
units would be meaningless to me until converted into bars.  Likewise solar
masses (not kilograms!) for the masses of stars, clusters, and galaxies.
Likewise solar units for their light output. 

To summarize, use the units that make sense in a particular context.  We are
not likely to be using acres for surfaces, feet for length, or troy ounces
for masses in any astronomical context.  Let's not cause big problems where
few or none presently exist. 

Ben Zellner - speaking only for myself.
Computer Sciences Corporation
Space Telescope Science Institute

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From: [email-masked] (Stefano Casertano)
Date: 7 Sep 1993 15:01:03 GMT

>in this forum would be welcomed. 

 [response from Zellner omitted]

While I generally agree with the sentiment expressed by Ben Zellner, one
should be careful not to exaggerate the importance of habit.  _If_ there
were good reasons to use SI, I am sure all scientists involved would be
able to switch in a relatively short time. 

However, there is a good reason _not_ to use SI in astronomy, namely
that SI units would cause a very significant loss of precision in some
fields of astronomy.  For example, masses can be measured in units of
Earth or Sun masses much more accurately than in SI units, because of
the poorly known value of G.  While the accuracy is in practice good
enough for galactic and extragalactic work, there is an intrinsic
ugliness in having to include poorly-known conversion factors in your
results.  And of course, for solar system work the precision falls short
by several orders of magnitude.  The 'easy-out' solution, DEFINING the
value of the constant of gravity (as has been done for the speed of
light), will not work in this case, as the value of G is constrained by
the values of Kilograms, Newtons and Meters, all of which can be
measured in the lab with better accuracy than G. 

Given this problem, I do not see compelling enough reasons to _force_
astronomers to use SI units.  They can if they want to...

By the way, I hope Mr.  Zellner's mention of American units (acres,
feet, troy ounces) as unsuitable for astronomy does not mean he
advocates their use in other scientific contexts!  There IS, IMHO, a
good reason for eliminating such units altogether from science AND
engineering - but this is another thread.

Stefano Casertano
[email-masked]

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From: tordm@vana (Tord G.M. Malmgren)
Date: Tue, 7 Sep 1993 16:28:28 GMT

>in this forum would be welcomed. 

[Reponse from Zellner partly deleted]
>units would be meaningless to me until converted into bars.

 I agree with everything you write, but not with this. A pascal (Pa) is
just a factor of 10^{something} so that wouldn't cause any problems.
And you mention "cm" in one context, which is, in my opinion, a stupid
way of speaking against SI units. 

---------------+--------------------------------+----------------------------
 Tord Malmgren | InterNet: [email-masked] | These opinions are my OWN,
               | BITNet  : TordM@SESUF51        | and NOT of this department!
---------------+--------------------------------+----------------------------
 Department of Physics, University of Stockholm -- Sweden (Scandinavia)

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From: [email-masked] (Ben Weiner)
Date: 7 Sep 93 18:18:07 GMT

To paraphrase those well-known PhDs Wayne and Garth, SI units in
astronomy - NOT!  The mental overhead would be just too high.  Some
might say that this is a bad argument which has been used in the U.S.
to delay adopting the metric system.  I disagree - there are some
traditional astronomical units which have a certain intrinsic
rationale - solar masses, for example.  Even aside from the
precision/value of G issue, if, for example, you tell me the mass of a
globular cluster in solar masses, that's one number, which relates
closely to the number of stars.  If you tell me the mass in
peta-peta-kilograms, I also have to keep remembering that low-mass
stars are on order 1 to 2 peta-peta-kilograms.  The prefixes alone
argue against trying to use SI ...  anyone care to recalculate all
mass-to-light ratios in SI rather than solar units?

SI is based on fundamental units which are derived, more or less, from
earth-bound, practical experience, which is a good thing.  They just
don't adapt very well to "ordinary" astronomical quantities.  This
creates annoyances: for example, doing galactic dynamics, I often find
myself fumbling for the conversion between km/sec and pc/yr (about
10^6).  Nor is there any reason, really, why we should use pc.  But we
need some unit of about that length.  I don't see why astronomers must
martyr themselves to conversion; high energy physicists, for example,
are not about to start reporting particle masses in Joules/c^2, much
less in kg.

Another problem with converting to SI is that it wouldn't fix the one
system of astronomical units which is really arcane, confusing, based
on thoroughly antiquated roots, and difficult to get comfortable with
- because that system is dimensionless (except for a reference
distance, and what do we do about _that_?)  I'm referring, of course,
to the magnitude system.