He finds, its apparent Motion was not made in a Just great Circle, but deviated considerably from it; and conceives, that every Comet falls to this deviation, when this apparent Motion grows slow, and the Star becomes Stationary (which, as he saith, it doth in respect of the Ecliptick, not its own Orbite,) Here he observes, That from Decemb. 8/18, to Decem. 30. Jan. 9. its course was almost a great Circle: but that then it began to deflect from that Circle towards the North; so that afterwards, with a very notable and conspicuous Curvity, it directed its course towards Primam Arietis: Of which deflection, he ventures to assign the cause from the Cometical Matter, the various position and the distance of the Comet from the Earth and the Sun, the annual Motion of the Earth, and the impressed Motion, and the inclination of the discus of the Cometical Body.
He is pretty positive, that without the annual Motion of the Earth, no rational Account can be given of any Comet, but that all is involved with perplexities, and deform'd by absurdities.
He inquires, since all Comets have the peculiar Ingenite Motion, what kind of Line it is, they describe by that Motion of their own? whether circular, or streight, or curve, or partly streight and partly curve? And if curve, whether regular or irregular? if regular, whether Elliptick, or Parabolar, or Hyperbolical? He answers, That this Motion is Conical; and judgeth, that by the Conick path all the Phænomena of Comets can, without any inconveniency, be ready solved; even of that, which (by History) in fifty daies, passed through more then the 12 Signs in the Zodiack: And of that, which in two daies ran through eight Signs: and of another, which in 48 daies posted through all the Signs, contra seriem. Which how it can be explicated upon the supposition of the Earths standing still, and upon the denying of the annual Motion thereof, he understands not at all.
He refers to his Cometography these Disquisitions: whether all Comets (in their innate Motion) move equal spaces in equal Times? which is the swiftest, and which the slowest Motion they are capable of? what the cause of this acceleration and retardation of their true Motion?
He puts it out of doubt, that they are in the Sky it self, producing reasons for it that are very considerable, and alledging amongst others, That the Parallaxes doe clearly evince it, which he finds far less in Comets, then in the Moon, yea then sometimes in the Sun it self. Where he also represents, That he hath deduced the Horizontal Parallax of this very Comet from one onely Observation, made Feb. 4. st. n. by which he found, That then it was distant from the Earth 5000 Semidiameters of the same, or 4300000 German miles. From this distance from the earth, he deduces, That on that Day when it was so remote from the Earth, its true Diameter was 2560 German miles, which is three times bigger then the Diameter of the Earth, and almost six times bigger then that of the Moon, whose Diameter, according to his Theory, is 442 German miles.
He finds the Matter of Comets to be in the Æther it self, making the Æther and the Air to differ only in purity, and esteeming, That the Planets do emit their Exhalations, and have their Atmospheres like unto our Earth. Where he affirms, That the Sun alone may cast out so much Matter at any time in one year, as that thence shall be produced not one or two Comets, equallizing the Moon in Diamiter, but very many; which if so, what contribution may not be expected from the other Planets?
Of this Cometical Matter, he thinks, That first it is by little and little gathered together, then coagulated and condensed, and thereby reduced to a less Diameter; but then, after a while it resolves again, and grows dilute and pale, and at last is dissipated. And accordingly he affirms, That he hath observed the Head of this Comet at first more confused, thin and pale, afterwards clearer and clearer.
He conceives, That all Comets do respect the Sun as their King and Centre, as Planets do, making them a kind of Spurious Planets, that emulate the true ones in their Motion almost in all things.