[LEAPSECS] Far past and far future

Clive D.W. Feather clive at davros.org
Thu May 26 17:29:46 EDT 2011

Rob Seaman said:

> > Would the earth be slowing down so fast without the moon? There's some tidal coupling in the earth-sun system but isn't it much smaller?


> Wonderful questions. I'll bet there are lurkers here who could speak authoritatively :-)

There is no way that I can speak authoritatively about this. But I took
some numbers from the Williams paper that was cited and played with them in
a spreadsheet. These suggest that about 90% of the terrestrial rotational
angular momentum reappears as lunar orbital angular momentum. On the other
hand, I can't see where the rest of it would go - apparently observations
show no indication that the Earth's orbit is growing.

> The tides themselves are only a means to an end mediating the transfer of terrestrial rotational angular momentum to lunar orbital angular momentum. The efficiency of this is presumably an "interesting" (and perpetually varying) combination of many factors,

It's suggested in my reading that the present period has an abnormally high
transfer rate because of the present arrangement of the continents and,
therefore, the oceanic tides. Certainly the present rate cannot have
happened for long periods - projecting back they make the moon collide
with the earth about 1 to 2 Gyr ago.

> Tides are an inverse-cube effect suggesting that the coupling was stronger in past aeons since the Moon was closer. The deceleration ought be decelerating in a smoothed long-term trend.

That's not what the numbers appear to show.

Clive D.W. Feather | If you lie to the compiler,
Email: clive at davros.org | it will get its revenge.
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