Dante and the early astronomers . astronomers in placing Saturn, Jupiter, andMars, beyond the sun, in order of the periods of theirdeferents, which were all longer than a year. Venusand Mercury, however, had periods the same as thesun: on which side of him, then, must they be placed ?Some modern authors, Ptolemy says, thought they werebeyond the sun, but he agrees with the most ancient,and places them between sun and moon. The generaldisposition of the heavenly bodies according to Ptolemyis shown in Fig. 35, with the periods of epicycles anddeferents, and the directions of the several movement
Dante and the early astronomers . astronomers in placing Saturn, Jupiter, andMars, beyond the sun, in order of the periods of theirdeferents, which were all longer than a year. Venusand Mercury, however, had periods the same as thesun: on which side of him, then, must they be placed ?Some modern authors, Ptolemy says, thought they werebeyond the sun, but he agrees with the most ancient,and places them between sun and moon. The generaldisposition of the heavenly bodies according to Ptolemyis shown in Fig. 35, with the periods of epicycles anddeferents, and the directions of the several movements,but epicycles and spaces between deferents are all mad**equal. It will be noted that the sun has no epicycleand that the moon turns on hers in a reverse directionthat the centres of those of Mercury and Venus are ina line with the sun, while the lines joining Mars, Jupiter,and Saturn to the centres of theirs are in each caseparallel with the line joining Earth and Sun. Beyond star Spheia Saturn Jupiter :^ Oiurnal Revo/ut/on -.^. 30,000 years. 29^ years. nearly 12 years. nearly 2 years. One siderealyear. One siderealyear. One siderealyear. One siderealmonth (27^ dys.) Fig. 35. The Ptolemaic system. The arcs of circles are portions of the deferents which carry round the smaller circles(the epicycles) in th^ periods named at the side. 152 PTOLEMY. all the wandering stars is the sphere of the fixed stars,moving in its vast period of 36,000 years; and thewhole system is carried round Earth in one great re-volution of a day and a night. And here we have to note one astonishing the planets were all beyond reach of measure-ment, it was not so with the moon, which the Greeksrightly recognized as our nearest neighbour. They hadactually achieved the long-desired feat of measuringher real size and distance. The fundamental principle on which they worked iseasily explained. If we look out of a window at a treein the garden, it appears against a background of somedistant scen
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