Dante and the early astronomers . lled backwards against thesigns until October 26th, when it stopped again,reversed its direction once more, and during the restof the year moved rapidly forward. Fig. 29 shows the principle on which Ptolemy wouldhave explained this curious track. Each planet wassupposed to be fixed on a small circle, the epicycle,and this was fixed upon a large circle, or deferent, uponwhich it travels in the direction shown by the arrow,at a uniform speed, returning to the same place inthe sidereal period of the planet. Thus Mars, as seenfrom Earth, which is near C the centre
Dante and the early astronomers . lled backwards against thesigns until October 26th, when it stopped again,reversed its direction once more, and during the restof the year moved rapidly forward. Fig. 29 shows the principle on which Ptolemy wouldhave explained this curious track. Each planet wassupposed to be fixed on a small circle, the epicycle,and this was fixed upon a large circle, or deferent, uponwhich it travels in the direction shown by the arrow,at a uniform speed, returning to the same place inthe sidereal period of the planet. Thus Mars, as seenfrom Earth, which is near C the centre of the deferent,makes a great circle through all the zodiac in twoyears, Jupiter in twelve, and so on. But meanvrhilethe epicycle is rotating round its own centre, Cl, andwhen the planet reaches the point marked S, the 6°4° t0 40 20 0* 40 20* 19 Iscium • 0 ^--^ Decs •x • MCeti • ^ 4°6 Au^22• \ • (Opposition) >-0ct2e • • • ?**—,.^June29 Fig. 28. The Path of Mars among the Stars, Fig. 29. The method of accounting for movements such as those shown in Fig. 28,
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