Paschal full moon
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In the course of the third century Easter tables came in use. In the beginning of the third century computists of some churches, among which the church of Rome and the one of Alexandria, had gone to calculate their own periodic sequences of dates of Paschal full moon, to be able to determine their own dates of Easter Sunday. To be able to develop those Easter tables, owing to the then incalculability of the Jewish calendar, one had been forced to substitute the dates of the fourteenth day of Nisan, preparation day of Pesach, for dates of Paschal full moon adapted to either (e.g. in the case of the church of Rome) the Julian calendar or (e.g. in the case of the church of Alexandria) the Egyptian calendar. During four centuries the sequences of dates of Paschal full moon plied by the different churches could show great differences, which was the main cause of the fact that the Easter tables propagated by the different churches all too often differed strongly and did not lead to the same dates being eligible for the celebration of Easter Sunday.