Ask A Scientist© Astronomy Archive

Question:  Why was the matter that formed the earth spinning?  Why does everything
in the universe, from galaxies down to atoms, spin?
Do you have any thoughts on this?
alice e packard

Answer 1:  Wow, what a neat question.  I think it is related to the effect that
we have seen when a skater who has arms extended and is slowly spinning
suddenly pullls the arms into the body surface.  The result is that
spin rate increases.  To even spin faster the skater will be standing
straight with the arms up over the head.  This makes the distributions
of mass as close toe the spin axis as possible and makes the spin
as large as possible.  The main idea is that angular momentum is conseved
, ie, cannot change.  Angular momentum is the product of the spin rate
adn the moment of inertia ( a measure of the distribution of the mass about
the axis of rotation)  The product is constant.  If mass is widely dispersed
as before condensing into a planet, and is very slowly rotating, then
when it condenses into a planet (making a small moment of inertia) the
rate of rotation could be large.  So condensing amplifies whatever
rotation there was initially.  NOw, what are the chances of something
not rotating exactly before it condenses?  Apparently, very small since
everything seems to have some rotation.  I am not sure this gets to your
question of why, but it has been a great question to think about. Thanks.
 samuel p bowen

Answer 2:  Good answer Dr. Bowen!
I considered responding to this question but didn't know how
to address the central issue:
"since everything seems to have some rotation"
Why?
I think it's kinda like asking why does everything have mass,
or charge, or linear momentum...
I'm not sure how to answer except to say that these are all
fundamental properties of all matter.

Answer 3:  One possible answer to your question is this :
Consider objects condensing out of a gas cloud, as they collapse
the tidal force of one lump can induce rotation in a nearby lump.
As angular momentum is conserved, there is no overall rotation in
the system ( Universe ) but matter at all scales will have some
rotation. Total angular momentum must add to zero.

Tidal force is a short range force that essentially arises from
variation of some force ( here gravitation ) over the size of the
object we are interested in. To see how this can lead to transfer
of angular momentum, consider the example of Earth Moon system.
Here, tidal force of Earth has slowed down rotation of the Moon
and now the Moon is doing the same thing to the Earth. Such a
transfer requires nonspherical distribution of mass - even a small
departure from sphericity can lead to a large transfer of
angular momentum over a long enough time.
Jasjeet

Answer 4:  As far as spinning stars in spiral galaxies are concerned,
there is a simpler answer : Differential rotation of
the galaxy makes gas clouds rotate at a slow rate. This
can also explain the definite sense of rotation in the solar system.
Jasjeet