On 1/9/2012 2:40 PM, Ian Batten wrote:
So long as those all tick the same thing, its relationship to the rotation of
the earth is, +/- several hours, irrelevant. No-one cares what the
relationship between their watch/clock/computer and the sun is at anything
other than the grossest scale
The foregoing statement is just not true. The time of sunset and sunrise
is important
in many applications other than astronomy. It is important in all
outdoor activities
where artificial light sources will not be carried. It's important for
pointing some
solar arrays, and modeling the output from the same.
Of course, changing the disseminated global time scale from UTC as presently
defined will not cause an immediate problem for these applications. A
common
rule of thumb is that a measurement standard should have a tolerance no
more than 1/3 of the device under test. If the device under test is a
wrist watch,
and it is required to have a tolerance of 1 minute, then the measurement
standard
should have a tolerance no worse than 20 seconds. This would suggest
sunset/sunrise
tables that are to be enforced with a wrist watch should have calculations
that are accurate to no worse than 20 seconds. Treating the difference
between
UTC or its successor and mean solar time as negligible becomes inadequate
for use with wrist watches in about 30 years.
Gerry Ashton
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