_B._ The cause of that can be no other, than that the earth, which is _l m_, hath the like motion to that which I suppose the sun to have in L M, compounded of straight and circular from west to east in a day, as the annual motion hath in a year; so that, not reckoning the eccentricity, it will be moved through the ecliptics in one revolution, as Copernicus proveth, about one degree. Suppose then the whole earth moved from H to I, (which is half the year) circularly, but falling from I to _i_ in the same time about thirty minutes, and as much in the other hemisphere from H to _k_; then draw the line _i k_, which will be equal and parallel to H I, and be the diameter of the equator for the next year. But it shall not cut the diameter of the ecliptic B Z in A, which was the equinoctial of the former year, but in _o_ thirty-six seconds from the first degree of Aries. Suppose the same done in the hemisphere under the plane of the paper, and so you have the double of thirty-six seconds, that is seventy-two seconds, or very near, for the progress of the vernal equinox in a year. The cause why I suppose the arch I _i_ to be half a degree in the ecliptic of the earth, is, that Copernicus and other astronomers, and experience, agree in this, that the equinoctial points proceed according to the order of the signs, Aries, Taurus, Gemini, &c. from west to east every hundredth year one degree or very near. _A._ In what time do they make the whole revolution through the ecliptic of the sky? _B._ That you may reckon. For we know by experience that it hath proceeded about one degree, that is sixty minutes, constantly a long time in a hundred years. But as one hundred years to one degree, so is thirty-six thousand years to three hundred and sixty degrees. Also as one hundred years to one degree, so is one year to the hundredth part of one degree, or sixty minutes; which is (60)/(100), or thirty-six seconds for the progress of one year; which must be somewhat more than a degree according to Copernicus, who, (lib. iii. cap. 2) saith, that for four hundred years before Ptolomy it was one degree almost constantly. Which is well enough as to the natural cause of the precession of the equinoctial points, which is the often-said compounded motion, though not an exact astronomical calculation. _A._ And it is a great sign that his supposition is true. But what is the cause that the obliquity of the ecliptic, that is, the distance between the equinoctial and the solstice, is not always the same?