The planetary system of Ptolemy

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The planetary system of Ptolemy


It was the geocentric astronomical model of Ptolemy which depicted and forecasted the orbits of the Sun, Moon and known planets on the ecliptic level.

In the astronomical system of Ptolemy the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn revolved around the Earth, while the remainder stars were standing on an outer sphere.

In the present reconstruction, the three views of Ptolemy have also been applied for the explanation of the ostensible motion of the planets: a) the eccentricity of the Earth, b) the epicycles (which had already been proposed by Apollonios of Perga and Hipparchos for the justification of retrograde motion of the planets) and c) the equant (the blue point from where a hypothetical observer would see the planet that revolved around him on an epicycle, covering equal angles at equal time intervals).

This provident model was so precise that it dominated for 1500 years until Copernicus established, irreversibly, the previous heliocentric system of Aristarchos of Samos.

SOURCES: "Ptolemy, The Great Syntaxis of Astronomy (Almagest)", "Pappus of Alexandria, Annotation in the Books 5 and 6 of the Mathematical Syntaxis".