section_path: New ideas/Astronomy/Heliocentrism
section_name: Heliocentrism
paragraph_text: Heliocentrism
  
  For almost five millennia, the geocentric model of the Earth as the center of the universe had been accepted by all but a few astronomers. In Aristotle's cosmology, Earth's central location was perhaps less significant than its identification as a realm of imperfection, inconstancy, irregularity, and change, as opposed to the "heavens" (Moon, Sun, planets, stars), which were regarded as perfect, permanent, unchangeable, and in religious thought, the realm of heavenly beings. The Earth was even composed of different material, the four elements "earth", "water", "fire", and "air", while sufficiently far above its surface (roughly the Moon's orbit), the heavens were composed of a different substance called "aether". The heliocentric model that replaced it involved the radical displacement of the Earth to an orbit around the Sun; sharing a placement with the other planets implied a universe of heavenly components made from the same changeable substances as the Earth. Heavenly motions no longer needed to be governed by a theoretical perfection, confined to circular orbits.
  
  Copernicus' 1543 work on the heliocentric model of the Solar System tried to demonstrate that the Sun was the center of the universe. Few were bothered by this suggestion, and the pope and several archbishops were interested enough by it to want more detail. His model was later used to create the calendar of Pope Gregory XIII. However, the idea that the Earth moved around the Sun was doubted by most of Copernicus' contemporaries. It contradicted not only empirical observation, due to the absence of an observable stellar parallax, but more significantly at the time, the authority of Aristotle. The discoveries of Kepler and Galileo gave the theory credibility.
  
  Kepler was an astronomer who is best known for his laws of planetary motion, and Kepler´s books Astronomia nova, Harmonice Mundi, and Epitome Astronomiae Copernicanae influenced among others Isaac Newton, providing one of the foundations for his theory of universal gravitation. One of the most significant books in the history of astronomy, the Astronomia nova provided strong arguments for heliocentrism and contributed valuable insight into the movement of the planets. This included the first mention of the planets' elliptical paths and the change of their movement to the movement of free floating bodies as opposed to objects on rotating spheres. It is recognized as one of the most important works of the Scientific Revolution. Using the accurate observations of Tycho Brahe, Kepler proposed that the planets move around the Sun not in circular orbits but in elliptical ones. Together with Kepler´s other laws of planetary motion, this allowed him to create a model of the Solar System that was an improvement over Copernicus' original system.
  
  Galileo's main contributions to the acceptance of the heliocentric system were his mechanics, the observations he made with his telescope, as well as his detailed presentation of the case for the system. Using an early theory of inertia, Galileo could explain why rocks dropped from a tower fall straight down even if the Earth rotates. His observations of the moons of Jupiter, the phases of Venus, the spots on the Sun, and mountains on the Moon all helped to discredit the Aristotelian philosophy and the Ptolemaic theory of the Solar System. Through their combined discoveries, the heliocentric system gained support, and at the end of the 17th century it was generally accepted by astronomers.
  
  This work culminated in the work of Newton, and his Principia formulated the laws of motion and universal gravitation which dominated scientists' view of the physical universe for the next three centuries. By deriving Kepler's laws of planetary motion from his mathematical description of gravity, and then using the same principles to account for the trajectories of comets, the tides, the precession of the equinoxes, and other phenomena, Newton removed the last doubts about the validity of the heliocentric model of the cosmos. This work also demonstrated that the motion of objects on Earth and of celestial bodies could be described by the same principles. His prediction that the Earth should be shaped as an oblate spheroid was later vindicated by other scientists. His laws of motion were to be the solid foundation of mechanics; his law of universal gravitation combined terrestrial and celestial mechanics into one great system that seemed to be able to describe the whole world in mathematical formulae.