136 LIGHT SCIENCE FOR LEISURE HOURS. 



length of time occupied by Uranus in circling round 

 his orbit, the astronomer labours under a difficulty 

 distinct in character from the difficulties which have 

 already been considered. As Jupiter and Saturn circle 

 on their wide orbits they exhibit to us the former 

 in the course of eleven years, the latter in the course 

 of twenty-nine and a half years all those varying pre- 

 sentations which correspond to the seasons of these 

 planets. Jupiter, indeed, owing to the uprightness of 

 his axis (with reference to his path) presents but slight 

 changes. But Saturn's globe is at one time bowed 

 towards us, so that a large portion of his north polar 

 regions can be seen, and anon (fifteen years later) is so 

 bowed, that a large portion of his southern polar 

 regions can be seen ; while between these epochs we 

 see the globe of Saturn so posed that both poles are 

 .on the edge of his disc, and then only does the shape 

 of his disc indicate truly the compression or polar 

 flattening of the planet. 



But although similar changes occur in the case of 

 Uranus, they occupy no less than eighty-four years in 

 running through their cycle, or forty-two years in com- 

 pleting a half cycle during which, necessarily, all 

 possible presentations of the planet are exhibited. 

 Now it is commonly recognised among telescopists that 

 the observing time of an astronomer's life that is, the 

 period during which he retains not merely his full skill, 

 but the energy necessary for difficult researches 

 continues but about twenty-five years at the outside. 

 So that few astronomers can hope to study Uranus in 



