As for the magnitude of the circles, and the times in which they are described by the bodies which are in them, I will suppose them to be such as shall seem most agreeable to the phenomena in question. [Sidenote: Possible causes of the motions annual and diurnal; and of the apparent direction, station, and retrogradation of the planets.] 6. The causes of the different seasons of the year, and of the several variations of days and nights in all the parts of the superficies of the earth, have been demonstrated, first by Copernicus, and since by Kepler, Galileus, and others, from the supposition of the earth's diurnal revolution about its own axis, together with its annual motion about the sun in the ecliptic according to the order of the signs; and thirdly, by the annual revolution of the same earth about its own centre, contrary to the order of the signs. I suppose with Copernicus, that the diurnal revolution is from the motion of the earth, by which the equinoctial circle is described about it. And as for the other two annual motions, they are the efficient cause of the earth's being carried about in the ecliptic in such manner, as that its axis is always kept parallel to itself. Which parallelism was for this reason introduced, lest by the earth's annual revolution its poles should seem to be necessarily carried about the sun, contrary to experience. I have, in art. 10, chap. XXI, demonstrated, from the supposition of simple circular motion in the sun, that the earth is so carried about the sun, as that its axis is thereby kept always parallel to itself. Wherefore, from these two supposed motions in the sun, the one simple circular motion, the other circular motion about its own centre, it may be demonstrated that the year hath both the same variations of days and nights, as have been demonstrated by Copernicus. For if the circle _a b c d_ (in fig. 3) be the ecliptic, whose centre is _e_, and diameter _a e c_; and the earth be placed in _a_, and the sun be moved in the little circle _f g h i_, namely, according to the order _f_, _g_, _h_, and _i_, it hath been demonstrated, that a body placed in _a_ will be moved in the same order through the points of the ecliptic _a_, _b_, _c_, and _d_, and will always keep its axis parallel to itself.