History and Philosophy of Science » Early Greek

Early Greek

Aristotle encapsulates in his writings many of the traditional Ancient Greek views of physics. Important in his relation to later thinkers such as Galileo is the view that the natural state of an object is at rest. His theory of motion along with many other theories would later be rejected. However because of the encorporation of Aristotelian doctrine into religious canon proving opposing theories was both dangerous and difficult.

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This animation shows epicyclic motion. Ptolemy used epicycles to explain the apparent backward motion of the planets during the year. In actuality this occurs because of the relative speeds of the planets. The concept was that the planets during their regular circular orbit would also orbit around their orbit. Thus at regular intervals they would be observed to move backward. This model predicted planetary motions with a high rate of success.

This animation illustrates the equant. Ptolemy used the equant to explain the solstices. It explained the differing length of day throughout the seasons without breaking from circular motion. Ptolemy surmised that the sun orbited around an invisible circular point next to earth. This point the equant (red) was equidistant from a balance point (blue) and the earth.

This is an animation of the full Ptolemaic system. Moving out from earth is the Moon, Mercury, Venus, the Sun and finally Mars. The Sun is on an equant. Mercury Venus and Mars exhibit epicyclic motion. This is a model at 24 hours per second.
Ptolemaic Model: