I may add besides, to confirm the probability of this simple motion, that as almost all learned men are now of the same opinion with Copernicus concerning the parallelism of the axis of the earth, it seemed to me to be more agreeable to truth, or at least more handsome, that it should be caused by simple circular motion alone, than by two motions, one in the ecliptic, and the other about the earth's own axis the contrary way, neither of them simple, nor either of them such as might be produced by any motion of the sun. I thought best therefore to retain this hypothesis of simple motion, and from it to derive the causes of as many of the phenomena as I could, and to let such alone as I could not deduce from thence. It will perhaps be objected, that although by this supposition the reason may be given of the parallelism of the axis of the earth, and of many other appearances, nevertheless, seeing it is done by placing the body of the sun in the centre of that orb which the earth describes with its annual motion, the supposition itself is false; because this annual orb is eccentric to the sun. In the first place, therefore, let us examine what that eccentricity is, and whence it proceeds. [Sidenote: The cause of the eccentricity of the annual motion of the earth.] 8. Let the annual circle of the earth _a b c d_ (in fig. 3) be divided into four equal parts by the strait lines _a c_ and _b d_, cutting one another in the centre _e_; and let _a_ be the beginning of Libra, _b_ of Capricorn, _c_ of Aries and _d_ of Cancer; and let the whole orb _a b c d_ be understood, according to Copernicus, to have every way so great distance from the zodiac of the fixed stars, that it be in comparison with it but as a point. Let the earth be now supposed to be in the beginning of Libra at _a_. The sun, therefore, will appear in the beginning of Aries at _c_. Wherefore, if the earth be moved from _a_ to _b_, the apparent motion of the sun will be from _c_ to the beginning of Cancer in _d_; and the earth being moved forwards from _b_ to _c_, the sun also will appear to be moved forwards to the beginning of Libra in _a_; wherefore _c d a_ will be the summer arch, and the winter arch will be _a b c_. Now, in the time, of the sun's apparent motion in the summer arch, there are numbered 186¾ days; and, consequently, the earth makes in the same time the same number of diurnal conversions in the arch _a b c_; and, therefore, the earth in its motion through the arch _c d a_ will make only 178½ diurnal conversions. Wherefore the arch _a b c_ ought to be greater than the arch _c d a_ by 8¼ days, that is to say, by almost so many degrees. Let the arch _a r_, as also _c s_, be each of them an arch of two degrees and 1⁄16. Wherefore the arch _r b s_ will be greater than the semicircle _a b c_ by 4⅛ degrees, and greater than the arch _s d r_ by 8¼ degrees. The equinoxes, therefore, will be in the points _r_ and _s_; and therefore also, when the earth is in _r_, the