Thanks Bill for your clear reasoning and explanation. That's what I meant to say buried deep between the lines! [If anyone's interested, here's a detailed explanation of AU: http://neo.jpl.nasa.gov/glossary/au.html ]
You're right too in saying we shouldn't need that kind of comparison quite as much where we're measuring things on a human scale. I bet I'm not the only person sick of reading about molecules and nanometer-scale objects being described as so many times smaller than the width of a human hair. (As if all hair were the same width in the first place....) I used the Mars example for an unstated reason. Generally speaking, you're right that rounding off distances between planets to the nearest Gm or Tm shouldn't bother anyone but astronomers or precision-freaks, but in this case it was the measurement of the closest distance between Earth and Mars in 59,619 years, so that level of accuracy was unusually meaningful. I believe it comes within 56 Gm of Earth (rounded up) much more frequently. David Shatto Los Angeles On Wed, 10 Sep 2003 16:29:38 -0700 "Bill Potts" wrote: > 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 > > >
