revolve round the sun, since the centre of the epicycle did not always lie in the direction of the sun, but might be anywhere in the ecliptic. One peculiarity, however, in the motion of any of the superior planets might easily have suggested their motion round the sun, and was either completely overlooked by Ptolemy or not recognised by him as important. It is possible that it was one of the clues which led Coppernicus to his system. This peculiarity is that the radius of the epicycle of the planet, j j, is always parallel to the line e s joining the earth and sun, and consequently performs a complete revolution in a year.
Fig. 45.—The epicycle of Jupiter.
This connection between the motion of the planet and that of the sun received no explanation from Ptolemy's theory. Now if we draw e j' parallel to j j and equal to it in length, it is easily seen[1] that the line j' j is equal and parallel to e j, that consequently j describes a circle round j' just as j round e. Hence the motion of the planet can equally well be represented by supposing it to move in an epicycle (represented by the large dotted circle in the figure) of which j' is the centre and j' j the radius, while the centre of the epicycle, remaining always in the direction of the sun, describes a deferent (represented by the small circle round e) of which the earth is the centre. By this method of representation the motion of the superior planet is exactly like that of an inferior planet, except that its epicycle is larger than its deferent; the same reasoning as before shows that the motion can be represented simply by supposing the centre j' of the epicycle to be actually the sun. Ptolemy's epicycle and deferent are therefore capable of being replaced, without affecting the position of the planet in the sky, by a
- ↑ Euclid, I. 33.
