John Cowan wrote:
> I accidentally specified sidereal rather than mean solar days by
> using the wording "the Earth rotates".
"Rotate" is as perfectly good a word to use relative to our nearby star
as to the distant ones :-) The solar system is chock full of nifty
synodic
periodicities and resonances.
> No, that won't last forever, but neither will any
> other scheme -- when we get to the 36-current-hour day, the connection
> with solar time will *have* to be broken, unless we have evolved (or
> evolved ourselves) to cope with very different sleep-wake cycles.
We have evolved, we are evolving, we will evolve. Unlike other species,
however, we have the opportunity to control the context of that
evolution.
This doesn't have to rise to the level of Morlock versus Eloi to be
significant.
One can well imagine that a diurnal wake-sleep cycle will be selected
for
at a significant level. The corresponding adaptations may be varied and
wonderful between species, however.
> BTW, are we now in a position to give a reasonable figure for the
> mean and standard deviation of the Earth's deceleration, or do
> we not have enough data yet?
The day was 23 "SI hours" long during the Jurassic and about 22 "SI
hours"
during the Devonian period, the "age of the fishes" about 350 Mya.
Taking
outrageous license, one can estimate that the day will be 36 "SI
hours" long
about 2100 Mys hence (no compensation in this estimate for the lessening
of the effect as the Moon recedes).
But, as Steve Allen has pointed out, the length of the day has always
been
exactly 24 hours by definition. The notion of "SI hours" is spurious.
Since we've been arguing for seven years about a daily effect at the
level
of 2 ms, not 12 hours, one can assume that a consensus will need to be
formed some time sooner than the time it will take new evolutionary
clades
as significant as the fishes or reptiles (including birds) to appear
during
some "Futurassic" period to come.
Would suggest that a consensus on the character of the problem be
established, before a consensus on its solution is sought. In
particular,
if we're interested in evolutionarily significant spans of time, one
suggests
that we should not kowtow to the imaginary needs of makers of early
third
millennium technical geegaws.
As far as the measuring the slope, try starting with the first figure
from
http://www.ucolick.org/~sla/leapsecs/dutc.html. Three slopes are
overplotted
on the data from the past 2500 years. The most shallow, 1.4 ms/cy,
corresponds to the backwards extrapolation from recent behavior. The
most steep, 2.3 ms/cy, is derived from direct measurements of Lunar
recession - the angular momentum has to balance. A fit to the
overall trend
is intermediate at 1.7 ms/cy. The short term and long term shape to the
trend line (with the interesting hint of a ~1200 year period) is
presumably
affected by numerous geophysical issues such at the continuing rebound
of the continents since the glaciers receded. It can also give you
some idea
of an intrinsic "width" of any estimate. Hard to know how to comment on
"standard deviation" when there are clearly effects yet to be
accounted for.
Would think that the lunar value provides the best handle on the long
term
behavior since there are orders of magnitude of natural smoothing in the
orbital angular momentum versus the rotational. I doubt the point need
be emphasized that I'm no expert :-) Would be delighted if this
list's silent
experts were to correct my gaffs and omissions.
Rob
NOAO
Received on Fri Aug 04 2006 - 13:01:21 PDT