96 LENGTH OF THE CIVIL YEAR. [SECT. xn. 



of the tropical year, so that it would retrograde through the 

 different seasons in a period of about 1507 years. The 

 Egyptian year began with the heliacal rising of Sirius, and 

 contained only 365 days, by which they lost one year in 

 every 1461 years, their Sothaic period, or that cycle in which 

 the heliacal rising of Sirius passes through the whole year 

 and takes place again on the same day. The commence- 

 ment of that cycle is placed by ancient chronologists in the 

 year 1322 before the Christian era. The division of the 

 year into months is very old and almost universal. But the 

 period of seven days, by far the most permanent division of 

 time and the most ancient monument of astronomical know- 

 ledge, was used by the Brahmins in India with the same 

 denominations employed by us, and was alike found in the 

 calendars of the Jews, Egyptians, Arabs, and Assyrians. It 

 has survived the fall of empires, and has existed among all 

 successive generations, a proof of their common origin. 



The day of the new moon immediately following the 

 winter solstice in the 707th year of Rome was made the 1st 

 of January of the first year of Julius Caesar. The 25th of 

 December of his forty-fifth year is considered as the date of 

 Christ's nativity ; and the forty-sixth year of the Julian Ca- 

 lendar is assumed to be the first of our era. The preceding 

 year is called the first year before Christ by chronologists, 

 but by astronomers it is called the year 0. The astronomical 

 year begins on the 31st of December at noon ; and the date 

 of an observation expresses the days and hours which have 

 actually elapsed since that time. 



Since solar and sidereal time are estimated from the pas- 

 sage of the sun and the equinoctial point across the meridian 

 of each place, the hours are different at different places : 

 while it is one o'clock at one place, it is two at another, three 

 at another, &c. ; for it is obvious that it is noon at one part 

 of the globe at the same moment that it is midnight at an- 

 other diametrically opposite to it : consequently an event 

 which happens at one and the same instant of absolute time 



