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SIE FRANK W. DYSON^ M.A.^ F.R.S., ON 



positions that the stars ought to have quite different aspects. 

 You can illustrate this for yourselves very easily from any point 

 where you have a view of objects at different distances. If 

 you change your position by a few yards the nearer objects 

 are seen projected differently against the more distant 

 landscape. In the slide on the screen, for example, there are 

 shown two rough views of Edinburgh from different parts of the 

 grounds of the observatory, which is, say, about two miles away. 

 For example, in the picture on top the chimney in front is 

 shown to the right of the spire of St. Giles' Cathedral and in 

 the picture below it appears to the left. Again the Grange 

 Church spire appears to the right of the Castle in one picture, 

 and to the left in the other. Surely, said the opponents of the 

 Copernican system, we ought to see similar effects among the 

 stars : the stars nearest to us ought to shift their positions 

 relatively to more distant ones. This was perfectly sound argu- 

 ment : it admitted of only one reply, namely, that the stars are 

 at such great distances that these changes of position are too 

 small to be perceived by us. We have all grown up with the 

 idea of the great distances of the stars, and perhaps do not fully 

 perceive how great this difficulty was to the astronomers of the 

 I7th century. They were convinced that Galileo and Copernicus 

 were right, but for two centuries the}^ looked in vain before 

 they found the changes for which they were in search. This is 

 not surprising, for the nearest star, we know now, is more than 

 250,000 times as far away as the sun. Suppose ourselves at 

 King's Cross Station, and let us represent the distance from the 

 earth to the sun by half of the distance between the railway 

 lines. That is, supposing we are looking northwards, in January 

 we look along the line nearer to the platform and in July along 

 the line further from the platform. If instead of being parallel 

 the lines met somewhere between Grantham and Doncaster, we 

 should have drawn to scale the lines from the earth to the nearest 

 star as seen by us from two opposite sides of the sun. Perhaps 

 it is not surprising that it took astronomers and instrument 

 makers two centuries before they could measure angles with 

 sufficient accuracy. 



Another way of looking at the matter may show you wdiat a 

 difficult task was in front of astronomers The diameter of the 

 sun is 30'. The nearest star to us is at such a great distance that 

 the change of its position amounts to only xtW P^^'^ ^^^i^- 

 Before any attempt could be successful, it was necessary that 

 astronomical instruments should be improved to such an extent 

 that this email angle could be appreciated and measured. 



