Dante and the early astronomers . from the Sun. On September 24, when Marswas in the middle of his retrograde arc in Pisces, thesun was exactly opposite, in the constellation of is found to be always the case, not only withMars, but with Saturn and Jupiter too. Wheneverone of these planets has the position 0 on its epicycle,and therefore is retrograding, the sun will be foundto be exactly opposite in the sky. Mars comes intothis position, and is opposite the sun, once in 780 days;this, therefore, Ptolemy called the period of hisepicycle, while a little less than two years was the L
Dante and the early astronomers . from the Sun. On September 24, when Marswas in the middle of his retrograde arc in Pisces, thesun was exactly opposite, in the constellation of is found to be always the case, not only withMars, but with Saturn and Jupiter too. Wheneverone of these planets has the position 0 on its epicycle,and therefore is retrograding, the sun will be foundto be exactly opposite in the sky. Mars comes intothis position, and is opposite the sun, once in 780 days;this, therefore, Ptolemy called the period of hisepicycle, while a little less than two years was the L 142 PTOLEMY. period of the deferent. The two periods of Saturnare 378 days and 29J years nearly; of Jupiter 399days and nearly 12 years. Venus and Mercury betray their dependence uponthe sun in more striking fashion, for since theseplanets simply oscillated from one side to the otherof the sun, their epicycles must be supposed to bekeeping pace with him all the way round the 31 shows their relation to one another. On. Fig. 31. Venus, Mercury, and the sun. September 25, 1911, Mercury was seen from Earthas a morning star as far west of the sun as it ispossible for him to travel, while Venus, after shiningas an evening star all the summer, had come into linewith the sun and become invisible.^ On December 7following, the rotation of the epicycles (Ptolemywould say) had brought both planets to new positions, * Venus had passed her inferior conjunction with the sun on Sej)t. 15. PTOLEMY. 143 Mercury now being an evening star at his greatestelongation east, and Venus a morning star. But thecentres of the two epicycles always remain in a linewith one another and the sun, and so their periods onthe deferents are the same as his, viz. one year. Theepicyclic periods, or intervals between two greatestelongations west or east, are 116 days for Mercury,584 days for Venus. We still explain the complicated course of the planetsby resolving it into two approximately circular motions
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