|MadSci Network: Astronomy|
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 spin. 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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