Notionally, the paschal full moon refers to the first ecclesiastical full moon of the northern spring used in the determination of the date of Easter. The name "paschal" is derived from "Pascha" which is a transliteration of the Greek word, which is itself a transliteration of the Hebrew pesach, both words meaning Passover. The date of Easter is determined as the first Sunday after the Paschal full moon. This full moon does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, determined from tables. It may differ from the date of the actual full moon by up to two days. The use of tables instead of actual observations of the full moon is useful and necessary since the full moon may occur on different dates depending where one is in the world.
The calculations to determine the date of the Paschal full moon are somewhat complex, but can be described briefly as follows: Nineteen civil calendar years are divided into 235 lunar months of 30 and 29 days each (the so-called "ecclesiastical moon".) The period of 19 years (the metonic cycle) is used because it produces a set of civil calendar dates for the ecclesiastical moons that repeats every nineteen years while still providing a reasonable approximation to the astronomical facts. The first day of each of these lunar months is the ecclesiastical new moon. The fourteenth day is the ecclesiastical full moon. Exactly one ecclesiastical new moon in each year falls on a date between March 8 and April 5 inclusive. This begins the Paschal lunar month for that year, and thirteen days later is the Paschal full moon. Easter is the Sunday following the Paschal full moon. In other words, Easter falls from one to seven days after the Paschal full moon, so that if the Paschal full moon is on Sunday, Easter is the following Sunday.
For a detailed discussion of the Paschal computations, see Computus.
By the middle of the third century AD computists of some churches, among which were the church of Rome and the one of Alexandria, had begun to calculate their own periodic sequences of dates of Paschal full moon, to be able to determine their own dates of Easter Sunday. The motivation for these experiments was a dissatisfaction with the Jewish calendars that Christians had hitherto relied on to fix the date of Easter. These Jewish calendars, according to their Christian critics, sometimes placed Nisan 14, the Paschal Full Moon and the day of preparation for the Jewish Passover, before the spring equinox (see Easter). The Christians who began the experiments with independent computations held that the Paschal Full moon should never precede the equinox.
The computational principles developed at Alexandria eventually became normative, but their reception was a centuries-long process during which Alexandrine Easter tables competed with other tables incorporating different arithmetical parameters. So for a period of several centuries the sequences of dates of the Paschal full moon plied by the different churches could show great differences (see Easter controversy).