IN DESCRIPTIVE ASTRONOMY. 131 



when Julius Caesar, who possessed no little astronomical knowl- 

 edge, called to his assistance a Greek astronomer named Sosig- 

 enes, and adjusted the civil year to the astronomical year. 

 By intercalating the extra day of leap year, he introduced 

 what is known as the Julian Calendar, which is still in use. 



The Persian Calendar, invented in the eleventh century as 

 a correction of the Julian, is remarkable for its accuracy; it 

 consists in making every fourth year bissextile seven times in 

 succession, and making the change for the eighth time in the 

 fifth year instead of the fourth. This is equivalent to reckon- 

 ing the tropical year as SGS^ days, which exceeds the period 

 determined by astronomers only by 0.0001823 of a day, or 

 only | of a second, so that it would require a great number of 

 centuries to displace sensibly the commencement of the civil 

 year. 



The Gregorian Calendar usually employed is somewhat less 

 exact, but it is more easily reduced to days, years, and centu- 

 ries, which is one of the most important objects of a calendar. 

 It consists in employing a bissextile year every fourth year, 

 suppressing three bissextiles in three centuries, and replacing 

 one in the fourth. Thus in every 400 years there are reckoned 

 only 97 leap years, making the length of the year 365^, 

 which exceeds the tropical year by 0.0002581 of a day, or very 

 nearly one second. 



If, following the analogy of the Gregorian Calendar, our 

 successors shall suppress a bissextile every 4,000 years, so as to 

 make 969 instead of 970 leap years in that interval, the length 

 of the year would become 365 9 ^ days, or 365.2422500 days, 

 instead of 365.242219 days, as determined by observation. 

 Haughton's Astronomy. 



84. How much is the Russian reckoning of time be- 

 hind ours ? 



The Russian reckoning is twelve days behind us. 



85. Is there any gain in having the astronomical and 

 the calendar year agree ? 



It is difficult to show what practical object is attained by 

 such coincidence. It is important that summer and winter, 



