Bradley J. Account of a new discovered motion of the Fix'd stars // Phil. Trans. 1728. 35. Р. 637-641.

Bradley J. Account of a new discovered motion of the Fix'd stars // Phil. Trans. 1728. 35. Р. 637-641.

Home   Other formats   <<<     Page 640   >>>

  637  638  639  640 641  642  643  644  645  646  647  648  649  650  651  652  653  654  655  656  657  658  659  660  661 

( 640 )

way from what it would have been, had it proceeded from an annual Parallax of the Star : But being now pretty well satisfied, that it could not be entirely owing to the want of Exadtnefs in the Observations ; and having no Notion of any thing else, that could cause such an apparent Motion as this in the Star; we began to think that fome Change in the Materials, &c. of the Instrument itself, might have occasioned it. Under these Apprehensions we remained fome time, but being at length fully convinced, by several Trials, of the great Exactness of the Instrument, and finding by the gradual Increase of the Stars Distance from the Pole, that there must be Some regular Caufe that produced it ; we took care to examine nicely, at the Time cf each Observation, how much it was: and about the Beginning of March 17x6, the Star was found to be zo" more Southerly than at the Time of the first Observation. It now indeed seemed to have arrived at its utmost Limit Southward, because in several Trials made a-bout this Time, no sensible Difference was observed in its Situation. By the Middle of April it appeared to be returning back again towards the North ; and about the Beginning of June, it pasted at the same Distance from the Zenith as it had done in 'December, when it was first: observed.

From the quick Alteration of this Star’s Declina-nation about this Time (it increasing a Second in three Days ) it was concluded, that it would now proceed North ward, as it before had gone Southward of its prefent Situation ; and it happened as was conjectured : for the Star continued to move Northward rill September following, when it again became stationary,

( 641 )

tionary, being then near 2,0" more Northerly thau in Juney and no less than 39" more Northerly than it was in March. From September the Star returned towards the South, till it arrived in 'December to the same Situation it was in at that time twelve Months, allowing for the Difference of Declination on account of the Preceflion of the Equinox.

This was a sufficient Proof, that the Instrument had not been the Caufe of this apparent Motion x>i the Star, and to find one adequate to such an Effe<9: seemed a Difficulty. A Nutation of the Earth’s Axis was one of the first rhings that offered itself upon this Occasion, but it was fbon found to be insufficient; for though it might have accounted for the change of Declination in y Draconis yet it would not at the same time agree with the Phsenomena in other Stars; particularly in a finall one almost opposite in right Ascension to y Draconis, at about the fame Distance from the North Pole of the Equator: For, though this Star seemed to move the same way, as a Nutation of the Earth’s Axis would have made it, yet it changing its Declination . but about half as much as y Draconis in the same time ( as appeared upon comparing the Observations of both made upon the same Days, at different Seafons of the Year) this plainly proved, that the apparent Motion of the Stars was not occasioned by a real Nutation, since if that had been the Cause, the Alteration in both Stars would have been near equal.

The great Regularity of the Observations left no room to doubt, but that there was fome regular Cause that produced this unexpe&ed Motion, which did not depend on the Uncertainty or Variety of the

CLnqc Seafons

Hosted by uCoz