Mechanics, Planetary Motion and the Modern Science Revolution 55
planets, the Moon and the Sun also had magnetic properties and that
these bodies mutually attracted each other like magnets.
Kepler was greatly influenced by Gilbert and incorporated these
ideas into his cosmological system. He called the force emanating
from the Sun effluvium magnetieum. Kepler’s system did not include
inertia. It explained the motion of the planets in terms of a positive force
from the Sun, which presumably accounts for the fact that the closer
a planet is to the Sun the faster it moves.
In the world systems of Copernicus and Kepler the space between the
heavenly bodies is empty. In the Cartesian world system like the
Aristotelian one, the space between the Sun and the planets is filled with
a solid aether. Loberval, a follower of Descartes and the first to propose
a universal gravitation attraction claimed in 1643 that the planets did not
fall into the Sun because of the solid aether between them.
Borelli in 1665 came closer to the Newtonian picture by claiming
that the planets did not fall into the Sun because the centrifugal tendency
due to their circular motion was counterbalanced by the positive
attraction of the Sun. With the suggestion of Borelli all the ingredients
for the Newtonian breakthrough had been assembled. They included the
Copernican heliocentric theory of the solar system, Kepler’s three laws
of planetary motion, Descarte’s formulation of inertia, Borelli’s concept
of the centrifugal force as well as the concept of a universal gravitation
attraction of all heavenly bodies. Descartes had also formulated the
concept of momentum conservation, which he arrived at by arguing that
God would not waste motion.
Newton’s Revolutionary Mechanics
Newton put the pieces of the puzzle together. Newton made two
enormously important contributions. First he tied together all of the
concepts that his predecessors had struggled to achieve and placed
them within a consistent and coherent framework. Secondly he expressed
the entire scheme in a precise, elegant mathematical language, which
enabled one to calculate exactly the behaviour of mechanical systems.
By assuming that the mutual gravitational attraction of two bodies is
proportion to their masses and inversely proportional to the square of the
distance between them, he was able to derive Kepler’s three laws of
planetary motion. He had realized the Pythagorean dream of expressing
in terms of numbers the motion of the bodies of the universe. In so doing