The Greek astronomy
From a historical point of view, Greek astronomy was dominated by two characters, Aristotle and Ptolemy, who introduced ideas and incorrect models that would dominate scientific thought for almost two millennia.Aristote
The key figure is Aristotle, a philosopher of the fourth century BC, who leaned on the ideas of one of his predecessors, Plato. According to him, the world must have a spherical shape and movement of all celestial bodies must be circular and uniform, ie at constant speed.
In the system of Aristotle, as in that of Pythagoras, the Earth was motionless at the center of the world and surrounded by a series of crystalline spheres. The problem of the Pythagorean model was the fact that each planet was associated with a single sphere, which could not explain the irregularities of the apparent motions.
Aristotle overcame this problem by creating a more complex system containing 55 spheres nested into each other. Each planet was then associated with a group of spheres whose movement is superimposed. Combining different rotations allowed to give each planet a complex movement that could be adjusted to match the one that was observed in the sky.
With a combination of 55 spheres, Aristotle came relatively well to reproduce the apparent motions of the planets. His system was nonetheless a major flaw: it was unable to explain the apparent changes in brightness of the planets.
We now know that these changes are due to the change in distance between the Earth and each planet. But in the system of Aristotle planets were at a fixed distance from the Earth and the brightness changes remained unexplained.
Aristotle also introduced a more philosophical concept that came to be accepted until the sixteenth century: the distinction between the Earth and the heavens. For him, the interior of the lunar orbit, which included the Earth and its atmosphere, represented the realm of imperfection and change. Beyond the Moon, was the kingdom of perfection and immutability.
Ptolemy
The main flaw of Aristotle's system was its inability to explain the brightness variations of the planets. For this reason, an Alexandrian astronomer Claudius Ptolemy, changed the system in the second century AD, but without undermining the principles enunciated by Plato and Aristotle.
For Ptolemy, the heavenly bodies were not related to crystalline spheres centered on the Earth. Indeed, each planet was moving on a small circle, called epicycle, the center itself was moving in a great circle centered on the Earth, called deferent.
By adjusting the size and position of all circles involved, Ptolemy got a system that can accurately reproduce the apparent motions of the heavenly bodies. He was more able to explain the variations in brightness of the planets, since they now saw their distance from Earth varies.
This double success explains that the Ptolemaic system, which improved the Aristotelian in form but not the spirit, was accepted until the sixteenth century.
Heracleides
Despite the dominance of Aristotle and Ptolemy, two other Greek philosophers proposed much more realistic systems. At the time of Aristotle, Heraclitus advanced that the Earth was not stationary, but actually turned on itself. The apparent rotation of the sky 24 hours then explained much more natural way. The explanation was good, but it was not accepted.
Later, to explain the specific movements of Mercury and Venus, which seemed to hover around the Sun, Heracleides forward these two planets do not turn around the earth but around the sun. He thus obtained a more realistic description, even if he thought the rest of the heavenly bodies, including the Sun, still revolved around the Earth.
0 commentaires: