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A. M. W. DOWNING, H.A., D.SC, F.K.S., OX 



year should be adopted. The Pope referred the matter to a 

 commission, the principal member of which was a German 

 Jesuit named Schliissel, better known by his Latinised name of 

 Clavius. It was decided, in order to bring up the spring equinox 

 to what was considered to be the proper date, that the day after 

 October 4, 1582, should be called October 15, and in order to 

 correct for the assumed error in the length of the mean Julian 

 year, of three days in 400 years, that the centennial years 

 should be counted as leap years only when the number of 

 centuries is divisible by four. Thus the years 1700, 1800, and 

 1900. which in the Julian calendar are leap years, are common 

 years in the reformed calendar, whilst the year 2000 is a leap 

 year in both calendars. The Gregorian calendar was immediately 

 adopted in Eoman Catholic countries, but the old style remained 

 in force in England until 1752. The accumulation of error 

 in the Julian reckoning having by that time amounted to eleven 

 days, it was decided that the day after September 2 in that year 

 should be called September 14. It will be noted that this 

 change does not involve any change in the week-days, but only 

 in their numeration as days of the month. Wednesday, 

 September 2. was followed by Thursday, September 14. And 

 in Eussia and Greece, where the old style is still continued, the 

 day of the week is the same as with us, only the day of the 

 month is different. Thus Monday, March 15, new style, 

 corresponds to Monday, March 2, old style, the difference of 

 the styles now amounting to 13 days. 



It will be found that the mean length of the Gregorian year 

 is 365*2425 days. The actual length of the tropical year being 

 365*2422 days, the error of the mean Gregorian year amounts to 

 3 ten-thousanths of a day, or 26 seconds,^?' annum, or to one 

 day in about 3,300 years. This is sufficiently accurate for 

 practical purposes. It may, however, be pointed out that as the 

 error of the mean Julian year amounts, with great exactness, to 

 one day in 128 years, greater accuracy would have been attained 

 by following the rule that one intercalary day should be dropped 

 in every such period. But the practical inconvenience of this 

 arrangement would be much greater than that of the Gregorian 

 rule, for which the increased accuracy would scarcely be a 

 sufficient compensation. 



It must be understood that the difference of styles causes a 

 great deal of trouble, and is always a possible source of confu- 

 sion to those who have to take account of it. And many a 

 time astronomers and chronologists are constrained to wish that 

 Pope Gregory and his advisers had adopted the alternative 



