On Oct 12, 12:11 am, davee wrote:
On Oct 11, 2:09 am, qzectb wrote: Venus does not have moon to spin it like Earth, there are no competing
differential forces so yes it has a dark side just like our moon
(although it may be illuminated at some time due to rotation)

I was comparing most other planets in our solar system with moons
against those
without those with moons appear to have far shorter rotational periods
than those without.

That is more because there are comparatively few planets without
moons. If you looked at the spin rates of all the various known
asteroids you would find that despite being singletons they mostly
have fairly rapid rotation. It is a direct consequence of conservation
of angluar momentum when lumps of stuff collide and aggregate to form
a larger one. Exact head on collisions are rare - anything else and
there is necessarily some spin imparted.

Uranus has moons but is pretty oddball. The planets spin axis is in
the plane of the solar system.

The Earth does not need the moon to "spin it". Actually, tidal forces
by the moon act as a form of drag on the earth's rotation, as the moon
only orbits the earth once every 28 days. It's because of the Earth's
strong tidal action on the moon that the moon is now "locked" with one
side toward the earth, although it certainly didn't start out that
way.

Would the Earth be spinning now if it did not have one?

Yes. And it would be spinning quite a bit faster since without the
moons tidal friction acting for billions of years it would not be
losing 1.5ms per century from the day length. The fossil record shows
that 500MY ago the day length was about 22h with about 400 of them in
a year. It is estimated to have been around 6h at formation. eg

http://physics.fortlewis.edu/Astrono...ML/AT30706.HTM

The same should have applied to the Earth then if frictional forces of
the atmosphere were the only thing that causes these things, after all
it has been around 4.3 billions years since Earths creation.

The Earth behaves like a spinning top on near perfect friction free
bearings and would continue to spin for all eternity in the abscence
of any other external forces. Tidal drag from the suns gravitation
will also gradually slow its spin rate. And the sun has already
tidally locked the unfortunate Mercury. Wikipedias description is
quite good:

http://en.wikipedia.org/wiki/Tidal_locking

Earth and Moon are a couplet pair that revolve around each other that
follow an average orbital path and not a planet that follows an
orbital path with a moon rotating around it.

Having a nice big moon makes the Earths axis of spin more stable and
stronger more variable tides which makes the place more habitable for
life but that is all.

Over time, the earth's rotation is slowing down due to the moon, and
over time the moon's orbit is gaining angular momentum from the earth,
which is putting it into a higher orbit.

Generally speaking, every planetary body comes into existence with
considerable rotation.

Why?

Conservation of angular momentum. Essentially to end up with exactly
zero net angular momentum requires a very special set of initial
conditions that are almost never satisfied.

The same reason that the ballerina spins faster when she folds her
arms in.

Regards,
Martin Brown