654 POPULAR SCIENCE MONTHLY. 



extent of the solar system was supposed contrary to all probability. 

 The actual infinity of space; the consideration that one had only to 

 enlarge his conceptions a little to see spaces a thousand times the size 

 of the solar system look as insignificant as the region of a few yards 

 round a grain of sand, does not seem to have occurred to anyone. 



Considerations drawn from photometry were also lost sight of, be- 

 cause that art was still undeveloped. Kepler saw that the sun might 

 well be of the nature of a star; in fact, that the stars were probably 

 suns. Had he and his contemporaries known that the light of the sun 

 was more than ten thousand million times that of a bright star, they 

 would have seen that it must be placed at one hundred thousand times 

 its present distance to shine as a bright star. If, then, the stars are 

 as bright as the sun, they must be one hundred thousand times as 

 far away, and their annual parallax would then have been too small for 

 detection with the instruments of the time. Such considerations as 

 this would have removed the real difficulty. 



The efforts to discover stellar parallax were, of course, still con- 

 tinued. Bradley, about 1740, made observations on y Draconis, which 

 passed the meridian near his zenith, with an instrument of an accuracy 

 before unequalled. He thus detected an annual swing of 20" on each 

 side of the mean. But this swing did not have the right phase to be 

 due to the motion of the earth; the star appeared at one or the other 

 extremity of its swing when it should have been at the middle point, 

 and vice versa. What he saw was really the effect of aberration, 

 depending on the ratio of the velocity of the earth in its orbit to the 

 velocity of light. It proved the motion of the earth, but in a different 

 way from what was expected. All that Bradley could prove was that 

 the distances of the stars must be hundreds of thousands of times that 

 of the sun. 



An introductory remark on the use of the word parallax may preface 

 a statement of the results of researches now to be considered. 



In a general way, the change of apparent direction of an object 

 arising from a change in the position of an observer is termed parallax. 

 More especially, the parallax of a star is the difference of its direction 

 as seen from the sun and from that point of the earth's orbit from 

 which the apparent direction will be changed by the greatest amount. 

 It is equal to the angle subtended by the radius of the earth's orbit, as 

 seen from the star. The simplest conception of an arc of one second 

 is reached by thinking of it as the angle subtended by a short line 

 at a distance of two hundred and six thousand times its length. To 

 say that a star has a parallax of 1" would therefore be the same thing 

 as saying that it was at a distance of a little more than two hundred 

 thousand times that of the earth from the sun. A parallax of one-half 

 a second implies a distance twice as great; one of one-third, three times 



