April 7, 1916] 



SCIENCE 



479 



tion of the earth must produce an annual 

 parallax of the stars. Tycho Brahe re- 

 jected the Copernican system because he 

 could not find from his observations any 

 such parallax. However, the system was 

 generally accepted as the true one and the 

 determination of stellar parallax or the dis- 

 tance of the stars became a live subject. 

 Picard in the latter half of the seventeenth 

 century, using a telescope and a micrometer 

 in connection with his divided circle, showed 

 an annual variation in the declination of 

 the pole star amounting to 4'0". In 1674 

 Hooke announced a parallax of 15" for 

 y Draconis. About this same time Flam- 

 steed announced a parallax of 20" for- 

 a Ursae Minoris, but J. Cassini showed that 

 the variations in the declination did not 

 follow the law of the parallax. 



The period which we have now reached is 

 so admirably treated by Sir Frank W. 

 Dyson, Astronomer Royal, 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 estab- 

 lished that the stars were at least 20,000 or 30,000 

 times as distant as the sun. Halley did not suc- 

 ceed in finding their range, but he made an im- 

 portant discovery which showed' that three of the 

 stars were at sensible distances. In 1718 he con- 

 tributed to the Eoyal Society a paper entitled 

 "Considerations of the Change of the Latitude of 

 Some of the Principal Bright Stars. ' ' While pur- 

 suing researches on another subject, he found that 

 the three bright stars — ^Aldebaran, Sirius and 

 Areturus — 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 ex- 

 eluded, 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 positive 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 twenty-six 

 years old. He was admitted to membership 

 in the Roj^al Society the same year that 

 Halley's paper was presented. He was ex- 

 ceedingly 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, y Draconis, crossed the meridian. 

 This instrument is called a zenith sector. The di- 

 rection of the vertical is given by a plumb-line, 

 and he measured from day to day the angular dis- 

 tance of the star from the direction of the vertical. 

 Prom December, 1725, to March, 1726, the star 

 gradually moved further south; then it remained 

 stationary for a little time; then moved north- 

 wards until, by the middle of June, it was in the 

 same position as in December. It continued to 

 move northwards until the beginning of Septem- 

 ber, then turned again and reached its old position 

 in December. The movement was very regular and 

 evidently not due to any errors in Bradley's ob- 

 servations. 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 move- 

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

 plain 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 the direction of the wind, as indi- 

 cated 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 Philo- 

 sophical Transactions for December, 1728: 



' ' When the year was completed, I began to ex- 

 amine 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 the nutation of the earth's a.xis. The next 



