On 2011-02-02 16:47, Stephen Colebourne wrote in part:
Solar
- a solar-day is a measured period of time
- the length of a solar day in in SI-seconds varies over time
The length in "SI seconds" of a time interval during which
UT1 increases by 1 d ("solar-day") is not a well-defined notion
unless you state how you measure that interval. "SI second" is
just the unit -- but a length in SI seconds depends on how you
measure it (and this is not prescribed by the unit).
You probably mean a comparison with TAI (a kind of average proper
time on the geoid) or with TT. Thus, a concise way to define this
"length of a solar-day" uses the rate of the two time scales that
are involved: d(TAI)/d(UT1) d (invoking the mean value theorem).
Of course, your statement is totally correct as stated. I just
think it might be misunderstood by non-astronomers like me.
Michael Deckers.
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