16 Astronomy. [Lecture 23. 



passed through the twelve signs of the zodiac. 

 The year, therefore, consists of 365 days and 

 about .. It was at first estimated at only 365 

 days. But as it was found that, while the earth 

 makes a complete revolution in its orbit, it makes 

 with respect to the sun 365 turns and nearly f 

 upon its own axis, which makes the year to in- 

 clude 365 days and six hours, and also that the 

 equinoxes receded about the value of a day in 

 every four years ; to remedy this inconvenience, 

 was agreed to employ the six supernumerary or 

 intercalary hours in giving an additional day to 

 every fourth year, which on that account is called 

 leap year ', and contains 366 days. 



The year, however, consists only of 365 days, 

 5 hours, 48 minutes, and 45^ seconds. In adding, 

 therefore, to every fourth year a whole day, we 

 give 11 minutes and 14^- seconds too much. 

 This quantity, though small in itself, must in the 

 course of many years amount to a considerable 

 sum. In the pontificate of Gregory XIII, the 

 equinoxes had advanced ten days, so that the 

 vernal equinox, instead of the 20th, fell on the 

 30th of March. This irregularity caused much 

 inconvenience with respect to the festivals of the 

 church. The pope, therefore, with the, advice of 

 the ablest astronomers, reformed the calendar. 

 Having calculated that the surplus time, viz. 

 eleven minutes and fourteen seconds, which had 

 been added to the leap years, must amount to a 

 whole day in 133 years, and that, consequently, 



