I propose a work of Professor Livia Giacomini on worldview and its models.
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cinematic vision
Kinematics is the study of how the planets move. Why study the kinematics? It 'an important historical example of the evolution of the scientific method and the concept of scientific model.
In this introduction, we deal mainly of European science: from the Greeks to Newton.
- The history of the kinematics of the solar system
The first models geocentric In the model
Aristotle (384-322 BC) each planet, the sun and moon are fixed on concentric crystalline spheres that move around the Earth. This first model is thus geocentric and the motion of planets and 'round and smooth.
Since then, with increasing accuracy of observations of the positions of the planets, astronomers continued to develop models more and more 'complex to explain the observed motion of the planets.
The big problem of these early models was the fact that we observed an effect of retrograde orbit of the planets. This optical effect 'due to the fact that the Earth moves (when the outer planet, more 'slowly, is overcome by the Earth, there is an apparent retrograde motion) and can not' be explained by a simple motion and uniform circular motion ...
The retrograde motion as seen from Earth (http://www.astronomynotes.com)  |
The Ptolemaic model to explain these observations, in the later models were introduced additional motions of the planets, called epicycles , which overlap the circular motion of revolution and evenly around the planet Earth, can explain the existence of retrograde motion.
| The Ptolemaic model (observed epicycles) |
 | After numerous corrections and additions, we arrive at the Ptolemaic model (85-165 AD) where the Earth 'was slightly shifted from the center and each planet has a bike and that' the juxtaposition of a global motion of revolution (circular and uniform) epicycles and other (a simple representation of this motion and 'the drawing above). The Ptolemaic model and 'was present for about 1500 years (its strength and' was to be updated on the comments most 'recent and accurate information and its publication in the colossal work of 13 volumes). |
The formulation of this model and 'an example of the scientific method. In fact, this theoretical model is adapted to explain the experimental observations, the basic concept of the scientific method that will 'be delivered later.
An important consideration, however, 'the evolution of the concept of a scientific model. In this case, in fact this model was based on the presence of transparent spheres which were based on the planets. The planets themselves had to 'make the epicycles, moving from the surface of the sphere itself. This vision was clearly instrumentalist, that 'this was only an accurate computer model of the motion but not a true representation of reality' (because making the epicycles, the planets would frnatumato crystal balls ...). That 's a difference substantial with the current concept of scientific model that should be a mirror of reality ', true to its nature and its funzionamneto.
The Copernican revolution
 | Copernicus (1473-1543 AD) found many flaws in the Ptolemaic model (which had been added and many many epicycles to explain the observations more and more accurate). He may make a heliocentric model (ie putting 'the Sun at the center) while trying to give' the idea of \u200b\u200ba uniform circular motion of the planets (thus using more of epicycles to explain the experimental observations). Copernicus 'and' the author's other findings: introducing the concept of the astronomical unit (average distance earth sun) is not measured. He uses this important concept for esprimerein accurately the relative distances in the solar system (by calculating these distances as simple applications of trigonometry). Copernicus also determined that the outer planets are slower, thus explaining the retrograde motion. |
Copernicus, unlike his predecessors, he thought his model as a description of reality '(a true current scientific model). His contemporaries interpreted it instead as another good calculator of motion, not accepting the idea that the Sun was physically in the center. One of the major objections that the model of Galileo's contemporaries muovaveano rigurdava the lack of parallax in the observations of distant stars (which is actually 'due to the remoteness of the stars themselves, which at that time was ignored).
Parallax:
 | If we consider an observer looking from the Earth of the stars close enough, they will appear in different positions (compared to the sky reference) according to the position of Earth in its orbit ( in design, at two different times of the year). This effect, and 'called parallax. If increases the distance of stars from the sun distance (d) The parallax error decreases and then becomes less obvious. At the time of Copernicus it was thought that the stars were much 'closer than they really are. So, it was thought that if the Earth had indeed been moving around the Sun, one should see a large parallax error on the position of the stars. The absence of this effect was a reflection of the fact that the Earth could not be in motion around the sun ... |
Kepler's Laws
The orbits of the planets around the Sun are still described as a first approximation by three laws called laws of history Keplero.La formulation of Kepler's laws and 'very interesting and it' an example of the scientific method.
 | all begins by Tycho Brahe (1546-1601) who built 'a model of fully equivalent in terms of motion, the model of Copernicus, but with the Earth at the center. To refute the Copernican model, calculation 'that if the Earth had been in motion, then the star had to be distant from Saturn about 700 times the distance from Saturn to Sole What seemed impossible at the time although we now know that the nearest star and 'to 28500 times that distance. Brahe and 'also makes an excellent experimental observations with an uncertainty of some arcominuto (about 10 times better than a person with a good view with the naked eye!). only drawback of the entire pattern: observations on the orbit of Mars (1546-1601) could not be reconciled with the theoretical model!. |
 | Now comes into play Kepler (1571-1630) who was hired by Brahe to accommodate the mathematical details that does not fit in his model. Instead of working on this road, Kepler works to improve the Copernican model (which is not allowed to explain the motion of the planets, however, because of the perfectly circular orbits). After several years of work has brilliant idea: the orbit of the planet is not 'circular but elliptical ... His contemporaries (including Galileo) did not agree ... |
Newton and the birth of dynamic
's just more' later that Isaac Newton (1642-1727) has shown these laws as an application of the law of universal gravitation approximated to a case (a 2-body problem ...)
fact the solar system as a first approximation may 'be summarized as a two-body system where the Sun, of much more massive planets, and' detention, and where the planets, given point, do not interact with each other in lora orbit around the sun.
With this demonstration, Newton invented the calculus ...
Galileo
 | Later, Galileo (1564-1642) takes up this heliocentric model by presenting it as a true representation of reality (a true scientific model now), arguing first that the Sun was actually located at cnetro the solar system. For this, Galileo had some problems with the Church dell'eopca ... Galileo is usually presented as the father of modern science. Often the birth of the scientific method is attributed to its model of the solar system, which has replaced the old geocentric model based on clear experimental evidence (Galileo first used the telescope to make observations). |
In reality there is no next step, namely the reconstruction experiment in the laboratory (which is not always easy in astrophysics!). We have to wait a while longer to get some conclusive tests of the earth's motion around the Sun (Foucault's Pendulum ...)
- Kepler's Laws: capiamole with animations ...
The orbits of the planets around the sun are described by three empirical laws (based only on experimental observations!) Formulated for the first time by Johannes Kepler (1571-1630).
| Kepler's First Law: |
| It 's just an illustration. In reality 'orbits are much less flattened. Some important consequences:
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| Kepler's second law: |
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 | A very important consequence of this second law, 'the speed' of the planet is not 'constant. Since the elliptical orbit, the distance sun and planet 'constant. Since the swept area must be constant, the planet must speed when approaching perihelion (see animation below). |
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 | This third law implies that the period of the planets increases rapidly with increasing radius of the orbit and therefore with the distance from the sun: the planets 'revolve much more distant' slowly. This is why the planets' distance have "years" a lot more 'long most of the planets' interiors. See the following page for an interesting experiment: |
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