174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



The period which we have now reached is so admirably treated 

 by Sir Frank W. Dyson, Astronomer Koyal, in his Halley lecture 

 delivered at Oxford on May 20, 1915, that I ask your indulgence 

 while I quote rather freely from that source : 



Thus in Halley's time it was fairly well established that the stars were at 

 least 20,000 or 30.000 times as distant as the sun. Halley did not succeed in 

 finding their range, but he made an important discovery which showefl that 

 three of the stars were at sensible distances. In 1718 he contributed to the 

 Royal Society a paper entitled " Considerations of the Change of the Latitude 

 of Some of the Principal Bright Stars." While pursuing researches on another 

 subject he found that the three bright stars — Aldebaran, Sirius, and Arcturus — 

 occupied positions among the other stars differing considerably from those 

 assigned to them in the Almagest of Ptolemy. He showed that the possibility 

 of an error in the transcription of the manuscript could be safely excluded, and 

 that the southward movement of these stars to the extent of 37', 42', and 33' — 

 i. e., angles larger than the apparent diameter of the sun in the sky — were 

 established. * * * 



This is the first good evidence — i. e.. evidence which we now know to be 

 true — that the so-called fixed stars are not fixed relatively to one another. It 

 is the first iiositive proof that the distances of the stars are sensibly less than 

 infinite. 



At the time of the appearance of Halley's paper there was coming 

 into notice a young astronomer, James Bradley, then 26 years old. 

 He was admitted to membership in the Royal Society the same year 

 that Halley's paper was presented. He was exceedingly eager to 

 attack the problem of the distances of the stars. At length the 

 opportunity presented itself. To quote again from Sir Frank 

 Dyson : 



Bradley designed an instrument for measuring the angular distance from the 

 zenith, at which a certain star. 7 Draconis, crossed the meridian. Tliis in- 

 strument is called a zenith sector. The direction of the vertical is given by a 

 plumb line, and he measured from day to day the angular distance of the 

 star from the direction of the vertical. From December, 1725, to March, 1726, 

 the star gradually moved farther 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 re:iched its old position in 

 December, The movement 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 

 farthest 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 woiild not explain the phenomena. 

 The true explanation, it is said, although I do not know how truly, occurred 

 to Bradley when he was sailing on the Thames and noticed that tlie direction 

 of the wind, as indicated by a vane on the masthead, varied slightly witli the 

 course on which the boat was sailing. An account vf the observations in the 



