176 SIR DAVID GILL^ ON THE SIDEEEAL UNIVEliSE. 



each other. We know nowadays that the stars not only all 

 move, but that if they had been originally created at rest they 

 would now be in motion in consequence of their mutual attrac- 

 tions. Tobias Mayer in 1760, was the first to recognise that if 

 our sun moved amongst the stars, tlie mere effect of that motion 

 must be to produce apparent motions amongst the stars. He 

 used the illustration, and a very excellent one it is, that if a man 

 walks through a wood not too closely planted with trees, he 

 would observe that all the trees in front of him appear to open 

 out as he proceeds, that those he leaves behind will close behind 

 him, that those near him to the right and left will appear to 

 move backwards as he moves forward. If the sun moves, then 

 the stars right and left of the direction of the sun's motion must 

 appear to move in an opposite direction to the sun ; those stars 

 in front must be opening outwards and those behind closing up. 

 This will be easier to realise if I draw your attention to fig. 1 

 (p. 189). In this figure, if the line AB is taken to represent the 

 movement of the sun and the crosses represent stars about the 

 same distance from the sun in various directions, then the ap- 

 parent movement of each star as the sun passes from A to B will 

 be represented by the dotted lines. Those stars at right-angles to 

 the direction of the sun's motion would apparently move the 

 fastest (that is, through the greatest angles), while for those stars 

 more in front and behind the resulting motion would be 

 smaller. It is evident also that as the sun approaches a group 

 of stars they open out, while those left behind close up. fig. 1 

 gives you a good idea of what would take place if you could see 

 certain stars now, and again at a sufficiently later point of time, 

 But, in order to enable you to realise at all what these move- 

 ments are, it has been necessary to draw this figure on an enor- 

 mous scale of time. Suppose that the stars indicated by a cross 

 are distant from the sun by the average distance of the first 

 magnitude stars, then it would take half a million years to 

 produce the changes in the apparent directions which are shown 

 in the figure. If these stars were at the average distance of 

 9th magnitude stars, it would require five million years to 

 produce a like change. 



The first observations, sufficiently accurate for determining 

 the proper motions of a number of stars, were made by Bradley 

 150 years ago, and have been made unceasingly ever since. 

 Now, although it would be easy to find the direction of the sun's 

 motion through space, if the stars were fixed as they are repre- 

 sented on fig. 1, you can easily realise what a much more com- 

 plicated matter it must be to find the sun's motion through space 



