STARS. 



iltuation cf the afcending node of the moon's orbit when he 

 firll began to make his obfervations, he fufpcfted that the 

 moon's aftion upon the equatorial parts of the earth might 

 produce thefe efFeas. For if the preceffion of the equinox 

 be, according to fir Ifaac Newton's principles, caufed by 

 the adions of the fun and moon upon thofe parts ; the 

 plane of the moon's orbit being, at one time, above ten 

 degrees more inclined to the plane of the equator than at 

 another, it was reafonable to conclude, that the part of the 

 whole annual preceflion which arifes from her iclion, would, 

 in different years, be varied in its quantity ; whereas the 

 plane of the ecliptic, in which the fun appears, keeping al- 

 ways nearly the fame inclination to the equator, that part 

 of the prccefTion which is owing to the fun's aftion, may 

 be the fame every year ; and from hence it would follow, 

 that althout^h the mean annual preceffion, proceeding from 

 the joint adions of the fun and moon, were 50" ; yet the 

 apparent annual preceffion might fometimes exceed, and 

 fometimes fall iliort of that mean quantity, according to the 

 various fituations of the nodes of the moon's orbit. 



In the year 1727, the moon's afcending node was near 

 the beginning of Aries, and confequently her orbit was as 

 much inclined to the equator as it can ?X any time be ; and 

 then the apparent annual preceffion was found, by the doc- 

 tor's firft year's obfervations, to be greater than the mean ; 

 which proved, that the Itars near the equinodial coliire, 

 whofe declinations are mod of all affeded by the preceffion, 

 had changed theirs above a tenth part more than a pre- 

 ceffion of 50" would have caufed. The fucceeding year's 

 obfervations proved the fame thing ; and in three or four 

 years time the difference became fo confiderablc, as to leave 

 no room to fufped it was owing to any imperfeftion either 

 of the inftrument or cbfervation. 



But fome of the ftars that were near the folllitial colure 

 liavint? appeared to move, during the fame time, in a manner 

 contrary to what they ought to have done, by an increafe of 

 the preceffion ; and the deviations in tliem being as remark- 

 able as in the others; it was evident that fcmething more than 

 a mere change in the quantity of the preceffion would be re- 

 quifite to folve this part of the phenomenon. Upon com- 

 paring the obfervations of liars near the folllitial colure 

 that were almoil oppofite to each other in right afcenfion, 

 they were found to be equally affeded by this caufe. For 

 whilft y Draconis appeared to have moved northward, the 

 fmall fl'ar, which is the 3Jth Camelopardah Hevelii in the 

 Britifh Catalogue, feemed to have gone as much towards 

 the fouth ; which fhewed, that this apparent motion in both 

 thofe ftars might proceed from a nutation of the earth's 

 axis ; whereas the companfon of the dodor's obfervations 

 of the fame ftars, formerly enabled him to draw a different 

 conclufioii with refpeft to the caufe of the annual aberra- 

 tions arifing from the motion of hght. For the apparent 

 alteration in 7 Draconis, from that caufe, being as great 

 again a.i in the other fmall ftar, proved that that did not 

 proceed from a nutation of tlie earth's axis ; as, on the con- 

 trary', this may. Upon making the like comparifon between 

 the obfervation of other ftars that lie nearly oppofite in 

 right afcenfion, whatever their fituations were with refped 

 to the cardinal points of the equator, it r.ppeared that 

 their change of dechnation was nearly equal, but contrary ; 

 and fuch as a nutation or motion of the earth's axis would 

 effed. 



The moon's afcending node being got back towards the 

 beginning of Capricorn in the year 17,52, the ftars near the 

 equinodial colure appeared about that time to change their 

 declinations no more than a preceffion of 50" required ; 

 whiHl f >me of thofe near the folftitial colure altered theirs 

 above 2" in a year lefs than they ought. Soon after, the an- 



6 



nual change of declination of the former was perceived to 



be diminifhed, fo as to become lefs than 50" of preceffion 

 would caufe ; and it continued to diminifh till the year 

 1736, when the moon's afcending node was about the be- 

 ginning of Libra, and her orbit had the leail inclination to 

 the equator. But by this time, fome of the ifars near the 

 folllitial colure had altered their declinations 18" lefs fince 

 the year 1727, than they ought to have done from a pre- 

 ceffion of 50". For "/ Draconis, which in thofe nine years 

 would have gone about 8'' more foutherly, was obferved, in 

 1736, to appear 10" more northerly than it did in the year 



1727- 



As this appearance in y Draconis indicated a diminution 

 of the inclination of the earth's axis to the plane of the 

 ecliptic, and as feveral aftronomers have fuppofed that in- 

 clination to diminifh regularly ; if this phenomenon de- 

 pended upon fuch a caufe, and amounted to 18" in nine 

 years, the obliquity of the ecliptic would, at that rate, alter 

 a whole minute in thirty years ; which is much faller than 

 any obfervations before made would allow. The dodor 

 had therefore reafon to think, that fome part of this motion 

 at lealt, if not the whole, was owing to the moon's adion 

 upon the equatorial parts of the earth, which, he conceived, 

 might caufe a libratory motion of the earth's axis. But as 

 he was unable ro judge, from only nine years' obfervations, 

 whether the axis would entirely recover the fame pofition 

 that it had in the year 1727, he found it neceffary to con- 

 tinue his obfervations through a whole period of the moon's 

 nodes ; at the end of which he had the latisfadion to fee, 

 that the itars returned into the fame pofitions again, as if 

 there had been no alteration at all in the inclination of the 

 earth's axis, which fully convinced him that he had gueffed 

 rightly as to. the caufe of the phenomenon. This circum- 

 flance proves likewife, that if there be a gradual diminution 

 of the obliquity of the ecliptic, it does not arifc only from 

 an alteration in the pofition of the earth's axis, but rather 

 from fome change in the plane of the ecliptic itfelf ; becaufe 

 the liar-, at the end of the period of the moon's nodes, ap- 

 peared in the fame places, with refped to the equator, as 

 tiiey ought to have done if the earth's axis had retained the 

 fame inclination to an invariable plane. 



The dodor having communicated thefe obfervations, and his 

 fufpicion of their caufe, to the late Mr. Machin, that excellent 

 geometer foon after fent him a table, containing the quantity 

 of the annual preceffion in the various pofitions of the moon's 

 nodes, as alfo the correfponding nutations of the earth's 

 axis ; which was computed upon the fuppofition that the 

 mean annual preceffion is 50", and that the whole is go- 

 verned by the pole of the moon's orbit only ; and, therefore, 

 Mr. Machin imagined, that the numbers in the table would 

 be too large, as, in fad, they were found to be. But it ap- 

 peared that the changes which Dr. Bradley had obferved, 

 both in the annual preceffion and nutation, kept the fame 

 law, as to increafing and decreafing, with the numbers of 

 Mr. Machin's table. Thofe were calculated upon the fup- 

 pofition, that the pole of the equator, during a period of 

 the moon's nodes, moved round in the periphery of a little 

 circle, whofe centre was 23° 29' diftant from the pole of the 

 ecliptic ; having itfelf alfo an angular motion of 50" in a 

 year about the fame pole. The north pole of the equator 

 was conceived to be in that part of the fmall circle which 

 is fartheit from the north pole of the ecliptic, at the time 

 when the moon's afcending node is in the beginning of 

 Aries ; and in the oppofite point of it, when the fame node 

 is in Libra. 



If the diameter of the little circle, in which the pole of 

 the equator moves, be fuppofed equal to 18", which is the 

 whole quantity of the nutation, (which fee,) as coUeded from 



Dr. 



