MadSci Network: Astronomy

Re: Why do astronomical bodies spin on their axes

Area: Astronomy
Posted By: John Haberman, Space Scientist, NASA Goddard Space Center, Greenbelt MD
Date: Thu May 1 15:45:54 1997
Area of science: Astronomy
ID: 862242264.As
What a great question and one that has always proven difficult to answer,
at least in a way that doesn't involve a complex mathematical model.

One of the foundations of the science of mechanics is the conservation of 
momentum, one of the three natural laws clearly stated for the first time 
by Sir Issac Newton and published in 1686.  The three dimensional 
equivalent of this rule is the conservation of orbital angular momentum.  
This is the explanation for and the reason why all astronomical bodies 

Everyone agrees that astronomical objects are created by gases and dust in 
space collecting to form larger bodies: a process called accretion.  Each 
of the objects is traveling in some direction at some speed and may be 
spinning.  As these individual particles come together and grow in size, 
the conservation of momentum rule must be obeyed.  One way for the new, 
larger, object to obey this rule is to have some of the momentum change 
into rotation or spin.  

In addition, all objects are subjected to the forces of gravity and to the
electromagnetic forces that are present throughout space.  The constraints
added by these forces also can be explained by having the object spin.

Other attempts to respond to this question can be found at:

Why do Planets Spin?
Why does Jupiter spin so fast?
Why does the Earthspin?

Interesting Observations:

There does not seem to be a reasonable explanation for the rates of 
rotation (spin) of objects in space.  Some objects spin very fast while 
others rotate quite slowly.  The spin of only a few objects, Earth for 
example, seem to have been affected by gravitational and tidal forces.

At least one theoretical model for rotation predicts that an accreted 
object in a circular orbit can only have a retrograde spin.  If the orbit 
is elliptical, the object can have either a retrograde or a prograde spin.

John Haberman, Space Scientist
NASA/Goddard Space Flight Center
Greenbelt, MD USA

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