One Half of this Difference may perhaps be excused, if Refraction were not allowed in this Case by Tycho; yet 2 Min. in such a Star as Syrius, is somewhat too much for him to be mistaken in.
But a more evident Proof of this Change is drawn from the Observation of the Application of the Moon to Palilicium, An. Chris. 509. Mar. 11. when, in the Beginning of the Night, the Moon was seen to follow that Star very near, and seemed to have eclipsed it, ἐπέβαλλε γὰρ ὁ ἀστηρ τῳ πᾶρα την διχοτομίαν μέρει τῆς κυ'ρτυς περιφειας τõυ πεφωτισμένου μερους, i.ve. Stella apposita erat parti per quam bisecabatur limbus Lunæ illuminatus, as Bullialdus, to whom we are beholden for this ancient Observation, has translated it. Now, from the undoubted Principles of Astronomy, this could never be true at Athens, or near it, unless the Latitude of Palilicium were much less than we at this Time find it[AM]."
[AM] Vide Bulialdi Astr. Philolaica, p. 172.
The [AN]Motion of Arcturus seems further confirmed, from the Observations of Tycho Hevelius and Flamstead; for Hevelius sets down the Distance of that Star from Lyra 4′ greater than Tycho had observed it seventy-two Years before him, and Flamstead twenty-two Years after measured the Distance betwixt the same two Stars, still 3′ greater than Hevelius found it; so that if Lyra had stood still all that while, there was an Appearance of Arcturus's having gone 7' out of his Place in the Space of an hundred Years. See Dr. Long's Astronomy, p. 274.
[AN] These are the nearest and greatest of the fixed Stars, the Motion of the others not having been observed, or being at too great a Distance, are either imperceptible, or have not been taken notice of.
It is further to be observed, in Confirmation of the Motion of one of these Stars, that Flamstead found the Distance of Arcturus, from the Head of Hercules 3′ greater than it is set down by the Prince of Hesse; and that his Distance from the Lion's Tail was a little decreased with 5½′ less Latitude than Tycho had observed. Hence, to make these Observations agree, one or both of them must have moved together equal to 7′. This Change of Place, which is quite contrary to all known Causes proceeding from the Earth, must therefore be occasioned either by the Motion of the Sun, or by a particular Motion of their own; but if, amongst themselves, they must all move, and if all be in Motion, the Sun must also move.
If these Observations, delivered down to us by very able Astronomers, be either true or near it, as great Allowances have been made for the Ignorance of the Ages in which they were taken, and the Inaccuracy of the Instruments, we may naturally conclude, that these Stars must have a Motion; and if they move, as has been before observed, the Sun must also; hence he cannot now be in the original Plane of the Earth's annual Direction, or at least in the same identical Place he was at first possessed of: And if so, the Stars must also have the like Motion, though in different Directions, and all may thus be governed by the same impulsive Power.
To illustrate this primitive Motion of the Stars, and at the same time to show that the Variety which appears in the Quantity of Motion can be no Objection to it,
See PLATE XVIII. Fig. 2.
Where A represents the Eye of an Observer, and B, E, F, H, various Systems, moving in different Directions thro' the mundane Space; it is evident that the Sphere B, having moved from C, and that of E, not having appeared to move at all, there must be a sensible Change in the new Position of these two Systems to one another, and so of the rest; and tho' the apparent Motion of H, be much more than that of F, from the Point A, yet from C, they will appear less different, and from B, they will appear nearly equal. And farther, as the Direction from H, is in the Line I, H, and that of F, in the Line K, G, those two Systems will appear to approximate, and the Magnitude of the Star in the first will be increased, and in the latter diminished. Thus, many of the Stars in the oldest Catalogues, which were said to be of the second Magnitude, are now become of the first, and several of the first are now judged to be of the second, &c.