The International Astronomical Union (IAU) has a policy that calls
almost exclusively for the use of SI units. It does permit the light
year, though, if my memory is correct. The American Astronomical Society
(AAS) uses CGS metric units. The AAS represents the vast majority of the
money and publication pages in the field of astronomy and so they are
impervious to the wishes of the IAU.
I once asked my department head (an astronomer) at the College of
Charleston why American astronomers use Gausian CGS and he claimed that
the SI doesn't provide the units necessary to correctly give magnetic
quantity values and that the SI units for magnetics (tesla, weber,
ampere meter, etc.) are unusable. He's wrong, of course, but that
reflects the bias of AAS members.
This all goes back to the various ways that physicists have modeled
magnetic fields, a subject not meant for the faint-hearted and certainly
beyond easy description in an email. Ironically, those American
astronomers have no trouble teaching electromagnetism in lower division
physics courses in SI terms!
So, chalk up the use of CGS metric (instead of SI metric) and the use of
parsecs, light years, etc. in American astronomy to bias and tradition.
Jim
On 2011-04-08 2310, Paul Rittman wrote:
John,
Thanks for the clarification. Yes I see that using the meter (or its
powers to the ten) could preserve scale that the terms /astronomical
units/ or /parsecs/ don't.
It would be nice to see metric units used more often with inter-stellar
systems. I wonder why they aren't used more often. I'm sure there are at
least a few astrophysicists and astronomers who are metric-conversant.
Anyways I'll keep looking at this--I realize there is a massive scale
here, going from our own solar system all the way up to the 92 billion
light-year-wide--err... that is, 900 billion petameter-wide universe, so
it it very well might be the case that metrically-minded astronomers
might want to use two metric prefixes (one for shorter and the other for
longer distances).
Are gigameters and petameters the two most commonly used ultra-metric
measurements?
[USMA:50264] re: astronomical measurements
John M. Steele
Sun, 03 Apr 2011 07:41:49 -0700
No. I find comparing in meters (with a suitable prefix) gives me a better
sense
of scale than mixing AU, light years, parsecs, and other obscure units with
obscure inter-relationships. Determining that a light year is some 63000 AU
is
not particularly helpful.
As a general comparison, it is much easier to learn a few metric prefixes,
usable with all units, to provide a short hand alternative to scientific
notation for scale than to learn all the absurd interrelationships of Customary
units.
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
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