Classical Astronomy

Classical Astronomy

The Greeks related the movements of the stars to each other and devised a cosmos of spherical shape, whose center occupied an igneous body and around it revolved the Earth, the Moon, the Sun and the five known planets; the sphere ended in the sky of the fixed spheres: To complete the number of ten, which they considered sacred, they imagined a tenth body, the Anti-Earth.

The bodies described, according to them, circular orbits, which kept definite proportions in their distances. Each movement produced a particular sound and all together originated the music of the spheres.


They also discovered that the Earth, in addition to the movement of rotation, has a movement of translation around the Sun, however, this idea failed to thrive in the ancient world, tenaciously clinging to the idea that the Earth was the center of the Universe.

Eudoxius and his pupil Calipus proposed the theory of homocentric spheres, able to explain the kinematics of the solar system. The theory was based on the fact that the planets revolved in perfect spheres, with the poles located in another sphere that in turn had its poles in another sphere. Each sphere rotated regularly, but the combination of speeds and the inclination of one sphere in relation to the next resulted in an irregular planet movement, as observed. To explain the movements I needed 24 spheres.


Calipo improved his calculations with 34 spheres. Aristotle presented a model with 54 spheres but considered them with real existence of his own, not as calculation elements like his predecessors. Hipparchus reduced the number of spheres to seven, one for each planet, and proposed the geocentric theory, according to which the Earth was in the center, while the planets, the Sun and the Moon revolved around it.

Claudio Ptolemy adopted and developed the Hipparchus system. The number of periodic movements known at that time was already enormous: it took about eighty circles to explain the apparent movements of the heavens. Ptolemy himself came to the conclusion that such a system could not have physical reality, considering it a mathematical convenience. However, it was the one that was adopted until the Renaissance.


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