When the speed of rotation gets to half it's present level there'll be 48
hour days and only about 183 days in a year.

On Mon, 04 Dec 2006 22:31:51 -0500, Uncle Clover wrote:

What sort of most-probable events would cause the slowing of the
Earth's rotation, and what kind of effect would such slowing have at
various rates?

Naturally if the Earth were to -instantly- stop spinning, I think the
crust would shatter and turn into magma in pretty short order. But in
more realistic slowing scenarios (pick a rate of decelleration, _any_
rate of decelleration), what would happen?

One thing I'd heard as a child is that gravity would stop working &
everything would go flying off into space. I'm not sure - I think
things may go flying, but I don't think the gravity would be affected
if I understand gravity well enough to conceptualize it in that
scenario. Gravity isn't caused by the spin, but by the mass. Such
theories seem to be suggesting gravity as a sort of "vacuum-pull"
produced by the moving of atoms in the earth away from another object,
and as with the color transparency, was the kind of theory I'd even
heard suggested by high school physics teachers. Given those two
gaffes, were the physics teachers in my high school incompetent or
ignorant? If so, might explain where I got some of -my- bizarre
notions.

Just curious...
--

After reading the very well informed responses, I notice that one is
missing: during the "El Nino" phenomenon several years ago, the speed
of Earth's rotation slowed by a few milliseconds per day. This was due
to the fact that the sustained wind direction shift caused by El Nino
had to be countered by a slight slowing in the rotational velocity of
the Earth itself, because the Earth and atmosphere together must
conserve angular momentum.

Uncle Clover <UncleClover@SpamMeNot.com> wrote in
news:ejp9n21gfspeodlfvigtl9lbuan7qech35@4ax.com:

What sort of most-probable events would cause the slowing of the Earth's
rotation, and what kind of effect would such slowing have at various
rates?

Tidal effects are the cause. Both the Moon and the Sun are slowing the
rotation of the Earth. Eventually the Earth will become tidally locked to
the Moon just as the Moon is now tidally locked to the Earth. The former
will however take billions of years.

Naturally if the Earth were to -instantly- stop spinning, I think the
crust would shatter and turn into magma in pretty short order. But in
more realistic slowing scenarios (pick a rate of decelleration, _any_
rate of decelleration), what would happen?

The only real physical thing that could do it fast is the close passage of
a Neutron star or black hole but for the tidal forces to be large enough to
rapidly stop the Earth's rotation, the Earth would have to be within the
Neutron star's Roche limit. That would basically mean that the Earth would
be ripped apart. In the less dramatic case of the eventual tidal locking to
the Moon you would end up with the Earth rotating in sync with the Moon's
orbit, however the moon would be be orbiting a bit further out than it does
now, as the Earth's angular momentum would have been transferred to the
Moon's orbit. It means that the day would be much longer than it is now.

One thing I'd heard as a child is that gravity would stop working &
everything would go flying off into space. I'm not sure - I think
things may go flying, but I don't think the gravity would be affected if
I understand gravity well enough to conceptualize it in that scenario.
Gravity isn't caused by the spin, but by the mass.

Correct. But spin does affect gravity in a way opposite to what you are
considering . The Earth's rotation means that it is actually easier to get
satellites into orbit from a launch site near to the Earth's equator, than
from the poles. The Earth's rotation gives the launch vehicle a boost. If
the Earth were not rotating, the space shuttle would actually have a harder
time getting into orbit from the Cape. There is a more subtle effect of
spin with respect to gravity (see link below). In fact even pressure
affects gravity, as pressure is one of the components of the stress-energy
tensor. This is important enough to be a factor with neutron stars. As far
as the subtle spin issue goes, read about Gravity Probe B:

http://einstein.stanford.edu/

Such theories seem
to be suggesting gravity as a sort of "vacuum-pull" produced by the
moving of atoms in the earth away from another object, and as with the
color transparency, was the kind of theory I'd even heard suggested by
high school physics teachers. Given those two gaffes, were the physics
teachers in my high school incompetent or ignorant? If so, might
explain where I got some of -my- bizarre notions.

I think either you, they, or both are confused ;-). If they really said
that there would be no gravity if the Earth wasn't spinning then that is
completely wrong. Isaac Newton worked out all that stuff around three
hundred years ago. Maybe you are misremembering what they said.