vi INQUIRY AND INTERPRETATION 149 



the dimension which has to be measured in order to 

 find the distance of the nearest star. 



The minuteness of the angle may be understood by 

 considering the movement of the minute hand of a 

 clock or watch. If the hand moves at a perfectly 

 uniform rate there will be a difference of position at the 

 beginning and end of a second or any smaller interval 

 of time. The angle between two positions separated 

 by about the five-hundredth part of a second represents 

 the angle due to the movement of the earth around the 

 sun when the nearest star is observed ; and this notwith- 

 standing the fact that the earth is ninety-three millions 

 of miles from the sun, so that by its revolution around 

 the sun its position with regard to the stars is changed 

 by nearly two hundred million miles. 



Using an illustration due to Prof. E. C. Pickering, the 

 apparent change of position of the nearest star, as the 

 earth moves to points nearly two hundred million miles 

 apart, is equal to the height of a man at a distance of 

 two hundred miles. In other words, the problem of 

 determining the apparent displacement due to change 

 of the earth's position in its orbit is like measuring the 

 height of a man two hundred miles away. So accurate 

 are the methods now used by astronomers that the 

 height of the man could be determined with an uncer- 

 tainty of only one inch. Yet in spite of the marvellous 

 precision with which measurements can be made, only 

 about forty stars have been found near enough to our 

 system for any angular displacement to be detected 

 sufficiently large to admit of the determination of 

 their distances. Few stars are less than a million 

 times farther from us than the sun, and most of them 

 are probably thousands of times more distant than 

 the nearest. 



