on 4/26/2002 9:21 PM, Duncan Bath at [EMAIL PROTECTED] wrote: > I grant that Pm, Tm, nm etc. can > be handy for specialists where such numbers are dealt with frequently.
Of course! No one would suggest that anyone should become familiar with these units except those for whom they are useful. That can be said for ANY SI unit. (It can be said for any non-SI unit as well). Sometimes the beautiful order and logic of SI makes us want to teach it all to the uninitiated so that they can see how complete and orderly it really is. It is perhaps worthwhile that people should generally be aware that a variety of other prefixes are available, but they need only be taught to use those that they will find useful. Now there are some questions as to which units and which prefixes are useful. I think astronomers should be making greater use of terrametres (Tm), petametres (Pm), exametres (Em), zettametres (Zm) and yottametres (Ym) for appropriate distances, but many of them resist doing so because they are so attached to their light-years (l.y.), parsecs (ps*),kiloparsec (kps) and megaparsecs (Mps). That is unfortunate. For reference, here are the equivalents of all the above units, in order from smaller to larger. You'll notice that the light year and parsec (even with the commonly used kiloparsec and megaparse included) still doesn't provide the wide range of unit sizes as SI does. 1 Tm = 1 x 10^12 m 1 l.y. = 9.5 x 10^12 m 1 ps = 3.1 x 10^13 m 1 Pm = 1 x 10^15 m 1 kps = 3.1 x 10^16 1 Em = 1 x 10^18 m 1 Mps = 3.1 x 10^19 1 Zm = 1 x 10^21 m 1 Ym = 1 x 10^24 m Regards, Bill Hooper physics professor (retired), Florida, USA *I'm not positive of the accepted symbol or abbreviation for the non-SI parsec. I think it is either ps or pc, or it may be something else. ======================== Keep It Simple - Make It Metric! ========================
