106 TIMEKECKONING. 



the lunar month, and the day. All other divisions of time, as the 

 civil month, the week, the hour, the minute, and the second, although 

 long in general use, are arbitraiy, conventional and artificial. 



The employment of the lunar month for reckoning time is not 

 general, although some nations, such as the Turks, Jews and Chinese, 

 have preferred a lunar chronology. In China the age of the moon 

 and the day of the month ai-e identical. 



The period measured by the diurnal movement of the earth on its 

 own axis constituted the first space of time reckoned by the human 

 race, and is undoubtedly the most important to man in all stages of 

 civilization. It involves the most familiar phenomena of light and 

 darkness, and embraces the constantly recurring periods of wakeful- 

 ness and sleep, of activity and rest. 



A day is the shortest measux'e of time aflforded by nature. It is 

 denoted by the revolution of the earth, and although the motion of 

 the earth is uniform, we have three kinds of natural days all varying 

 in length — the solar, lunar, and sidereal. 



A solar day is the period occupied by a single revolution of the 

 earth on its axis in relation to the sun. 



A lunar day is the interval of time occupied by a revolution of the 

 earth on its axis in relation to the moon. 



A sidereal day is the period required for a complete revolution of 

 the earth on its axis in relation to any one fixed star. 



Of these three natural days, the sidereal day is the only one uniform 

 in length. The lunar day, on account of the irregular and complicated 

 motion of the moon in the heavens, is never employed as a measure of 

 time. The solar day is variable in length on account of the ellipticity 

 of the earth's orbit. Solar time is that shown by a sun-dial. 



Although the sidereal day is uniform in length, inasmuch as it 

 has no relation to the daily return of light and darkness, it is not 

 employed for civil purposes. The commencement of the sidereal day 

 is constantly changing throughout the year ; at one period it comes 

 at midnight, at another period at high noon. 



It has been found convenient, therefore, to establish an artificial 

 day, uniform in length, designated the mean solar day. 



The mean solar day, as its name implies, is the average length of 

 all the natural solar days in a year, and is the time intended to be 

 indicated by ordinary clocks and watches. 



