I'm surprised nobody has yet commented on what AU stands for -- astronomical unit.
Given that it is defined as the mean distance between the Earth and the Sun, it certainly offers a short way of expressing a distance in terms of a multiple of the Earth-Sun distance. An objection to the use such a comparative unit doesn't actually advance the SI argument. One can express astronomical distances in gigameters, terameters, petameters, etc. (for which nobody actually has a real feeling), and add clarification in terms of the AU (for which the layperson does have some general feeling -- as long as s/he knows the definition). On the other hand, where we're measuring things on a human scale, from the size of a molecule to the distance between cities, we shouldn't need that kind of comparison quite as much (although saying something [e.g., a cruise ship] is the length of so many football fields does have a certain appeal). Regarding the distance to Mars, the use of gigameters needn't involve the use of 6 decimal places. To anyone but an astronomer, "56 Gm" is close enough. Bill Potts, CMS Roseville, CA http://metric1.org [SI Navigator] >-----Original Message----- >From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] >Behalf Of David Shatto >Sent: Wednesday, September 10, 2003 12:48 >To: U.S. Metric Association >Subject: [USMA:26935] RE: NASA NEO report > > >My comments below. > >On Wed, 10 Sep 2003 19:48:33 +0100 "Terry Simpson" wrote: >> I think 'million km' beats AU. I am familiar with metres and see them all >> around me but have no idea about AU. > >Normally I'd agree, but in a report about Near Earth Objects I think it's >appropriate to use a Earth-centric measurement unit that gives "ballpark" >distance approximations. More precise measurements would be done in km >of course. > >This is a technical report made by working scientists for working >scientists, so they don't concern themselves with the fact that the >public doesn't understand what an "AU" is. I'm sure you realize that if >it WAS written for the public, they'd be using feet and miles no doubt! >Sad but true! > >> Incidentally, 'million km' is not the only SI compliant way of >expressing a >> distance of that magnitude. However, people appear reluctant to apply the >> prefixes beyond kilo to metres and grams. Does anybody have any idea why >> that is? > >Yes, that's irked me terribly too. In astronomy they always use millions >or billions of km (mostly without any regard to the difference between >American billions and English billions, but that's another problem, isn't >it?) I have never seen Mm or Gm distances, and I really think they >should be used. [Then again, I'm only casually involved in space >science, so maybe there are some groups out there I don't know about that >use these prefixes regularly.] > >I think that large km distances are used to avoid decimalized >measurements (eg saying "Mars was recently 55,758,006 km from Earth" is >better than saying "... 55.758006 Gm from Earth"). This follows the same >reasoning for using only mm and m as standard units for measurement in >the trades. > >David Shatto >Los Angeles >
