From Bill.Gawne@launchpad.unc.edu Sun Sep 5 23:01:40 1993 From: Bill.Gawne@launchpad.unc.edu (William C. Gawne) Newsgroups: sci.astro Subject: Astronomy and the SI (was Re: NASA and metric) Date: 5 Sep 1993 23:07:28 GMT Organization: University of North Carolina Extended Bulletin Board Service NNTP-Posting-Host: lambada.oit.unc.edu Summary: Why tie ourselves to a restrictive system? Keywords: SI, units_of_measure Along the lines of this discussion, there's the letter in the latest AAS Newsletter from Hale Bradt asking for a "unified and rapid" move away from the use of such units as parsecs, light-years, Janskys, etc... and toward use of the SI units in all AAS journal articles. While I appreciate the SI for a great number of reasons, I hate the thought of sacrificing all the truly useful units of astronomical measurement on the altar of small-minded conformity. Surely the people who read ApJ and AJ are sufficiently talented that they can make conversions from any system of measurements to any other with ease given the conversion factors. -Bill -- The opinions expressed are not necessarily those of the University of North Carolina at Chapel Hill, the Campus Office for Information Technology, or the Experimental Bulletin Board Service. internet: laUNChpad.unc.edu or 152.2.22.80 From jaa101@gorton.anu.edu.au Mon Sep 6 09:18:49 1993 From: jaa101@gorton.anu.edu.au (James Ashton) Newsgroups: sci.astro Subject: Re: NASA and metric Date: 6 Sep 1993 02:53:28 GMT Organization: The Australian National University, Canberra Distribution: world NNTP-Posting-Host: 150.203.15.124 In article <1993Sep1.172041.1231@qualcomm.com>, karn@unix.ka9q.ampr.org.qualcomm.com (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 VK1XJA Department of Systems Engineering Voice +61 6 249 0681 Research School of Physical Sciences and Engineering FAX +61 6 249 2698 Australian National University Email James.Ashton@anu.edu.au Canberra ACT 0200 Australia From zellner@stsci.edu Tue Sep 7 21:20:28 1993 Newsgroups: sci.astro From: zellner@stsci.edu (BEN) Subject: SI units in astronomy Organization: Space Telescope Science Institute 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 From stefano@hal.pha.jhu.edu Tue Sep 7 21:21:43 1993 From: stefano@hal.pha.jhu.edu (Stefano Casertano) Newsgroups: sci.astro Subject: Re: SI units in astronomy Date: 7 Sep 1993 15:01:03 GMT Organization: HCF - Johns Hopkins University, Balitmore, Maryland, USA Distribution: world NNTP-Posting-Host: hal.pha.jhu.edu In <1993Sep6.203756.1@stsci.edu> zellner@stsci.edu (BEN) writes: >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. [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 stefano@mds.pha.jhu.edu From tordm@vana Tue Sep 7 21:22:22 1993 Newsgroups: sci.astro From: tordm@vana (Tord G.M. Malmgren) Subject: Re: SI units in astronomy Nntp-Posting-Host: vana.physto.se Reply-To: TordM@VanA.PhySto.SE Organization: Department of Physics, University of Stockholm -- Sweden Date: Tue, 7 Sep 1993 16:28:28 GMT In article <1993Sep6.203756.1@stsci.edu>, zellner@stsci.edu (BEN) writes: >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 partly deleted] >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. 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: TordM@VanA.PhySto.SE | These opinions are my OWN, | BITNet : TordM@SESUF51 | and NOT of this department! ---------------+--------------------------------+---------------------------- Department of Physics, University of Stockholm -- Sweden (Scandinavia) From bweiner@electron.rutgers.edu Tue Sep 7 21:23:30 1993 From: bweiner@electron.rutgers.edu (Ben Weiner) Newsgroups: sci.astro Subject: Re: SI units in astronomy Date: 7 Sep 93 18:18:07 GMT Organization: Rutgers Univ., New Brunswick, N.J. 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. From jfgonzal@ens-lyon.fr Thu Sep 9 13:22:24 1993 From: jfgonzal@ens-lyon.fr (Jean-Francois Gonzalez) Newsgroups: sci.astro Subject: Re: NASA and metric Date: 6 Sep 1993 07:25:17 GMT Organization: Ecole Normale Superieure de Lyon, France Distribution: world Reply-To: jfgonzal@ens-lyon.fr NNTP-Posting-Host: carl9000.ens-lyon.fr In article 9nm@manuel.anu.edu.au, jaa101@gorton.anu.edu.au (James Ashton) writes: > In article <1993Sep1.172041.1231@qualcomm.com>, karn@unix.ka9q.ampr.org.qualcomm.com (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 > VK1XJA Department of Systems Engineering > Voice +61 6 249 0681 Research School of Physical Sciences and Engineering > FAX +61 6 249 2698 Australian National University > Email James.Ashton@anu.edu.au Canberra ACT 0200 Australia The same thing happened in France. The numbers are the same, but hectopascal is the correct SI unit. Jean-Francois. From edgar@uwast.astro.wisc.edu Thu Sep 9 13:23:04 1993 From: edgar@uwast.astro.wisc.edu (Dick Edgar) Newsgroups: sci.astro Subject: Re: NASA and metric Date: 9 Sep 1993 13:06:30 GMT Organization: University of Wisconsin - Astronomy Department Distribution: world NNTP-Posting-Host: uwast.astro.wisc.edu 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 (edgar@uwast.astro.wisc.edu) 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 From zrzv0111@helpdesk.rus.uni-stuttgart.de Thu Sep 9 22:37:25 1993 From: zrzv0111@helpdesk.rus.uni-stuttgart.de (Bernd Lehle) Newsgroups: sci.astro Subject: Re: NASA and metric Date: 9 Sep 1993 15:55:26 GMT Organization: Pfaffenwald Observatory, U of Stuttgart, FRG Distribution: world NNTP-Posting-Host: helpdesk.rus.uni-stuttgart.de Dick Edgar (edgar@uwast.astro.wisc.edu) 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... -- > Bernd Lehle alias Wolfskin - Born to be wild * A supercompur < > Stuttgart University Computing Center Helpdesk * is a machine that < > e-mail: lehle@rus.uni-stuttgart.de * runs an endless < > Tel:+49-711-685-4828, private: +49-711-6874428 * loop in 2 seconds. <