4 BROWN : 



moon cannot happen before the 21st of March, and Easter 

 therefore cannot be earlier than the 22nd of March. 



Should the full moon happen on the 20th of March, then 

 the following would be the Paschal moon, but the full of this 

 moon falls 29 days after the 20th of March, or on the i8th of 

 April. Now if the i8th be a Sunday, then Easter is the fol- 

 lowing Sunday, 25th April, which is the latest date on which 

 Easter can fall. 



How mau}^ days, then, after the 21st of March will Easter 

 occur ? 



First, there will elapse a number of days from the 21st of 

 March to the 15th of the Paschal moon, which is the first daj^ 

 on which Easter can fall. 



Next there will elapse between the 15th of the Paschal 

 moon and Easter Sunday a number of days which is either o 

 or less than seven. Add this number to the above and get the 

 number of days from 21st of March to Easter. 



This last is called the Number of Direction ; and a step in 

 advance has been made, for it is now perfectly evident that we 

 must know how old the moon was on the 21st March in order 

 to get any farther. 



If there were means of knowing how old the moon was on 

 any given date, sa}' the ist of Januaiy, her age on the 21st of 

 March could at once be determined. 



A long step toward this determination was made in the 

 year 432 B. C. The great Greek astronomer Meton then dis- 

 covered that, given the relative position of the moon and the 

 earth and the sun on an}- day, that same relative position 

 would recur nineteen years ( nearly ) later. In fact 235 com- 

 plete lunations occupy nineteen years within about 90 minutes. 



This group of years is of course called the Metonic Cycle. 



The discovery' has been in full use ever since it was made ; 

 and one of the periods began with the first year before the 

 Christian Era. That is to say, the moon was new on the ist 

 of Januaiy of the year which is sometimes called the year '' o." 

 If then we add unitv to anv date of the Christian Era and 



