PLATE XVIII.
The Globe S is here supposed to represent the Sun, having changed its Situation by a local Motion from A to C, and B represents the Globe of the Earth in a permanent Position, with its principal Points and Circles, respecting the primitive Plane A, B, K. Now in Consequence of the Angle of Variation, A, B, C, it evidently appears that a new ecliptic Plane, will be produced, as C, B, and also a Variation in the greatest Declination of the Sun, North and South from the Line of the Equator D, L. Hence, as in this Figure, the Obliquity of the Poles P, N, and G, F, will naturally decrease, and is shewn in Quantity by the Line of Aberration H, I.
Here follows a Table of the Change observed in the Obliquity of the Ecliptic by Astronomers of different Ages.
Plate XVIII.
A Table of the Obliquity of the Ecliptic.
| Ante Christi | ° | ′ | |
| 124 | Arato | 24 | 00 |
| —— | Hiparchus | 23 | 51⅓ |
| 127 | Eratosthenes | 23 | 51½ |
| 140 | Ptolomy | 23 | 51⅓ |
| 749 | Abategnius | 23 | 35½ |
| 1070 | Airahel | 23 | 34 |
| 1140 | Alomean | 23 | 33 |
| 1300 | Profatiograd | 23 | 32 |
| 1458 | Purbacchio | 23 | 29½ |
| 1490 | Regiomontaus | 23 | 30 |
| 1500 | Copernicus | 23 | 28½ |
| 1592 | Tycho Brahe | 23 | 21½ |
| 1656 | Cassini | 23 | 29½ |
Now sure, if we consider this continual Decrease of the Sun's Declination, which can proceed from no other Cause than that of his having moved out of the primitive Plane; we need make no great Difficulty thus far, to think our Conjectures not irrational.
The following is a Citation from Dr. Edmund Hally, Astronomer-Royal. See Philosophical Transactions, No. 355. p. 736.
"But while I was upon this Enquiry (of the Obliquity of the Ecliptic) I was surprized to find the Latitudes of three of the principal Stars in the Heavens, directly to contradict the supposed greater Obliquity of the Ecliptic, which seems confirmed by the Latitudes of most of the rest; they being set down in the old Catalogues, as if the Plane of the Earth's Orbit had changed its Situation amongst the fixed Stars, about 20′ since the Time of Hipparchus, particularly all the Stars in Gemini are put down, those to the Northward of the Ecliptic, with so much less Latitude than we find, and those to the Southward, with so much more southerly Latitude; and yet the three Stars Palilicium, Sirius, and Arcturus, do contradict this Rule: For by it, Palilicium, being in the Days of Hipparchus, in about 10 gr. of Taurus, ought to be about 15′ more southerly than at present, and Sirius being then in about 15 gr. of Gemini, ought to be 20′ more southerly than now; yet Ptolomy places the first 20′, and the other 22′ more northerly in Latitude than we now find them: Nor are these the Errors of Transcribers, but are proved to be right by the Declination of them set down by Ptolomy, as observed by Timocharis, Hipparchus, and himself; which shew, that these Latitudes are the same as those Authors intended. As to Arcturus, he is too near the Equinoctial Colour, to argue from him concerning the Change of the Obliquity of the Ecliptic; but Ptolomy gives him 33′ more North Latitude than he is now found to have; and that greater Latitude is likewise confirmed by the Declinations delivered by the abovesaid Observations: So then these three Stars are found to be above half a Degree more southerly at this Time than the Antients reckoned them. When, on the contrary, at the same time, the bright Shoulder of Orion, has, in Ptolomy almost a Degree more southerly Latitude than at present, what shall we say then? It is scarce to be believed, that the Antients could be deceived in so plain a Matter, three Observers confirming each other. Again, these Stars being the most conspicuous in Heaven, are in all Probability the nearest to the Earth; and if they have any particular Motion of their own, it is most likely to be perceived in them, which in so long a Time as eighteen hundred Years, may shew itself by the Alteration of their Places, though it be intirely imperceptible in the Space of one single Century of Years: Yet, as to Syrius, it may be observed, that Tycho Brahe makes him 2 Min. more northerly than we now find him; whereas he ought to be above as much more southerly from his Ecliptic (whose Obliquity he makes 2′½ greater than we esteem it at the present) differing in the Whole 4′½.
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.
But as this apparent Motion of the Stars at the Earth, must, from its Nature, be very small, so as scarce to be discovered in some of them in less than an Age, with any Instrument by the nicest Observer, I judge it will be extremely proper in this Place to propose some Method, by which, in process of Time, the Truth of the Theory may be ascertained. The Way I think most likely to succeed is this.