eichelberger: distances of heavenly bodies 169 



pole. But this would not explain the phenomena. The true expla- 

 nation, it is said, although I do not know how truly, occurred to Bradley 

 when he was sailing on the Thames, and noticed that the direction 

 of the wind, as indicated by a vane on the mast-head, varied slightly 

 with the course on which the boat was sailing. An account of the 

 observations in the form of a letter from Bradley to Halley is published 

 in the Philosophical Transactions for December, 1728: 



When the year was completed, I began to examine and compare my obser- 

 vations, and having pretty well satisfied myself as to the general laws of the 

 phenomena, I then endeavored to find out the cause of them. I was already 

 convinced that the apparent motion of the stars was not owing to the nutation 

 of the earth's axis. The next thing that offered itself was an alteration in the 

 direction of the plumb-line with which the instrument was constantly rectified; 

 but this upon trial proved insufficient. Then I considered what refraction might 

 do, but there also nothing satisfactory occurred. At length I conjectured that 

 all the phenomena hitherto mentioned, proceeded from the progressive motion 

 of light and the earth's annual motion in its orbit. For I preceived that, if light 

 was propagated in time, the apparent place of a fixed object would not be the 

 same when the eye is at rest, as when it is moving in any other direction than 

 that of the line passing through the eye and the object; and that, when the eye 

 is moving in different directions, the apparent place of the object would be differ- 

 ent. 



When Bradley's observations of 7 Draconis were corrected 

 for aberration, they showed, according to himself, that the 

 parallax of that star could not be as much as 1".0, or that the 

 star was more than 200,000 times as distant from the Earth as 

 the Sun. 



On December 6, 1781 there was read before the Royal Society 

 a paper by Mr. Herschel, afterwards Sir William, on the Parallax 

 of the Fixed Stars. We read: 



The method pointed out by Galileo, and first attempted by Hook, 

 Flamstead, Molineaux, and Bradley, of taking distances of stars from 

 the zenith that pass very near it, though it failed with regard to paral- 

 lax, has been productive of the most noble discoveries of another nature. 

 At the same time it has given us a much juster idea of the immense 

 distance of the stars, and furnished us with an approximation to the 

 knowledge of their parallax that is much nearer the truth than we ever 

 had before. . . . 



In general, the method of zenith distances labours under the fol- 

 lowing considerable difficulties. In the first place, all these distances, 

 though they should not exceed a few degrees, are liable to refractions; 

 and I hope to be pardoned when I say that the real quantities of these 

 refractions, and their differences, are very far from being perfectly 

 known. Secondly, the change of position of the earth's axis arising 

 from nutation, precession of the equinoxes, and other causes, is so 

 far from being completely settled, that it would not be very easy to 

 say what it exactly is at any given time. In the third place, the aber- 



