The scientific revolution was a fundamental change in the direction of Western thought and scientific practice that began with the reassertion of the heliocentric model of the universe by Polish astronomer Nicolaus Copernicus (1473-1543) in 1543. The revolution culminated with English physicist Sir Isaac Newton's (1643-1727) publication of his profoundly influential 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). The scientific revolution was not merely a set of discoveries or technical advances; it brought about a fundamental change in the way the laws of nature were investigated. During this time scientists increasingly supplanted pure deduction with experimentation.
Prior to the seventeenth century, science and philosophy in the West had made relatively minor advances since the fall of the Roman Empire (ca. 400 CE). Despite significant technical advances in some fields, there were no significant systematic inquiries into the workings of nature. Many of the philosophical inquiries known to ancient Greek and Roman scientists were suppressed by theological doctrine. When translations of ancient Latin texts once again reawakened interest in the Aristotelian scientific/philosophical systems of the classical world, these were based almost purely upon deductive reasoning. Not until the early seventeenth century--and the research of Italian physicist Galileo Galilei (1564-1642)--were scientific theories based upon actual experimentation rather than philosophic ideals.
Astronomers, both ancient and modern, have used the heavens as a laboratory to understand the basic principles of nature. According to the theories of Aristotle (384-322 BCE), the Earth occupied the center of the universe; the heavens were the realm of perfection in which the Sun and the planets orbited the Earth in perfectly circular orbits at an unvarying rate of speed. To explain the retrograde motion of the planets, a complicated system of secondary circular orbits called epicycles were superimposed upon the main orbits. This system was perfected by the Greek astronomer Ptolemy (ca. 85-165 CE). Despite its complexity, it yielded reasonable results in predicting the positions of the planets.
Aristotle further postulated that all physical matter was a mixture of four basic elements (air, earth, fire, and water), each of which had certain intrinsic physical properties. For instance, bodies fell not because of gravity, but because the Earth had an inherent tendency to seek out the lowest possible level. Thus, heavier objects--those containing more of the basic element earth than others--would fall at a faster rate than lighter ones. Other than this "natural" motion, objects would remain always at rest, unless a force was constantly applied to them. Heavenly bodies constantly moved in circular orbits at a uniform rate of speed because this was an inherent property of the perfect material of which they were supposedly made, aptly named the "quintessence."
The Aristotelian system--based solely upon reasoning rather than experimentation--was deeply flawed as a scientific methodology. Regardless, it advanced the idea that the workings of the universe could be understood using only a small number of basic principles, which is still an underlying principle of scientific inquiry today.
Copernicus realized that planetary motion could be more simply explained by a system in which the planets orbited the Sun. However, laboring under the influence of Aristotle, he insisted on perfectly circular orbits and uniform rates of speed as an inherent property of the planets. While Copernicus's heliocentric theory yielded the correct frame of reference for further scientific inquiry, it was no more accurate than the Ptolemaic system in predicting the motions of the planets. Perhaps fearing persecution by the religious and scientific establishment of the time, he delayed publishing his theory in his work, On the Revolution of Planets, until just prior to his death.
Danish astronomer Tycho Brahe (1546-1601), rejecting both the Copernican and Ptolemaic systems, devised a geocentric model in which planets orbited the Sun, which in turn orbited the Earth, and he amassed a huge data set of planetary positions attempting to prove this theory. After Brahe's death, German astronomer and mathematician Johannes Kepler (1571-1630) used Brahe's data to formulate Kepler's laws of planetary motion. Kepler mathematically proved that planets orbit the Sun in elliptical orbits and that the square of their orbital periods is proportional to the cube of the semi-major axis of their orbit. In addition, Kepler was able to predict that the orbital speed of the planets was inversely proportional to their distance from the Sun-planet center of mass. Kepler's work effectively disproved the Aristotelian and Ptolemaic model of planetary motion.
In 1609, Galileo was the first person to use the telescope to observe the heavens. His discoveries included the mountains of the Moon, the moons of Jupiter, and the rings of Saturn. Perhaps most importantly, Galileo discovered that Venus displays crescent phases similar to that of the Moon. This last discovery conclusively disproved the validity of geocentric systems.
Galileo was prohibited by the Church from advancing heliocentric theory for complex reasons and forbidden to do more astronomical research. Under house arrest, in his old age he resumed studies of motion. His work included studies on the addition of velocities, the acceleration of falling bodies, and most importantly, the principle of motion. Galileo advanced the theory that if no friction acts upon a body set in motion, the body will continue to move forever, effectively overturning the Aristotelian idea that motion of an object is an inherent property of it. Most important of Galileo's contributions to science was the use of experimentation to prove or disprove any given hypothesis about underlying principles that govern nature.
Newton's development of the laws of motion and gravitation marked the beginning of what is now known as classical or Newtonian physics. In addition to developing calculus, Newton made tremendous advances in the understanding of light and optics. Without question, Newton's work dominated the Western intellectual landscape for more than two centuries. In particular, Newton made fundamental realizations regarding planetary motion from which Newtonian physics evolved. He realized that force could act at a distance--that is, gravity could act upon two bodies even if there was no physical contact between them. Newton defined this force as the interaction between two bodies, and each body experienced a force that was equal in magnitude and opposite in direction; this was a profound and innovative departure from Aristotle's approach. Newton also asserted that the acceleration of a body was equal to the net force applied to it divided by its mass. Moreover, Newton's law of universal gravitation explained and validated the elliptical orbits advanced by Kepler. According to Newton's law of gravity, the force of gravity had to be inversely proportional to the square of the distance between the Sun and the planets. To prove these theories, Newton--working independently of German mathematician Gottfried Wilhelm Leibniz (1646--1716)--developed calculus. The techniques of calculus advanced by Newton and Leibniz were immediately applied to a wide variety of problems in physics and astronomy.
The advancement of experimentalism during the scientific revolution and the use of calculus resulted in a new scientific methodology. This, in turn, led to the Age of Enlightenment and to an explosion of scientific inquiry into the natural world. These new methodologies eventually provided the mechanism for the development of more recent theories, such as quantum theory and relativity theory, that have essentially superseded classical physics.