On 02/03/2011 07:00, Clive D.W. Feather wrote:
Warner Losh said:
SI
- the SI-second is a standardised unit of measurement
- the SI-second is currently defined as a fixed number of transitions
of a caesium atom
- the current definition of the SI-second was ratified in 1967
Agreed. I'd also add
- the SI-second forms the basis for many other fundamental units of measure.
- an SI-based-minute is formed from exactly 60 SI-seconds
- an SI-based-hour is formed from exactly 60 SI-based-minutes and thus
exactly 3600 SI-seconds
- an SI-based-day is formed from exactly 24 SI-based-hours and thus
exactly 86400 SI-seconds
These are true only most of the time. On leap second day, they are all
false and off by 1, typically one low. It all depends on the timescale
that you use, and thus there can't be a universal definition here.
Actually, Stephen is right and you're wrong. The BIPM defines an SI minute,
hour, and day in exactly this form. See table 6, page 32, of:
http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
This, of course, implies that "SI-based-" is the same as "TAI-"
In my defense I'll note that they used to be non-SI units that were
recently added to SI due to their widespread use....
I wonder if the BIPM is aware of this inconsistency or not...
Warner
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