  The principle that celestial bodies must move on circular orbits or on paths
resulting from the composition of circular orbits has been assumed as a
constant guide in the astronomical thougth of the peoples facing the
Mediterranean sea as from the second century B.C. until the beginning of the
XVII century. The mathematical model based on such an assumption, the theory of
epicycles in all its versions and modifications, has been taken as a scheme for
all astronomical calculations during at least eighteen centuries, from
Hipparcus to Kepler. As it is known, in Astronomia Nova (1609), Kepler succeded
to establish the two laws which after him were named the first and the second
Kepler's laws. The revolution he performed by giving up the circle paradigm is
of fundamental importance and represents the indispensable premise to Newtonian
theory. This revolution, the result of what Kepler called his "war" against
Mars, is usually underestimated if one considers the break carried out with
respect to the pre-Kepler celestial kinematics. In "Astronomia Nova" we assist
to the attempt of Kepler to get for Mars an orbit consistent with the results
of Tycho Brahe's observations. Passing through several phases and step by step
discarding the hypoteses wich gave results in contrast with observations,
Kepler arrived at establishing that the orbit of Mars around the Sun is an
ellipse. Here an esposition is given of this work of analysis by Kepler wich
is, in the history of science, one of the early examples of rigorous setting up
of a model which correctly explains the experimental results.
