The birth of the *Principia* may be traced back to a discussion in 1684 at the
Royal Society. Astronomer Edmund Halley and architect Sir Christopher Wren suspected that
there was an inverse square relation governing celestial motions based on Kepler's Third
Law of elliptical orbits, but no one could prove it. They brought the question before
Newton's arch rival Robert Hooke, who claimed that he could prove the inverse square law
and all three of Kepler's laws. His claim was met with scepticism, and Wren offered a
forty-shilling book as a prize for the correct proof within a two-month limit. Hooke
failed to produce the calculation, and Halley travelled to Cambridge to ask for Newton's
opinion. Newton responded with a typical lack of interest in work that he had already
completed, that he had already solved the problem years before. He could not find the
calculation among his papers and promised to send Halley a proof. Halley, suspecting the
same bogus claim he had received from Hooke, left frustrated and returned to London. Three
months later he received a nine page treatise from Newton, written in Latin, *De Motu
Corporum*, or *On the Motions of Bodies in Orbit*. In it, Newton offers the
correct proof of Kepler's laws in terms of an inverse square law of gravitation and his
three laws of motion. Halley suggested publication, but Newton, reluctant to appear in
print, refused. At Halley's insistence, Newton finally began writing and, with typical
thoroughness, worked for 18 months revising and rewriting the short paper until it grew
into three volumes. The Royal Society, having exhausted available funds on an extravagant
edition of *De Historia Piscium*, or *The History of Fishes*, could not pay
for the publication and so it was at Edmund Halley's expense that *Philosophić
Naturalis Principia Mathematica* was finally published.

*The Mathematical Principles of Natural Philosophy*, or *The Principia*
as it came to be commonly known, begins with the solid foundation on which the three books
rest. Newton begins by defining the concepts of mass, motion (momentum), and three types of forces: inertial, impressed
and centripetal. He also gives his definitions of
absolute time, space,
and motion, offering evidence for the existence of
absolute space and motion in his famous "bucket
experiment". These absolute concepts provoked great criticism from philosophers
Leibnitz, Berkeley, and others, including Ernst Mach centuries later. The three Laws of Motion are proposed, with consequences derived from them.
The remainder of *The Principia* continues in rigorously logical Euclidean fashion
in the form of propositions, lemmas, corollaries and scholia. Book
One, *Of The Motion of Bodies*, applies the laws of motion to the
behaviour of bodies in various orbits. Book Two continues with the motion of resisted
bodies in fluids, and with the behaviour of fluids themselves. In the Third Book, *The System of the World*, Newton applies the
Law of Universal Gravitation to the motion of planets, moons and comets within the Solar
System. He explains a diversity of phenomena from this unifying concept, including the
behaviour of Earth's tides, the precession of the equinoxes, and the irregularities in the
moon's orbit.

*The Principia* brought Newton fame, publicity, and financial security. It
established him, at the age of 45, as one of the greatest scientists in history.

Isaac Newton's *Principia* 1687, Translated by Andrew
Motte 1729

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Last edited 26/07/98

By gravity@thevortex.com