Deferent and epicycle

The epicycle theory states that the planets move in small circular orbits, the epicycles which in turn migrate ( called deferent ) on a large circular path around the Earth. The epicycles (Greek epi "on ," kyklos "circle" ) is thus a " circle on the circle." This theory was representing some 2,000 years - from the 3rd century BC to the 17th century.

The concept of epicycles was probably erected by Apollonius of Perga at the end of the 3rd century BC. In the Ptolemaic system was introduced as a systematic theory in astronomy to explain the speed and direction of changes in the movements of moon, sun and planets in the sky. In particular, epicycle theory tries to explain why the planets seem at times to run on their loop path backwards. Background of the epicycle theory is the study of Aristotle, that the only perfect motion is the circular path and that all movements outside the sphere of the moon's orbit in circular paths run. The epicycle theory was first replaced by Kepler's theory of elliptical planetary orbits.

Content

Many computational models of antiquity has collected in the Almagest Ptolemy. Thus, he created a usable manual, which was binding on the subsequent generations of science in Arabic and Latin Middle Ages. The planetary movements are explained therein, among others, on the basis of the epicyclic motion.

After the epicycle, the planets moved in a small circle, the epicycle (Greek epíkyklos, incidental or Aufkreis ), which in turn moves along a larger circle, the deferent is called (also deferierender circle, from Latin Deferre " carry away ", " take "). The movement along the circuits are each in an easterly direction and approximately parallel to the plane of Earth's orbit ( the ecliptic plane ). The orbits of the planets in this system are similar epicycloids.

At the center of the deferent, the theory is after the earth. Seen of her from the planets would now move mostly to the east in the direction of the deferent. Half the time adds up to this movement, the eastward motion on the epicycle. The other time but would run the planet on the epicycle opposite to the motion of the deferent, thereby slowing its movement in the sky and would eventually turn into a retrograde through which the planet's orbit finally performed a loop.

However, this mathematical artifice was not sufficient to fully describe the observed motion of the planets in more detail. So it was necessary to add further levels of epicycles ( epicycles on epicycles ). The epicycle theory was also connected with the latest by Ptolemy Exzentertheorie Hipparchus and the earth thus pushed out of the center of the deferent. Furthermore, Ptolemy introduced the equant as a the apparent center of the uniform circular motion still must match the position of the earth, neither with the center of the orbit. The calculation of planetary orbits was, however, quite complicated by these additional terms and conditions.

Its very principle, one can consider the epicycle as an approximation of the actual planetary orbits by Fourier series. This parallel was discovered by Giovanni Schiaparelli latest and formally proved by Giovanni Gallavotti.

Overcoming

The heliocentric world view explains the grinding of the planetary orbits by superimposing the earth's motion and, therefore, seems to dispense with epicycles. Since the new model but still emanating from circular orbits for the planets had disagreements again be explained by the use of epicycles. Copernicus used in his world system still 34 epicycles, but could provide a conclusive explanation for the binding of Mercury and Venus web to the sun.

Only by Johannes Kepler, the epicycle was superfluous: the " natural" model of the planets on elliptical " Kepler orbits " around the sun does not require correction by superimposed epicycles.