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THE MEDIEVAL ARISTOTELIAN WORLD VIEW A geocentric system The Sun and the planets The sphere outside the moon orbit. The heavenly spheres which comprise the fifth, divine element, the aether. Here everything moves in perfect circles. No irregularities exist. All fixed stars and the planets exist in this sphere. The sphere inside the moon orbit. The earthly spheres. The four elements (earth, water, fire and wind) move towards their natural places (natural motions). Earth (still in the centre) The Moon The Ptolemaic system (from 100 AD) The movement of each planet was explained separately by its own circle system. The retrograde movements were explained by so called epi-cycles. Some problematic issues at that time The movements of the planets, especially the inner planets (Mercury, Venus, and Mars) were difficult to predict by using a system based on perfect circles. The planets had irregular orbital periods. The Sun did not appear at the same place next day as the day before. In one year the Sun had moved place in relation to the fixed stars. The Ptolemaic system contained errors that increased over time. Copernicus presented a new, heliocentric system Sun was put in the centre of the system instead of Earth. All planets moved around the sun. Also Earth moved around the sun and also around its own axis. However, the planets (including Earth) were still moving in perfect circles. Epi-circles were still used to explain the motions of the planet. A more uniform system, but did not result in better predictions of the motions of the planets. The system was equally complex as the Ptolemaic system. Copernicus’ system was published in 1543 (the same year as he died). Obvious facts that conflicted with the new system The concept of gravitational force did not exist. Loose objects would be thrown out in space if the Earth was moving around its on axis. Clouds, birds and other things in the air would lag behind. How could the Moon follow Earth if Earth was moving around the Sun? How could Earth move around the Sun? A tremendous force is needed to cause this movement. Such a force cannot exist. There were other astronomical observations that contradicted the new system. These contradictions could be explained if the distance to the stars was substantially increased; however, this was seen as an ad-hoc explanation. But most of all: the system challenged the world view at that time. It questioned the very basic ideas of cosmos and life. Despite the contradicting facts the heliocentric idea survived and developed thanks to many contributors. Some important are: Tycho Brahe: Contributed with astronomical observations of e.g. comets (but he did not accept the heliocentric system). Johannes Kepler: (a student to Brahe). Accepted heliocentric system and developed theories about the planetary motions. The circular orbits were replaced by ellipses. Galileo Galilei: Held on to the circular motions. Invented telescopes and observed many irregularities such as the irregular surface of the moon, sunspots, Jupiter’s moons, and more. Galilei’s observations supported the heliocentric system. He also developed laws of motion and a tide theory that further supported the new system. He wanted to appeal the common-sense and wrote in Italian direct to the people (Latin was the scientific language at that time). He was judged by the inquisition in 1633 to life imprisonment. His observations were not convincing, e.g. knowledge of optical laws and the nature of light did not exist at that time. René Descarte: Also wrote direct to the people in French. He eliminated the concept of ”natural motions” and introduced a pure mechanical model of the universe. Isaac Newton: His major work Matematika Principa was published in 1687. Modern laws of motion and the concept of forces were introduced. His theories were described in a mathematical way. He also developed the experimental science.