42 



HISTORY OF ASTRONOMICAL DISCOVERY. 



and being aware of the conclusion of Newton, that such bodies describe very eccentric 

 ellipses, his active mind began to study intently their phenomena, which resulted in a 

 prophecy that has immortalised his name. After cataloguing and comparing a consider 

 able number of comets, that of 1682 fortunately appeared. This he was led to regard as 

 identical with those of 1456, 1531, and 1607, between which there is nearly the same 

 interval. Hence he anticipated its return after the lapse of a similar period. " I dare 

 venture," said he, "to foretell that it will return again in 1758 ;" and, sanguine as to the 

 result, he called upon posterity to notice that it was an Englishman who had hazarded the 

 statement. This was a prediction announced in 1705, the accomplishment of which 

 ranks with the greatest achievements of modern astronomy, and will perpetuate the fame 

 of Halley to the remotest generations. He had been gathered to his grave in Lee church 

 yard seventeen years, when the celestial traveller re-appeared, at the time announced, to 

 verify his words, illustrate his sagacity, and invest him with undying honour. 



Bradley, the English Hipparchus, the model of observers, as he is styled by Laplace, 

 became the third astronomer royal upon the death of Halley. He had previously effected 

 one of his two great discoveries, the aberration of the stars, an optical illusion, arising 

 from the combined movement of the earth in space, and the progressive transmission 

 of light ; a discovery of the highest importance, requiring the greatest precision of 

 observation to detect. Ever since the doctrine of the earth's translation in space had 

 been received, astronomers had been anxious to find some parallax of the fixed stars, 

 as a sensible confirmation of the fact. Although the whole diameter of the earth's 

 orbit is relatively insignificant, it is yet absolutely vast. Hence it was deemed no 

 unreasonable expectation that some small apparent change of place in the heavens 

 would be discerned in the case of the fixed stars, when viewed from the two extre 

 mities of the earth's annual orbit, separated from each other by the mighty chasm 

 of a hundred and ninety millions of miles. To ascertain this, 

 if possible, Bradley commenced observing a particular star, 

 y Draconis, in connection with his friend the Hon. Mr. 

 Molyneux, in the house of the latter on Kew Green, which 

 was afterwards the residence of George III. The star in 

 question was selected from its passing very near the zenith 

 of their observatory. After a series of laborious observations, 

 which began towards the close of the year 1 725, this star was 

 found to have a southerly motion, a result which excited pro 

 found surprise, as it was in a direction opposite to what would 

 have been produced by an annual parallax. For instance, when 

 the earth is in its orbit at A, the place projected in the heavens 

 by the star B will be at c ; but when the earth is in its orbit at 

 D, provided that there be any sensible parallax, the 

 star will be seen in the direction E. Instead of this 



being the case, the observed appearance was that of the star appearing from D in 

 the direction F. The observed change of place, then, could not be explained 

 by means of parallax, or the earth's motion simply, because the change was in a 

 direction opposite to what would have been caused by the parallactic motion. 

 Suspecting some error, instruments were examined and observation renewed ; 

 but the fact was verified, and the star was found to move southerly from 

 December to April, then to move northerly again, returning to its original 

 place in the December following, after having described a small apparent 

 orbit in the heavens. The diagram represents the apparent course annually described by 

 y Draconis, and other stars similarly situated. Imagine the line a bent overhead, z being 



a 



