> 16] The Measurement of Time 17 



annual journey, being nevet more than about 47 and 29 

 respectively distant from it, on either side ; while the other 

 planets are not thus restricted in their motions. 



1 6. One of the purposes to which applications of 

 astronomical knowledge was first applied was to the 

 measurement of time. As the alternate appearance and 

 disappearance of the sun, bringing wiih it light and heat, 

 is the most obvious of astronomical facts, so the day is 



I H. 40m. 20 in. XI H * nu 



FIG. '/. The apparent path of Mercury from Aug. I to Oct. 3, 

 1898. The dates printed in capital letters shew the positions 

 of the sun ; the other dates shew those of Mercury. 



the simplest unit of time.* Some of the early civilised 

 nations divided the time from sunrise to sunset and also 

 the night each into 12 equal hours. According to this 

 arrangement a day-hour was in summer longer than a 



* It may be noted that our word " day " (and the corresponding 

 word in other languages) is commonly used in two senses, either for 

 the time between sunrise and sunset (day as distinguished from 

 night), or for the whole period of 24 hours or day-and-night. The 

 Greeks, however, used for the latter a special word, vvx^fJ^pov. 



2 



20m. 



