TIME-RECKO^'IN■G, ' 107 



The natural solar day is at one season of the year 14 minutes 32 

 seconds shorter, and at another 16 minutes 1 7 seconds longer than the 

 mean. Thus the extreme variation is half an hour and 49 seconds. 



The earth revolves in its orbit in about 365;^ days. To avoid 

 fractions of days, it has been found convenient to establish three 

 years in succession of 365 days, and each fourth year 366 days. The 

 latter are designated leap years. 



While an ordinary solar year has but 365 days, it has 366 sidereal 

 days. 



A solar day, therefore, exceeds the length of a sidereal by about 

 s-g^th part of a day, or nearly four minutes (3 minutes 55.9094 

 seconds). 



The mean solar day, according as it is employed for civil or astro- 

 nomical purposes, is designated the civil day, or the astronomical day. 

 The former begins and ends at midnight ; the latter commences and 

 ends at noon. The astronomical day is understood to commence 

 twelve hours before the civil day, but its date does not appear until 

 its completion, twelve hours after the corresponding civil date. The 

 two dates, therefore, coincide only during the later half of the civil 

 and the earlier half of the astronomical day. 



ANCIENT AND MODERN RECKONING OF TIME. 



It has been stated that all shorter periods of time than a day are 

 conventional and arbitrary, there being no measure less than a day 

 denoted by nature. The only exception is the interval marked by 

 the rising and setting of the sun ; a period of time varying with the 

 latitude and changing from day to day with the seasons. 



The sub-division of the day into parts has prevailed from the 

 remotest ages ; though different nations have not agreed, either with 

 respect to the epoch of its commencement, the number of the sub- 

 divisions, or the distribution of the several parts. 



The division of the day with which we are most familiar is that 

 which separates the whole space of time occupied by a diurnal revo- 

 lution of the earth into two equal parts ; one part extending from 

 midnight to noon, the other part from noon to midnight. These 

 half days are sub-divided into twelve portions or hour.s, and these 

 again into minutes and seconds. 



Astronomers do not divide the day into two sets of twelve hours. 

 The astronomical day, extending from noon to noon, is reckoned by 

 hours runuin<jr from one to twenty-four. 



