File:Epicycle and deferent.svg

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Summary[edit]

Description
English: Qualitative epicycle-and-deferent model as it was used by Apollonius, Hipparcus, Ptolemy and others in order to explain the apparent retrograde motion of planets in a geocentric model. The small circle in the center represents the Earth; the larger circle centered on the Earth is the deferent; the smaller circle centerd on a point of the deferent is the epicycle (its center is highlighted with a black dot in the drawing, but it was no physical body according to ancient astronomers); the filled red dot is the planet, which is centered on a point of the epicycle; the bold black line represents the actual motion of the planet, resulting from the composition of the double revolution of the deferent around the Earth and of the epicycle around a point of the deferent; the empty red dots are consecutive positions of the planet. As seen from the Earth, the planet seems to move forward (eastbound) from 1 to 2, to stop in 2, to move backwards from 2 to 3, to stop again in 3 and then to resume its normal forward motion.
Italiano: Modello epiciclo-deferente qualitativo. Modelli di questo tipo vennero impiegati da Apollionio, Ipparco, Tolomeo e altri per spiegare i moti retrogradi apparenti dei pianeti nei sistemi astronomici geocentrici. Il piccolo cerchio vuoto al centro rappresenta la Terra; il cerchio più ampio, centrato nella Terra, è il deferente; il cerchio più piccolo, centrato su un punto del deferente, è l'epiciclo (il suo centro è evidenziato con un punto nero nel disegno, ma ad esso non corrispondeva alcun punto fisico nelle concezioni astronomiche degli antichi); il cerchio rosso pieno è il pianeta, che è centrato su un punto dell'epiciclo; la linea nera spessa rappresenta il moto effettivo del pianeta, che risulta dalla composizione della doppia rivoluzione del deferente intorno alla Terra e dell'epiciclo intorno a un punto del deferente; i punti rossi vuoti rappresentano le successive posizioni del pianeta: visto dalla Terra, esso sembra muoversi in avanti (in direzione est) da 1 a 2, fermarsi in 2, muoversi di moto retrogrado da 2 a 3, fermarsi di nuovo in 3 e poi riprendere il suo moto normale.
Date
Source Based upon data from Thomas S. Kuhn, La rivoluzione copernicana (The Copernican Revolution), Einaudi, Torino, 2000, p. 78. Also based upon File:Epicycle et deferent.png by Julo
Author Own work MLWatts
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File:Epicycle et deferent.png by Julo

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current16:50, 27 June 2013Thumbnail for version as of 16:50, 27 June 2013560 × 500 (9 KB)MLWatts (talk | contribs)Corrected eastbound direction as seen from top of the Earth's North Pole
16:40, 27 June 2013Thumbnail for version as of 16:40, 27 June 2013560 × 500 (9 KB)MLWatts (talk | contribs)User created page with UploadWizard
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