On Tue, 18 Dec 2007 21:54:03 +0000, geoff wrote:

In message , Si
$3o&m writes
In message
, CS
writes
The only energy source that is totally reliable and predictable is
either tidal - as long as the moon stays in orbit!

There was a TV programme on BBC 4 on Monday evening in which it was
mentioned that the moon is slipping out of orbit at around 5cm/year.

There's one on now (power of the planet) where the person who actually
does the measurement every day says 4cm / year

(splitting hairs mode)


Quite so. The tidal energy is in fact coming from the minute rotational
slowing of the earth. In fact quite a bit of the energy is being
transferred to the moon kinetic energy as its orbit slightly increases
year on year.

So in true hair splitting mode tidal power is not sustainable. However
if you said it was good until the moon doubled it's orbit then that would
be 400,000,000 m / 4cm = 10,000,000,000 years which is longer than the
Sun should last!

The energy of the Earth's rotation is truly an immense source of power.
The speed is slow (and that's squared) but the moment of inertia is over
whelming.

If I remember my applied maths correctly then the energy of a rotating
sphere is 2mr²w²/5 = 0.4 x 6x10^(24) x (2 x PI / 86,400)² x (6,378,000)²
= 5 x 10 ^ 29 J = an infinitesimal fraction of which is more than all
the energy from the sun in an entire year.






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