ON ASTEONOMY. 



129 



But the foregoing result was not, perhaps, fully concurred in by 

 astronomers till Dr. Bradley, in 1725, discovered the aberration of 

 light, one of the most beautiful discoveries in the whole history of 

 science. Dr. Bradley, while prosecuting researches for the purpose of 

 detecting, if possible, the annual parallax of the stars, found that all 

 the stars were subject to an annual change of position, and that they 

 described orbits about their mean places having 40" of arc for their 

 major axis. Those situated in the ecliptic moved backward and forward 

 in a straight line. In proportion as they are removed from the ecliptic 

 the minor axes of the orbits increased, while the major axes of all 

 remain the same. And, what is quite singular, they are all displaced 

 or projected forward in the direction of that point in the heavens 

 towards which the earth is at the moment moving. The true expla- 

 nation of this fact was far from being obvious. It was at length sug- 

 gested to Dr. Bradley, on an occasion of being rowed across the 

 Thames. He observed that the streamer, at the top of the mast, indi- 

 cated a change in the direction of the wind while the boat was getting 

 under way. He inquired of the boatmen what made the wind change 

 when the boat began to move ? They could give no explanation, but 

 said they had often observed it. But, on reflecting upon the subject. Dr. 

 Bradley explained it on the principle of the parallelogram of forces 

 The streamer would necessarily take the direction of the diagonal of a 

 parallelogram which should have for one side the velocity of the 

 wind, and for the other the velocity of the boat. This at once 

 explained, also^ the displacement of the star. It must appear in the 

 direction of the diagonal of a parallelogram which should have for 

 one side the velocity of the earth, and tor the other the velocity of 

 light. 



E- 



In figure 16, if E B represents that portion of the earth's orbit 

 which is described in one second of time, and E S the distance traversed 

 by light in one second of time, then the star which is really at S will 

 appear to be at S', determined by the parallelogram formed on E B 

 and E _S as sides. The angle S B S', which measures the displace- 

 ment, is called the aberration, and when largest is always equal to 

 20" of arc. ^ "With every star this maximum occurs twice in the 

 year, at the time when the line of direction of the earth makes a right 

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