Issue 1: I think you missed a subtle point in my message, or perhaps I didn't express it well, Madan. The minute (min), hour (h), and day (d) are not SI units... but they are accepted for use with the SI, according to the SI brochure (as well as NIST SP 330, and industrial standards such as IEEE/ASTM SI 10). To me that means that nobody deserves to get jumped on for using them. If someone jumps on you for that, regard them as a fanatic responding to voices only they can hear.
By the way, notice that in your examples, the meter (m) and the watt (W) have prefixes attached, but not the hour (h). Nonetheless, I prefaced the remarks you quoted with a statement to the effect that the SI brochure is silent on prefixing min, h, d, �, ', and ". What I offered was a behind the scenes preference by those "in the business". There is nothing in the SI brochure that either condones it or condemns it. That gives it even less stature than the industrial standards and style guides (which are also silent on this) that I describe in http://www.cofc.edu/~frysingj/SIreferences.htm The reason that I built this expanded page recently is that I have realized the trouble people often have with recognizing levels and areas of authority pertinent to each standard. In summary, feel free to use the units and symbols in my first paragraph above. Prefixes for those five units (and their symbols) fall into the category of "it's best not done and we know that because none of the standards do it". The space between number and unit in a quantity is in a nearly similar situation. We put a space there because the SI brochure always does it that way, even though there is no explicit requirement in the brochure to do that. By contrast though, other standards do call for that space to be inserted. Issue 2: We don't use the Earth's rotation and revolution periods for time because they are not constant. Days vary one from another by milliseconds. Even a shift in global wind patterns from meridianal to zonal have been shown recently to have a measureable effect on the rotation rate of the Earth! The old, no-longer-used GMT was based on Earth's rotation. Now we use UTC which is based on atomic clocks and which is maintained by our friends at the BIPM. When UTC and GMT (now, called UT1) differ by 0.9 s, a leap second is added or subtracted to UTC by the International Earth Rotation Service (IERS) to keep it close to the current value of GMT. See these two pages: http://aa.usno.navy.mil/faq/docs/UT.html http://tycho.usno.navy.mil/leapsec.html Issue 3: Yep, and the various operating systems each have their own "start" time for this. That just adds spice to the game of trying to provide cross-platform links. Jim M R wrote: > > Jim wrote Issue 1: > "SI community (CGPM, CIPM, BIPM, etc.) did not favor > this practice at all, with a few exceptions". The > practice of using non-SI with SI unit. > > Hour is a non-SI unit, but km/h, kilowatt-hour are > used worldwide. In the same way, if someone writes > km/day or kilowatt-day, will anyone jump on them. Issue 2: > Meter is based on the distance from the pole to the > equator, so why not time be based on our planets > rotation. Just an argument. Issue 3: > FYI: > Internally the computers store the date in a 'single > number' which is '# of days' from a particular base > year and the time as a decimal part of the day. > For ex :- The date 2002-03-05 and time 13:35:00 will > be stored as 37320.57. This way the computers work > faster in calculations. > > Madan .... -- Metric Methods(SM) "Don't be late to metricate!" James R. Frysinger, CAMS http://www.metricmethods.com/ 10 Captiva Row e-mail: [EMAIL PROTECTED] Charleston, SC 29407 phone/FAX: 843.225.6789
