July 2, 1915] 



SCIENCE 



15 



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. He remarks : 



What shall we say then? It is scarce possible 

 that the antients could be deceived in so plain a 

 matter, three observers confirming each other. 

 Again these stars being the most conspicuous in 

 heaven are in all probability nearest to the earth, 

 and if they have any particular motion of their 

 own, it is most likely to be perceived in them, 

 which in so long a time as 1800 years may show 

 itself by an alteration of their places, though it be 

 utterly imperceptible in a single century of years. 



This is the first good evidence, i. e., evi- 

 dence which we now know to be true, that 

 the so-called fixed stars are not fixed rela- 

 tively to one another. It is the first posi- 

 tive proof that the distances of the stars are 

 sensibly less than infinite. This, then, is 

 the stage at which astronomers had arrived 

 less than two hundred years ago. The 

 stars are at least 20,000 or 30,000 times as 

 distant as the sun, but three of the brightest 

 of them are perceived to be not infinitely 

 distant. 



The greatest step in the determination of 

 stellar distances was made by another Ox- 

 ford astronomer, James Bradley. His un- 

 paralleled skill as an astronomer was early 

 recognized by Halley, who tells how 



Dr. Pound and his nephew, Mr. Bradley, did, 

 myself being present, in the last opposition of the 

 sun and Mars this way demonstrate the extreme 

 minuteness of the sun's parallax and that it was 

 not more than 12 seconds nor less than 9 seconds. 



Translated from astronomical language, 

 the distance of the sun is between 95 and 

 125 millions of miles. Actually the dis- 

 tance is 93 million miles. The astronomer 

 who so readily measured the distance of 

 the sun entered on the great research 

 which had baffled his predecessors — the dis- 

 tance of the stars. 



The theory of the determination of stellar 

 parallax is very simple : the whole difficulty 

 lies in its execution, because the angles are 

 so small that the slightest errors vitiate the 

 results completely. Even at the present 

 time with large telescopes, and mechanism 

 which moves the telescope so that the diur- 

 nal movement of the stars is followed and 

 they appear fixed to the observer in the 

 field of the telescope, and with the addi- 

 tional help of photography, the determina- 

 tion of the parallax of a star requires a 

 a good deal of care, and is a matter of great 

 delicacy. But in Bradley's time telescopes 

 were imperfect, and the mechanism for 

 moving them uniformly to follow the diur- 

 nal rotation of the stars had not been 

 devised. 



This was in some ways very fortunate, as 

 the method Bradley was forced to adopt 

 led to two most important and unexpected 

 discoveries. Every day, owing to the 

 earth's rotation, the stars appear to de- 

 scribe circles in the sky. They reach the 

 highest point when they cross the meridian 

 or vertical plane running north and south. 

 If we leave out all disturbing causes and 

 suppose the earth's axis is quite fixed in 

 direction, a star S, if at a great distance 

 from the earth, will always cross the merid- 

 ian at the same point S ; but, if it is very 

 near, its movement in the small parallactic 

 ellipse will at one period of the year bring 

 it rather north of its mean position and at 

 the opposite period an equal amount south. 



Bradley, therefore, designed an instru- 

 ment for measuring the angular distance 

 from the zenith, at which a certain star, y 

 Draconis, crossed the meridian. This in- 

 strument is called a zenith sector, and is 

 shown in the slide. The direction 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. From December, 1725, to March, 



