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supposed to move in the following manner. First of all it wasarranged that a point, iq (termed a deferent, or carrier), shouldbe conceived to travel uniformly round e in a circle, or cycle,ouce in every 365^ days. This large circle was called adeferent cycle. While this point, Pj, thus moved, it wasimagined that Mercury was in some way whirled round it,once in every 116 days, in an epicycle, or superimpcfsed andmuch smaller circle, which was carried along by Pj.
A similar arrangement, at a greater distance from the Earth ,was assumed for the planet Yenus, as is shown in the dia-gram. It was also supposed that the centres of the twoepicycles of Mercury and Yenus (viz., p x and p 2 ) moved so asalways to lie in a straight line joining the Earth and the Sun.
It is not hard to see that, if suitable velocities be assignedto Yenus and Mercury in their epicycles, such an arrangementwould account for the principal peculiarities in their move-ments which we have hitherto described. They would evi-dently appear to oscillate within certain limits on either sideof the Sun, which limits, as observation easily proved to be thecase, would be greater for Yenus than for Mercury, because ofthe larger epicycle belonging to the former. They would alsoalternately be brought nearer to the Earth , and removed fartherfrom it. And, supposing that the velocity of either of them inits epicycle, when nearest to the Earth , was sufficiently great,it is evident that the planet might, in spite of the onwardmotion of the centre of its epicycle, appear to move for atime in the reverse direction, as the arrows in the diagramindicate. In this way their periodical retrogressions ’might beexplained.
Any requisite variations of speed in describing the epicycle,as well as certain other irregularities of movement, were ac-counted for by placing additional epicycles upon those alreadymentioned; that is to say, instead of the planet Mercury moving in the epicycle shown in the diagram, another pointwas supposed to do so, which carried with it another smallercircle, or additional epicycle, in which the planet might movearound it. In this way several epicycles were successivelysuperimposed, as they were required. It was also found neces-