ON PRACTICAL ASTRONOMT. 539 



very little change of declination ; whence the time when tlie sun touches 

 either tropic is called the solstice. The vicissitudes of light and darkness 

 depending also on the solar day, it is necessary, for the regulation of the 

 civil calendar, to establish the proportion between the periods of the solar 

 dav and the tropical year; and since the tropical year exceeds the time of 

 365 days, by 5 hours, 48 nnnu*-es and 48 seconds, it is usual to add to the 

 common year an intercalary day once in about four years. The ancient 

 Egyptians reckoned only 365 days in a year, and their nominal new year 

 arrived continually earlier and earlier, so that after 1507 of their years, it 

 would have happened successively on each of the 365 days, and returned to 

 the original place: the same mode of computation was also adopted by 

 the Greek astronomers. The Romans inserted intercalary days, at first with- 

 out much regularity, according to the direction of their augurs, until the 

 time of Julius Caesar; who, observing that the year was almost 6 hours 

 longer than 365 days, added a day every fourth year to the calendar, by 

 reckoning twice the day in February called sexto calendas Martias, whence 

 this year of 366 days was denominated a bissextile year. The new moon 

 immediately following the winter solstice, in the 707th year of Rome, was- 

 made the first of January of the first year of Caesar; the 25th of December 

 in his 45th year is considered as the date of the Nativity of Christ, and 

 Caesar's 46th year is reckoned the first of our era. The preceding year is- 

 commonly called by astronomers the year 0, but by chronologists the year 

 before Christ. The correction introduced by Caesar was, however, t 

 great, the error being exactly 7 days in 900 years; so that in 1582 it amount- ■ 

 ed to about 12 days. This error was not wholly removed by Pope Gregory, 

 who reformed the calendar; he omitted 10 days only of the usual reckoning', 

 in order to bring back the course of the moveable feasts to the same state, in 

 which they had been established by the Nicene council, in the fourth cen- 

 tury. He determined at the same time that the last year of every century 

 should be passed without an intercalary day, excepting that of every fourth 

 century, which was still to be bissextile. Thus every year divisible by four, 

 without a remainder, is in general a bissextile or leap year, but the last 

 year of every century must be a common year, unless the number of the 

 century be divisible by 4; the year 1800 being a common year, and 2000 a 

 bissextile. In this manner 27 Julian bissextiles are omitted in 3600 years,, 

 while the true length of the year would require tlie omission of 28; but so» 



