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.