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Omar Reis asked, about calculating delta-T (the difference between Atomic
time and Greenwich time) for future years-

>I wonder why can't this value be deterministic,
>like everything else in astronomy ?
>
>I mean, the earth rotation is slowing down, as much
>as a second per year. But why is this amount variable
>and umpredictable ?

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Not everything in astronomy is deterministic. Even in a system, such as the
solar system, in which more than two bodies have a mutual gravitational
interaction, there's a limit to the long-term predictability of it all.
This is the famous "three-body problem".

In the case of the Earth, a small part (and a rather constant part) of the
Earth's spin-momentum is transferred to the Moon each year, dissipating a
certain amount of its spin-energy to the tides in doing so.

But a greater transfer of momentum is between the Earth's crust and
currents in the liquid parts of its interior, which show a slow turbulence.
Because the overall spin-momentum has to be conserved, changes due to these
currents must be matched by equal-and-opposite changes in the crust. This
effect can work either way, to slow down the speed of the crust or even to
speed it up. Even if we understood the basis of these turbulent motions
within the Earth, I doubt whether their future details would be
predictable.

Even changes in the East-West circulation of the atmosphere have to be
balanced against corresponding changes in the spin of the Earth, but I
think this effect is a small one.

Delta-t bumbled along without great dramatic changes in the two centuries
prior to 1900, since when it has been increasing rather steadily at about 1
second per year. However, over the last few years that regular increase
seems to have switched off, rather suddenly and dramatically. There are
things going on there, far beneath our feet, of which we know little.

George.

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