16 



SCIENCE 



[N. S. Vol. XLII. No. 1070 



1726, the star gradually moved further 

 south; then it remained stationary for a 

 little time; then moved northwards until, 

 by the middle of June, it was in the same 

 position as in December. It continued to 

 move northwards until the beginning of 

 September, then turned again and reached 

 its old position in December. The move- 

 ment was very regular and evidently not 

 due to any errors in Bradley's observations. 

 But it was most unexpected. The effect of 

 parallax — which Bradley was looking for 

 — would have brought the star furthest 

 south in December, not in March. The 

 times were all three months wrong. 

 Bradley examined other stars, thinking 

 first that this might be due to a movement 

 of the earth's pole. But this would not ex- 

 plain the phenomena. The true explana- 

 tion, 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 pub- 

 lished in the Philosophical Transactions 

 for December, 1728 : 



When the year was completed, I began to ex- 

 amme and compare my observations, 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 a nutation of the earth's axis. The next thing 

 that offered itself, was an alteration in the direc- 

 tion of the plumb-line, with which the instrument 

 was constantly rectified; but this upon trial proved 

 insufiicient. Then I considered what refraction 

 might do, but here also nothing satisfactory oc- 

 curred. At last I conjectured that all the phe- 

 nomena hitherto mentioned, proceeded from the 

 progressive motion of light and the earth's annual 

 motion in its orbit. For I perceived 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 direc- 



tion, than that of the line passing through the 

 eye and the object; and that, when the eye is mov- 

 ing in different directions, the apparent place of 

 the object would be different. 



This wonderful discovery of the aberra- 

 tion of light is usually elucidated by the 

 very homely illustration of how an um- 

 brella is held in a shower of rain. Suppose 

 the rain were falling straight down and a 

 man walking round a circular track: he 

 always holds the umbrella a little in front 

 of him — because when he is walking north- 

 ward the rain appears to eome a little from 

 the north, when he is going eastward it ap- 

 pears to come a little from the east, and 

 so on. 



Although the phenomena Bradley had 

 observed were almost wholly explained in 

 this way, there were still some residual 

 changes, which took nineteen years to un- 

 ravel; and he explained these by a nuta- 

 tion or small oscillation of the earth's axis, 

 which took nineteen years to complete its 

 period. I can not dwell on these two great 

 discoveries. For our present purpose, it 

 should be said that aberration and nutation 

 cause far greater changes in the apparent 

 positions of the stars than, we now know, 

 are caused by parallax. Until they were 

 understood and allowed for or eliminated, 

 all search for parallax must have been in 

 vain. Further, Bradley's observations 

 showed that in the case of y Draconis, at 

 any rate, parallax did not displace the star 

 by so much as 1.0" from its mean position, 

 or that the star was 200,000 times as dis- 

 tant as the sun. We may say that Bradley 

 reached to just about the inside limit of the 

 distances of the nearer stars. 



Let me now try to give some idea of what 

 is meant by a parallax of 1", which corre- 

 sponds to a distance 200,000 times that of 

 the sun. Probably many of you have looked 

 at the second star in the tail of the Great 

 Bear, Mizar, it is named, and have seen 



