Nicolaus Copernicus was born on February 19, 1473 in Torun, Poland. Circa 1508, Copernicus developed his own celestial model of a heliocentric planetary system. By 1508, Copernicus had begun developing his own celestial model, a heliocentric planetary system. Ptolemy had previously invented a geometric planetary model, which was inconsistent with Aristotle's idea that celestial bodies moved in a circular motion at different speeds around a fixed point, the earth. In an attempt to reconcile such inconsistencies, Copernicus's heliocentric solar system named the sun, rather than the earth, as the center of the solar system. Subsequently, Copernicus believed that the size of each planet's orbit depended on its distance from the sun. Though his theory was viewed as revolutionary and met with some controversy, Copernicus was not the first astronomer to propose such a theory; centuries prior, in 270 B.C., ancient Greek astronomer Aristarchus of Samos had identified the sun as the solar system's central unit. Still, Copernicus's heliocentric solar system proved to be more detailed and accurate than Aristarchus's, including a more efficient formula for calculating planetary positions throughout the year. Born on February 15, 1564, in Pisa, Italy, Galileo Galilei was a mathematics professor who made pioneering observations of nature with long-lasting implications for the study of physics. He also constructed a telescope and supported the Copernican theory, which supports a sun-centered solar system. In July 1609, Galileo learned about a simple telescope built by Dutch eyeglass makers, and he soon developed one of his own. n March 1610, he published a small booklet, The Starry Messenger, revealing his discoveries that the moon was not flat and smooth, but a sphere with mountains and craters. He found Venus had phases like the moon, proving it rotated around the sun. He also discovered Jupiter had revolving moons, which didn’t revolve around the earth. Soon Galileo began mounting a body of evidence that supported Copernican theory and contradicted Aristotle and Church doctrine. In 1612, he published his Discourse on Bodies in Water, refuting the Aristotelian explanation of why objects float in water, saying that it wasn’t because of their flat shape, but instead the weight of the object in relation to the water it displaced. In 1613, he published his observations of sunspots, which further refuted Aristotelian doctrine that the sun was perfect. That same year, Galileo wrote a letter to a student to explain how Copernican theory did not contradict Biblical passages, stating that scripture was written from an earthly perspective and implied that science provided a different, more accurate perspective. Francis Bacon was born on January 22, 1561 in London, England. Bacon served as attorney general and Lord Chancellor of England, resigning amid charges of corruption. His more valuable work was philosophical. Bacon took up Aristotelian ideas, arguing for an empirical, inductive approach, known as the scientific method, which is the foundation of modern scientific inquiry. From the time he had reached adulthood, Bacon was determined to alter the face of natural philosophy. He strove to create a new outline for the sciences, with a focus on empirical scientific methods—methods that depended on tangible proof— while developing the basis of applied science. Unlike the doctrines of Aristotle and Plato, Bacon's approach placed an emphasis on experimentation and interaction, culminating in "the commerce of the mind with things." Bacon's new scientific method involved gathering data, prudently analyzing it and performing experiments to observe nature's truths in an organized way. He believed that when approached this way, science could become a tool for the betterment of humankind. According to Bacon in Novum Organum, the scientific method should begin with the "Tables of Investigation." It should then proceed to the "Table of Presence," which is a list of circumstances under which the event being studied occurred. "The Table of Absence in Proximity" is then used to identify negative occurrences. Next, the "Table of Comparison" allows the observer to compare and contrast the severity or degree of the event. After completing these steps, the scientific observer is required to perform a short survey that will help identify the possible cause of the occurrence. Unlike a typical hypothesis, however, Bacon did not emphasize the importance of testing one's theory. Instead, he believed that observation and analysis were sufficient in producing a greater comprehension, or "ladder of axioms," that creative minds could use to reach still further understanding. Born on January 4, 1643, in Woolsthorpe, England, Isaac Newton was an established physicist and mathematician, and is credited as one of the great minds of the 17th century Scientific Revolution. With discoveries in optics, motion and mathematics, Newton developed the principles of modern physics. In 1687, after 18 months of intense and effectively nonstop work, Newton published Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). The work offers an exact quantitative description of bodies in motion in three basic laws: 1) A stationary body will stay stationary unless an external force is applied to it; 2) Force is equal to mass times acceleration, and a change in motion is proportional to the force applied; and 3) For every action, there is an equal and opposite reaction. These three laws helped explain not only elliptical planetary orbits but nearly every other motion in the universe: how the planets are kept in orbit by the pull of the sun’s gravity; how the moon revolves around Earth and the moons of Jupiter revolve around it; and how comets revolve in elliptical orbits around the sun. The laws also allowed Newton to calculate the mass of each planet, calculate the flattening of the Earth at the poles and the bulge at the equator, and how the gravitational pull of the sun and moon create the Earth’s tides. In Newton's account, gravity kept the universe balanced, made it work, and brought heaven and earth together in one great equation.