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