THK DISTANCES OP THE STARS. 



293 



Before the invention of the telescope, such a thing was quite 

 impossible. The greatest of astronomical observers before its 

 invention, Tycho Brahe, could measure angles of about 1'. Of 

 course, he did not know how very distant the stars were. He 

 tried, but could find no trace of movement, and even concluded 

 that the earth did not go round the sun. But the telescope has 

 increased our faculty of vision in at least three ways. It not 

 only enables us to see fainter objects, but it also magnifies the 

 small angles we have to measure, and thus makes it possible to 

 measure with far greater accuracy. Further, it made possible 

 a method of sighting vastly superior to anything that had been 

 available before its invention. And so after the time of Galileo, 

 when astronomers were convinced that the earth did travel round 

 the sun, they tried with more and more persistence to discover 

 the movement in the stars which would be a consequence of such 

 a movement. Hooke, a contemporary of Kewton and Wren, fixed 

 a long telescope, 36 feet long, in a vertical position and examined 

 a star called 7 Draconis, which passes near the zenith in the 

 latitude of London. Tlie idea was excellent, because it got rid 

 of the troublesome, and at that time uncertain, effect of the 

 refraction of light by our atmosphere. But Hooke did not 

 succeed. 



A great Danish astronomer, Eomer of Copenhagen, made an 

 attempt to find the distances of the two night stars, Sirius and 

 Yega. He found a change in the relative positions of these stars 

 in the spring and the autunm amounting to 1' of arc. He was 

 delighted with his success and published it in a dissertation 

 called " Copernicus Triumphans." But he was wrong, and 

 probably the error arose from small irregularities of his clock, 

 which was not compensated for changes of temperature. Con- 

 sequently he made errors in his determination of the times at 

 which Sirius and Vega were observed to be due south. 



The next attempt to which I will refer was made by Bradley 

 at Wanstead about 1750. He fixed his telescope in a vertical 

 position as Hooke had done, and observed 7 Draconis at the 

 times when it passed the meridian. By means of a plumb line 

 he determined the vertical, and with his long telescope measured 

 how far 7 Draconis was south of the zenith. The instrument 

 he used, called a zenith section, is still preserved at Greenwich. 

 He watched 7 Draconis from day to day for a year, and found a 

 real movement. But it was in the opposite direction to what 

 he anticipated. However, he succeeded in explaining the move- 

 ment. It was due to the fact that though light travels very fast, 

 it is only 10,000 times as fast as the earth's velocity round the 



