Sir Frank Dyson 87 



seventy-five to seventy-six years. He therefore predicted 

 its return in 1758 or early in 1759. The prediction was a 

 memorable one, because it was the first attempt to foretell 

 the appearance of one of these mysterious bodies, whose 

 visits seemed guided by no fixed law, they being always 

 regarded as visions of awful import. On Christmas Day 

 1758 the comet was detected, and in the following March 

 each night was lighted by its flaming splendour. 



Halley remained at his post until his death, and was 

 succeeded by James Bradley, already known through his 

 efforts to fix the distance of the sun from the earth. In 

 1719 he was convinced that it could not be more than 

 one hundred and twenty-five millions or less than ninety- 

 four millions of miles. This lower limit has since been 

 proved to be almost exact. But Bradley's greatest dis- 

 covery was what is called the ' aberration of light/ In 

 1667 Roemer, a Danish astronomer, had discovered that 

 light does not travel instantaneously from place to place. 

 Aberration is an apparent alteration in the position of a 

 fixed star, arising from the motion of the earth in its 

 orbit, combined with the time taken for light to travel. 



You can look at it in this way. When rain is falling 

 straight down, a drop entering the top of a stationary 

 tube goes right through and comes out at the bottom. 

 But if the tube be carried forward, still in the same 

 upright position, a drop entering the top will strike the 

 side a little way down. Bradley's great discovery was 

 that light from a star acts in similar fashion. 



Bradley did an immense amount of valuable work at 

 Greenwich. He observed the positions of more than three 

 thousand stars, he determined the exact longitudes of 

 Lisbon and New York, and his last work was the obser- 

 vation of the transit of Venus (the passage of the planet 

 Venus across the disc of the sun) in 1761. 



The next Astronomer Royal of note was Nevil Mas- 

 kelyne, who was an ancestor of the well-known conjurer 



