PHYSICAL ASTRONOMY. 151 
Gregorian Calendar. 
75. The Christians retained the astronomical arrangement of the Julian 
calendar in general, but established weeks of several days; January was 
the first month, December the last: January, March, May, July, August, 
October, and December, had 31 days, while April, June, September, and 
November, had 30. February had generally 28 days, except during the 
intercalary or leap year, when it had 29. At a later period, however, the 
length of the year was altered. Julius Cesar had originally assumed too 
great a length for the year. This required, even in the time of Augustus, a 
correction of three days, and in 1581 the error had again amounted to ten 
days. The then reigning pope decreed on the 24th of February, 1581, an 
improvement of the calendar, by which ten days should be omitted from 
October of that year (counting 4, 10, 15 October), and in four centuries 
three leap years should be omitted, by which the years 1700, 1800, 1900, 
2100, &c., were not leap years. This Gregorian calendar was soon intro- 
duced into all Catholic countries, while the Protestants retained the old 
system until 1699. In the year 1700, in February of the new improved 
calendar of the German Protestants, ten days were omitted, so that March 
Ist came immediately after February 18th. With respect to Easter, which 
in the Gregorian calendar of the Catholics was counted after the Epact, the 
Protestants assumed a purely astronomical mode of calculation, so that this 
always came on the Sunday after the first full moon succeeding the vernal 
equinox (thus between March 22 and April 25).. This improved calendar 
agreed for the most part with the Gregorian, but with respect to the different 
determinations of Easter, cases soon occurred in which one calculation 
made Saturday, and the other Sunday, the day which determined the 
celebration of Easter on the Sunday following. According to the latter 
calculation, therefore, this festival would happen eight days later than as 
determined by the former, which produced great confusion among those 
Catholics and Protestants living near each other. When, in 1778, this was 
about occurring, Frederick Il. of Prussia succeeded in persuading the 
Protestants of Germany to adopt the determination of Easter by the 
Epact. To this arrangement the other Protestant states of Europe have 
also conformed. | 
it will be necessary to add a few explanations of certain expressions which 
are connected with the determination of single days and festivals in our 
calendar. The determination of Sundays and week days depends upon the 
Sunday or Dominical Letter, and the Solar Cycle. The Sunday letter is 
that which falls upon the first Sunday, when we call the first day of the 
year A, and count from A to G. Should the year be leap year, count the 
Sundays after February 24th one further. It is necessary to know these 
letters to understand the construction of the perpetual calendar. 
The number of years after which the same week days fall upon the same 
date again, is determined by the solar cycle. Were there no leap years, 
then, as the date annually advances one on the days of the week, the same 
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