to A Short History of Astronomy [CH. I. 



noticed a few days later, they are found to be higher up 

 in the sky, and their place^ is taken by other stars at 

 first too low down to be seen. Such observations of 

 stars rising or setting about sunrise or sunset shewed to 

 early observers that the stars were gradually changing their 

 position with respect to the sun, or that the sun was 

 changing its position with respect to the stars. 



The changes just described, coupled with the fact that 

 the stars do not change their positions with respect to one 

 another, shew that the stars as a whole perform their daily 

 revolution rather more rapidly than the sun, and at such a 

 rate that they gain on it one complete revolution in the 

 course of the year. This can be expressed otherwise in 

 the form that the stars, are all moving westward on the 

 celestial sphere, relatively to the sun, so that stars on the 

 east are continually approaching and those on the west 

 continually receding from the sun. But, again, the same 

 facts can be expressed with equal accuracy and greater 

 simplicity if we regard the stars as fixed on the celestial 

 sphere, and the sun as moving on it from west to east 

 among them (that is, in the direction opposite to that of 

 the daily motion), and at such a rate as to complete a 

 circuit of the celestial sphere and to return to the same 

 position after a year. 



This annual motion of the sun is, however, readily seen 

 not to be merely a motion from west to east, for if so the 

 sun would always rise and set at the same points of the 

 horizon, as a star does, and its midday height in the sky 

 and the time from sunrise to sunset would always be the 

 same. We have already seen that if a. star lies on the 

 equator half of its daily path is above the horizon, if 

 the star is north of the equator more than half, and if south 

 of the equator less than half; and what is true of a star is true 

 for the same reason of any body sharing the daily motion of 

 the celestial sphere. During the summer months therefore 

 (March to September), when the day is longer than the night, 

 and more than half of the sun's daily path is above the 

 horizon, the sun must be north of the equator, and during 

 the winter months (September to March) the sun must be 

 south of the equator. The change in the sun's distance 

 from the pole is also evident from the fact that in the winter 



