106 TIME-RECKONING. 
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 arbitrary, 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 are 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 measure of time afforded 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 he 
indicated by ordinary clocks and watches. 
