Thanks for clear explanation Jim But the difference between astronomical and atomic time is so minute that it affects neither industrial nor government nor household activities.
I will like to see decimal time slowly creeping in our day today lives. Madan --- "James R. Frysinger" <[EMAIL PROTECTED]> wrote: > Date: Tue, 05 Mar 2002 14:50:44 -0500 > From: "James R. Frysinger" <[EMAIL PROTECTED]> > Organization: Metric Methods > To: "U.S. Metric Association" <[EMAIL PROTECTED]> > CC: "U.S. Metric Association" <[EMAIL PROTECTED]> > Subject: [USMA:18547] Re: Fwd: Re: Fwd: "How many > miles..." thread > Reply-to: [EMAIL PROTECTED] > > 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 > __________________________________________________ Do You Yahoo!? Try FREE Yahoo! Mail - the world's greatest free email! http://mail.yahoo.com/
