On 2/2/2011 11:47 AM, Stephen Colebourne wrote:
Statements so far - disgree or add please (in particular something on
UT1/UT/etc as I will only get it wrong...):
General:
- the terms seconds, minutes, hours and days are overloaded, thus
pedantic and explicit terms are used here
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
- 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
Solar
- a solar-day is a measured period of time
Some term other than SI-based minute/hour/day is needed. As another has
stated, unlike
TAI or UTC, SI seconds can be observed in other reference frames. Also,
the SI brochure
says nice things about UTC, without explicitly incorporating it into SI.
Thus it is not
cleaar that an SI minute cannot contain 61 SI seconds.
Solar days occurred before anyone was able to, or interested in,
measuring them.
Also, non-human animals and plants experience solar days without
measuring them.
Gerry Ashton
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