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To understand how theories about the movement of the planets developed,
visit the PhD thesis of Jeff Suzuki
(http://math.bu.edu/people/jeffs/thesis.html).
Suzuki writes about the work that Isaac Newton and
Pierre-Simon Laplace carried out in the 17th and 18th centuries to prove the
Solar System was stable. Before Newton, astrophysicists thought that eventually
the planets would spiral into the Sun.