SIDER1AL TIME. , 



CHAPTER VI. 



Dials and Dialing. 



' This shadow on the Dial's face, 

 That steals from day to day 

 With slow, unseen, unceasing pace, 

 Moments, and months, and years away, 

 Right onward, with resistless power, 

 Its stroke shall darken every hour, 

 Till Nature's race be run, 

 And Time's last shadow shall eclipse the sun. 



67 



IN the preceding chapter, we have made frequent use of the 

 word day, and have throughout meant what is called a mean Solar 

 day. We have already shown that the Siderial day is the time of 

 an exact revolution of the earth on its axis. This day is shorter 

 than the Solar day, by about 4 minutes. We have also alluded 

 to the apparent motion of the sun in the heavens, showing that if 

 to-day he came to the meridian at the same time with any particular 

 star, to-morrow the star would come to the meridian before the 

 sun, which had apparently changed its place in the heavens. Let 

 us consider to what the difference between Solar and Siderial time 

 is really owing, and see how much the Siderial day should be 



A 



shorter than the Solar, to do which we will have recourse to a 

 diagram 



