Sir Isaac Newton's work was the capstone of the Scientific Revolution, utilizing the advances made before him in mathematics, astronomy, and physics to derive a comprehensive understanding of the physical world. Johannes Kepler enunciated his laws of planetary motion in 1618. Galileo determined the laws of gravity and explored the laws of motion on earth. Newton first conclusively affirmed the laws of motion and linked them with Kepler's laws of planetary motion. Before Newton, no one had demonstrated conclusively that the movements of heavenly bodies were related to terrestrial physics. Galileo had suggested this, but was censored by the Church before he was able to do further work to prove his theories.

The first step in Newton's work was to solidify the laws of motion that Galieo had studied and hinted at without clearly expressing. The first law states that a body at rest tends to stay at rest; a body in motion tends to stay in motion unless compelled to change by an applied force. The second law states that the change in motion is proportional to the applied force and takes place in the straight line by which that force is applied. The final law states that for every action there is an equal and opposite reaction. Armed with these solidified theories of dynamics, Newton proved that the force that acted on planets and moons was the same force that caused a stone to fall to the ground: gravity. He first demonstrated this by calculating that if one extended the same gravitational force that acted on objects on the surface of the Earth to the distance of the moon, it predicted nearly exactly the same orbit that was observed.

The concept of universal gravitation--that every particle of matter attracts every other particle with a force proportional to the product of the two masses and inversely proportional to the square of the distance between them--is Newton's major contribution to science and the centerpiece of his work. The observed structure of the solar system was perfectly explained by assuming that the major organizing force among heavenly bodies was gravity. In order to apply the theory of universal gravitation to heavenly bodies with curved paths through space, Newton built upon the contributions of the mathematicians of the age and developed calculus. Using this tool he discovered that the attraction exerted by a spherical body on an external point could be calculated by assuming the mass of the body was concentrated at its precise center. This theory was the final step in producing accurate calculations, and soon the mechanisms of organization in the universe became clear to him.

In 1687, Newton set forth his findings in the most respected scientific work of
all time, *Philosphia Naturalis Principia Mathematica,* better known as the
*Principia.* This work established a model of the structure and functions
of the universe based on universal gravitation which remains in use today,
confirmed generation after generation by observation and calculation. The
scientific community immediately recognized Newton's findings as revolutionary
and proven with such clarity and logic as to be nearly indisputable by rational
argument. Gradually, his mechanical analysis of the heavens became widely
known, and accepted as the basis for all future astronomy.

Newton is quoted as saying, "if I have seen farther than others, it is because I
was standing on the shoulders of giants," by way of thanking his predecessors
for the contributions to science which made his *Principia* possible.
Indeed, Newton's work represents the finale in a long chain of theory and
discovery that evolved throughout the Scientific Revolution. The beginnings of
progress had come in the sixteenth century. Nicolas Copernicus suggested
that perhaps the ancient concept of the Earth's position in the universe was
flawed. Giordano Bruno went one step further to claim that the universe
itself was far different than the ancients and the Church perceived, and that it
stretched out infinitely. Next, Kepler reduced the motions of the planets to
intelligible mathematical rules. Galileo developed the system of earthly
mechanics that he hinted might be applied to the heavens. Newton's work was the
culmination of this chain of science, inspired by the ideas of these men and the
methods and tools developed by them and others of his predecessors. The
*Principia* linked the last two remaining pieces of the puzzle--Galileo's
physics and Kepler's astronomy--and emerged with the 'grand design' so many
before him had sought. The design seemed not to have been established by any
planning or simple geography, but rather by the interaction of the forces of
nature, principally gravitation, on an enormous scale.

At first, the full revolutionary extent of Newton's work was not recognized by
even Newton himself. But during the coming century it became evident. The
essence of Newton's revolution was that he had conceived not only a plausible,
but demonstrable model for the workings of the universe, solely relying on
mechanics and completely separate of any spiritual influence. When this became
clear, it was obvious that the *Principia* marked the most profound break
from the grip of the Middle Ages. One of the
reasons that Newton's theories gained only gradual acceptance is that he wrote
for mathematicians, and the full significance of his work was not accessible to
any but those who were most highly trained in mathematics. He needed the aid of
interpreters to bring the concepts of the *Principia* to the masses. One
of the most effective of these interpreters was
Voltaire, who invented the well known story
of Newton and the falling apple, and explained the Newtonian philosophy in a
1737 work, whose publication can be seen as the end of Aristotelianism, the pop
of the final bubble of breath from lips already cold.

Until the time of Newton, astronomy had been considered at best a theoretical
science, based little on direct evidence. It was more often referred to as a
philosophy, for its proponents were hypothesizers without proof, and they more
often than not invoked the divine plan of god, or the existence of a universal
soul in efforts to explain the structures and phenomena they observed. However,
Newton's *Principia* changed the image of astronomy drastically. His
theories were backed by solid mathematic tools, and his mechanical explanation
of the universe contained no hint of mysticism or spirituality. With the advent
of Newton, astronomy earned the right to be known as a science.