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But why did the Pope, in correcting the Julian calendar in 1582, not correct the whole error of thirteen days? Why did he leave the three days uncorrected? This question has been asked an hundred times, but a correct answer has never yet been given. Some say that the Pope did according to his best ability, and would make us believe that neither he nor his astronomers knew what the error was. This is not true, for history records the fact of the error, and just what that error was. He simply did not want to correct the three days, and for good reasons, which we shall endeavor to show; reasons which every churchman ought to know.
When Cæsar formed his calendar, 46 B. C., the vernal equinox fell on the 24th of March. At the meeting of the Council of Nice, in 325, it had fallen back to the 21st, the error being three days in about 400 years. Now it should be borne in mind that the Julian calendar was the only one in use at that time, and for the next 1257 years, when in 1582, it was corrected by Pope Gregory XIII. Easter, and all the movable feasts, had been unsettled during the 1257 years intervening, from the Council of Nice to Gregory, on account of the errors of the Julian calendar. The Easter question had been the cause of a good deal of discussion between the Eastern and Western churches during the second and third centuries, as they could not agree on the day of the week on which that event should be celebrated.
The Western churches observed the nearest Sunday to the full moon of Nisan. The Asiatics, on the other hand, adopted the 14th of Nisan upon which to commemorate the crucifixion, and observed the festival of Easter on the third day following, upon whatever day of the week that might fall. Finally, the Council of Nice was convened, and the matter came before that council, and a reconciliation was accomplished. It was then and there agreed by the two parties that Easter should be celebrated on the first Sunday after the full moon that falls upon or next following the day of the vernal equinox, and that the 21st of March should be accounted the day of the vernal equinox.
It has already been shown that the error in the Julian calendar is three days in 400 years; so that in 400 years from the Council of Nice the vernal equinox had fallen back to the 18th of March; in 800 years it had fallen back to the 15th; in 1257 years, that is in 1582, it fell on the 11th. Still the 21st of March, by the only calendar in use at that time, was accounted the date of the vernal equinox, by which date Easter was determined, so that, in 1582, when it was the 21st by the calendar, the correct date was the 31st. Hence, the error had been increasing at the rate of three days every 400 years until in 1582 it amounted to ten days.
Again it should be borne in mind that the Pope was a churchman and wished to abide by the decision of that council in celebrating the festival of Easter, so he drops the ten days and restores the vernal equinox to the 21st of March, its date at the meeting of the Council of Nice in 325, the date by which Easter day was determined. He not only made the correction, but he so reformed the calendar that the solar and the civil year are now made to coincide very nearly. Had he dropped the thirteen days, the vernal equinox would have been restored to the 24th of March, its date in the time of Cæsar, and the 24th would still be its date. But the Council of Nice decided that the 21st should be the date by which Easter day should be determined. Hence the reason for dropping the ten days instead of the thirteen is evident; and it is also evident that the Pope acted understandingly when he made the correction in 1582.