Paschal Moon

The Paschal Full Moon refers to the ecclesiastical full moon of the northern spring used in the determination of the date of Easter. The name "paschal" is derived from "Pascha", a transliteration of the Aramaic word meaning Passover. The date of Easter is determined as the first Sunday after the "paschal full moon" falling on or after the Spring Equinox (March 21). 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 19 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. Exactly one ecclesiastical new moon in each year falls on a date between March 8 and April 5, both inclusive. This begins the paschal lunar month for that year, and thirteen days later (that is, between March 21 and April 18, both inclusive) 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. Thus the earliest possible date of Easter is 22 March, while the latest possible date is 25 April.

Paschal Moon in Ruled Britannia

In 1598, the conflict between the Roman Catholic Gregorian Calendar and the Protestant Julian Calendar led to a disagreement to which full moon was the Paschal Moon. According to the Gregorian Calendar, the Paschal Moon fell on the same day as the vernal equinox but the Julian Calendar insisted was still winter. Thus, Protestants and Catholics marked Easter on different days and allowed the Spanish-backed English to root out hidden Protestants.