COMMON THINGS TIME. 



the displacement of the equinoctial points, the commencement of 

 the equinoctial anticipates that of the sidereal year ; the extent 

 of this anticipation, though very small at first, accumulating for 

 long series of years, causes the seasons to take place successively 

 at all imaginable parts of the sidereal year. 



For these reasons the sidereal year has never been adopted as 

 the civil year. 



If the annual displacement of the equinoctial point were 

 regular and constant, the precession of the equinoxes would bo 

 also constant ; and the equinoctial year, differing from the side-- 

 real year by an invariable quantity, would itself be invariable, and 

 as it is in accordance with the succession of the seasons, it would 

 be in all respects eligible as a standard measure of civil time. 



But it so happens that this displacement is rendered variable by 

 the operation of several causes. Its variations, however, are 

 circumscribed within narrow limits. It alternately increases and 

 decreases, and has a certain ascertainable average amount. On 

 account of this variation, the equinoctial year is of slightly variable 

 length, and is therefore not fit for a standard measure of time. 



98. This being the case, and the mean annual displacement of 

 the equinoctial point being accurately ascertained, a fictitious 

 equinoctial point is supposed to exist, which has this mean annual 

 displacement, and the interval between two successive returns of the 

 sun to this fictitious equinoctial point being invariable, is adopted 

 as the standard, and is called the MEAN SOLAR or CIVIL YEAR. 



Although rigorously this year does not therefore correspond 

 with the returns of the seasons, it never varies from them by any 

 interval great enough to be perceived or appreciated by any but 

 astronomers. 



The exact length of the mean solar or civil year is 



365 d 5 h 48 m 49 s -54, 

 being less than the sidereal year by 20 M 20 s -8. 



99. Such being then the actual length of such a year as would 

 always remain in accordance with the successive returns of the 

 seasons, let us see to what extent the year of the Julian calendar 

 differs from it, and how such difference would affect chronology. 



The Julian year being 365| days, the difference between it and 

 the mean solar year is easily found. 



D. H. M. S. 



Julian year 365 6 O'OO 



Mean solar year . .. 365 5 48 49 '54 



Difference . . . . 11 10-46 



100. It appears, therefore, that the Julian years would depart 

 from the course of the seasons at the rate of ll m 10"4G, or about 

 168 



