168 eichelberger: distances of heavenly bodies 



ture 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 dis- 

 covery which showed that three of the stars were at sensible distances. 

 In 1718 he contributed to the Royal Society a paper entitled Con- 

 siderations of the Change of the Latitude of Some of the Principle Bright 

 Stars. While pursuing researches on another subject, he found that the 

 three bright stars — Aldebaran, Sirius, and Arcturus — occupied posi- 

 tions 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- 

 cluded, 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 an- 

 other. 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 

 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, y Draconis, crossed the merid- 

 ian. This instrument 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 

 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 De- 

 cember. 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 



