But if, as I have supposed, the earth also be moved with simple circular motion in a plane that passeth through a, cutting the plane of the ecliptic so as that the common section of both the planes be in a c, thus also the axis of the earth will be kept always parallel to itself. For let the centre of the earth be moved about in the circumference of the epicycle, whose diameter is l a k, which is a part of the strait line l a c; therefore l a k, the diameter of the epicycle, passing through the centre of the earth, will be in the plane of the ecliptic. Wherefore seeing that by reason of the earth's simple motion both in the ecliptic and in its epicycle, the strait line l a k is kept always parallel to itself, every other strait line also taken in the body of the earth, and consequently its axis, will in like manner be kept always parallel to itself; so that in what part soever of the ecliptic the centre of the epicycle be found, and in what part soever of the epicycle the centre of the earth be found at the same time, the axis of the earth will be parallel to the place where the same axis would have been, if the centre of the earth had never gone out of the ecliptic.
Now as I have demonstrated the simple annual motion of the earth from the supposition of simple motion in the sun; so from the supposition of simple motion in the earth may be demonstrated the monthly simple motion of the moon. For if the names be but changed, the demonstration will be the same, and therefore need not be repeated.
The supposition of simple motion, why likely.
7. That which makes this supposition of the sun's simple motion in the epicycle f g h i probable, is first, that the periods of all the planets are not only described about the sun, but so described, as that they are all contained within the zodiac, that is to say, within the latitude of about sixteen degrees; for the cause of this seems to depend upon some power in the sun, especially in that part of the sun which respects the zodiac. Secondly, that in the whole compass of the heavens there appears no other body from which the cause of this phenomenon can in probability be derived. Besides, I could not imagine that so many and such various motions of the planets should have no dependance at all upon one another. But, by supposing motive power in the sun, we suppose motion also; for power to move without motion is no power at all. I have therefore supposed that there is in the sun for the governing of the primary planets, and in the earth for the governing of the moon, such motion, as being received by the primary planets and by the moon, makes them necessarily appear to us in such manner as we see them. Whereas, that circular motion, which is commonly attributed to them, about a fixed axis, which is called conversion, being a motion of their parts only, and not of their whole bodies, is insufficient to salve their appearances. For seeing whatsoever is so moved, hath no endeavour at all towards those parts which are without the circle, they have no power to propagate any endeavour to such bodies as are placed without it. And as for them that suppose this may be done by magnetical virtue, or by incorporeal and immaterial species, they suppose no natural cause; nay, no cause at all. For there is no such thing as an incorporeal movent, and magnetical virtue is a thing altogether unknown; and whensoever it shall be known, it will be found to be a motion of body. It remains, therefore, that if the primary planets be carried about by the sun, and the moon by the earth, they have the simple circular motions of the sun and the earth for the causes of their circulations. Otherwise, if they be not carried about by the sun and the earth, but that every planet hath been moved, as it is now moved, ever since it was made, there will be of their motions no cause natural. For either these motions were concreated with their bodies, and their cause is supernatural; or they are coeternal with them, and so they have no cause at all. For whatsoever is eternal was never generated.
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.
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 sun will appear in s. Wherefore the true place of the sun will be in t, that is to say, without the centre of the earth's annual motion by the quantity of the sine of the arch a r, or the sine of two degrees and 16 minutes. Now this sine, putting 100,000 for the radius, will be near 3580 parts thereof. And so much is the eccentricity of the earth's annual motion, provided that that motion be in a perfect circle; and s and r are the equinoctial parts. And the strait lines s r and c a, produced both ways till they reach the zodiac of the fixed stars, will fall still upon the same fixed stars; because the whole orb a b c d is supposed to have no magnitude at all in respect of the great distance of the fixed stars.
Supposing now the sun to be in c, it remains that I show the cause why the earth is nearer to the sun, when in its annual motion it is found to be in d, than when it is in b. And I take the cause to be this. When the earth is in the beginning of Capricorn at b, the sun appears in the beginning of Cancer at d; and then is the midst of summer. But in the midst of summer, the northern parts of the earth are towards the sun, which is almost all dry land, containing all Europe and much the greatest part of Asia and America. But when the earth is in the beginning of Cancer at d, it is the midst of winter, and that part of the earth is towards the sun, which contains those great seas called the South Sea and the Indian Sea, which are of far greater extent than all the dry land in that hemisphere. Wherefore by the last [article] of chapter XXI, when the earth is in d, it will come nearer to its first movent, that is, to the sun which is in t; that is to say, the earth is nearer to the sun in the midst of winter when it is in d, than in the midst of summer when it is in b; and, therefore, during the winter the sun is in its Perigæum, and in its Apogæum during the summer. And thus I have shown a possible cause of the eccentricity of the earth; which was to be done.
I am, therefore, of Kepler's opinion in this, that he attributes the eccentricity of the earth to the difference of the parts thereof, and supposes one part to be affected, and another disaffected to the sun. And I dissent from him in this, that he thinks it to be by magnetic virtue, and that this magnetic virtue or attraction and thrusting back of the earth is wrought by immateriate species; which cannot be, because nothing can give motion but a body moved and contiguous. For if those bodies be not moved which are contiguous to a body unmoved, how this body should begin to be moved is not imaginable; as has been demonstrated in [art. 7], chap. IX, and often inculcated in other places, to the end that philosophers might at last abstain from the use of such unconceivable connexions of words. I dissent also from him in this, that he says the similitude of bodies is the cause of their mutual attraction. For if it were so, I see no reason why one egg should not be attracted by another. If, therefore, one part of the earth be more affected by the sun than another part, it proceeds from this, that one part hath more water, the other more dry land. And from hence it is, as I showed above, that the earth comes nearer to the sun when it shines upon that part where there is more water, than when it shines upon that where there is more dry land.