THE SUN'S MOTION IN SPACE. 151 



of the existence of evidence that the so-called fixed stars did not 

 really occupy fixed positions, but were subject to movement, is to 

 be found in Edmund Halley's "Considerations on the change of 

 the latitudes of some of the principal fixed stars" published in 

 1717. 1 Comparing recent star places with Ptolemy's, Halley was 

 astonished to find that the latitudes of Aldebaran, Sirius, and 

 Arcturus, directly contradicted the greater obliquity of the ecliptic 

 indicated by the latitudes of most of the rest, and conjecturing 

 that in all probability these conspicuous stars are nearest to the 

 earth, 2 he remarked: — " if they have any particular motion of 

 their own, it is most likely to be perceived in them," that is to 

 say, in the nearer stars. Since the problem of solar motion, is a 

 problem of motion in relation to other stars, Halley must be con- 

 sidered in the passage quoted, to have, implicitly at least, raised 

 the whole question. Its full significance however does not appear 

 to have occurred to him. 



(5) Bradley, 17 J/7. — With Bradley 3 the conception took still 

 more definite shape, for in his paper in the Phil. Trans, of the 

 Royal Society in 1747, he discussed the consequences, in respect of 

 star places, of the alternative suppositions, viz., that the stars are 

 in motion, and the sun fixed; and that the stars are fixed and the 

 sun is in motion. Bradley clearly recognised that the problem 

 would, if at all, be solved by taking account of the large proper 

 motions of the nearer stars, and that a more exact knowledge of 

 precession, aberration, and nutation, was necessary, before the 

 problem could be properly attacked. That very knowledge, viz., 

 of the fundamental constants of astronomy, was afterwards 

 attained with a remarkable degree of precision, by the reduction of 

 Bradley's own observations and the comparison of them with 

 Bessel's observations and with others. 



1 Phil. Trans. Reprint, Vol. vi., pp. 329 - 330, Orig. Vol. xxx., 1717. 

 Halley was then Sec. Roy. Soc. 2 p. 330. 



3 James Bradley, t>.t>., f.r.s., Astron. Roy. — Phil, Trans. Reprint Vol. 

 ix., pp. 417-438, Orig. Vol. lv., 1747-8. 



