Dante and the early astronomers . nteresting to note that he does notexplain her varying velocity by an eccentric, as withthe sun and the planets. She has an eccentric, butPtojemy needed it for representing his own discovery,the evection, so he gave her an epicycle, using it inquite a different way from the epicycles of the epicycle also revolved while moving on theeccentric, but in the opposite direction, and there wasso little difference in speed between the two motionsthat it never brought the moon to a stop, nor reversedher direction, but simply increased and retarded hermotio
Dante and the early astronomers . nteresting to note that he does notexplain her varying velocity by an eccentric, as withthe sun and the planets. She has an eccentric, butPtojemy needed it for representing his own discovery,the evection, so he gave her an epicycle, using it inquite a different way from the epicycles of the epicycle also revolved while moving on theeccentric, but in the opposite direction, and there wasso little difference in speed between the two motionsthat it never brought the moon to a stop, nor reversedher direction, but simply increased and retarded hermotion alternately during her monthly , when the moon was at M, in what Ptolemy called 148 PTOLEMY. the upper apsis (or arc) of her epicycle, or as we shouldsay in her apogee, the motion on the epicycle was con-trary to her motion on the eccentric, and made it seemslower. When the epicycle had travelled halfway roundthe eccentric, it had also made nearly half a revolutionon its own axis: consequently the moon was at M^,. Fig. 34. The moons epicycle and deferent. near the lower apsis, or perigee, and the motion on theeccentric seemed to be accelerated. The slight difference in speed between the two motionsaccounted for the continuous displacement of the apogeein the zodiac, as may be seen from the diagram. Forsuppose that when in apogee at M the moon is seenfrom Earth among the stars in the middle of the return of the epicycle to this place next month,she has not yet quite completed a revolution on herepicycle but is at M^, and will not be in apogee untilthe centre of her epicycle is advanced 3° further inTaurus. After some time (five months), apogee does PTOLEMY, 149 not occur until the moon is in Gemini, and it will benine years before it occurs again in Taurus. These are the leading features of the system by whichPtolemy represented the motions of the planets, the sun,and the moon. He is uncomfortably conscious that itmay strike us as very complicated, an
Size: 1797px × 1390px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, booksubjectastronomy, booksubjectdantealighieri
