[LEAPSECS] Far past and far future

Rob Seaman seaman at noao.edu
Thu May 26 18:39:41 EDT 2011

Clive D.W. Feather wrote:

> Tony Finch 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?


> Solar tides are about 40% of lunar ones. But I don't know how that maps

> into tidal acceleration of orbits.

"Map" may not be a rich enough verb here. Consider that a tidally locked binary planet system - egg shaped A locked to oval B and vice versa - should be quite stable (in circular orbits and a naive analysis) without further trading of angular momentum. It isn't just the fact of tides, it's what purchase these provide to leverage spin against orbit.

> Rob Seaman said:

>>> But how long term is that? Does anyone have figures on how far back or

>>> forward I can project based on that figure? What was the length of the day

>>> in the time of the dinosaurs?


>> The abstract to the reference in Steve's reply rather suggests a number of 22 SI-hours at 620 MYA (ymmv).


> The whole paper suggests:

> 18.8 hours at 2450 Ma

> 20.8 hours at 900 Ma

> 21.9 hours at 620 Ma


> Converting the paper's numbers into smooth curves for angular momentum and

> lunar distance and then back-calculating, that projects into the future as:


> 30 hours at 1000 My

> 48 hours at 2100 My


> The latter, allegedly, is when increased solar radiation will boil the

> oceans and effectively stop tidal coupling.

Again - it would be delightful for somebody who knows this stuff to speak up. What are the relative scales of the coupling mediated by the oceans, the atmosphere, the crust, the mantle?

Also, I don't know if these numbers are correct, but it might make more sense to express them in angular velocity (proportional to the angular momentum) rather than length of day. Almost the definition of a moot point, but if the oceans boil into the atmosphere, how much of a change will that make to the Earth's moment of inertia - and in what direction?


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