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Easter occurs on the first Sunday after the “paschal full Moon.” In simple terms, this is the first full Moon immediately following the vernal (spring) equinox. However, that’s not the full story because Easter isn’t based on the actual Moon or equinox date! We’ll explain the curious connection between the Moon and Easter.
How Does Easter Relate to the Full Moon?
Easter is what’s known as a “movable feast”—in other words, a religious holiday that may fall on a different calendar date from year to year.
The date of Easter is tied to the relationship between the Paschal full Moon, whose dates are based on calculations made long ago, and the Church’s fixed date of the March equinox (also called the spring or vernal equinox in the Northern Hemisphere), which is March 21. Thanks to this, determining when Easter will be can get more than a bit confusing because of the occasional differences between these ecclesiastical dates and the astronomical dates.
Here’s a basic rule-of-thumb for finding the date:
“Easter is observed on the Sunday following the first full Moon that occurs on or after the March equinox.” Occasionally, the Church’s dates will not coincide with the astronomical dates, in which case this basic rule would not apply.
For example, if the equinox were to occur on March 21 and the full Moon were to occur two days later, on March 23, Easter would be observed on the first Sunday after March 23.
However, thanks to the motions of our planet and the Moon, as well as the inelasticity of calendars, calculating Easter’s date can get more complicated sometimes! Read on to learn more…
A Difference of Dates
The biggest cause of confusion regarding Easter is the tangled web of dates that are used to determine the holiday. If you take the rule given above at face value, things don’t always work out quite right.
This is exactly what happened in 2019. The March equinox occurred on March 20 at 5:58 P.M.EDT, with the full Moon reaching its peak four hours later, at 9:43 P.M. EDT. But wait—that means that the full Moon and the March equinox happened on the same date, which should have landed Easter on Sunday, March 24, right? Well, not quite.
The dates of the full Moon and the March equinox that are used to calculate Easter are not the astronomical dates of these events, but rather the ecclesiastical dates.
The astronomical dates of the full Moon and the March equinox are the actual, scientifically determined dates of these events. For example, the equinox occurs at the exact moment when the Sun crosses Earth’s equator, when day and night are approximately equal. Similarly, the full Moon occurs when the Moon reaches peak illumination by the Sun.
The ecclesiastical dates of the full Moon and the March equinox are those used by the Christian Church. They were defined long ago in order to aid in the calculation of Easter’s date, which means that they may differ from the astronomical dates of these events.
In A.D. 325, a full Moon calendar was created that did not take into account all the factors of lunar motion that we know about today. The Christian Church still follows this calendar, which means that the date of the ecclesiastical full Moon may be one or two days off from the date of the astronomical full Moon.
Additionally, the astronomical date of the equinox changes over time, but the Church has fixed the event in their calendar to March 21. This means that the ecclesiastical date of the equinox will always be March 21, even if the astronomical date is March 19 or 20.
Due to these rules, in 2019, the ecclesiastical full Moon occurred before the ecclesiastical Vernal Equinox, which meant that Easter would not be observed until after the next full Moon (the Paschal Full Moon) in mid-April. Thus, Easter was celebrated on Sunday, April 21, 2019.
Fun Fact: “Paschal” stems from Pascha, the Greek and Latin word for Passover.
When Is the Paschal Full Moon in 2023?
Technically, the Paschal Full Moon occurs on the 14th day of the lunar month that occurs on or after March 21 according to ecclesiastical calculations using the Metonic cycle. But in many cases, it coincides with the astronomical full Moon on or after the actual March equinox.
Using the astronomical dates for 2023, the vernal equinox in the Northern Hemisphere occurs on Monday, March 20. The first full Moon to occur after that equinox date is April’s full Pink Moon, which reaches peak illumination on Thursday, April 6, at 12:37 A.M. Eastern Time (or Wednesday, April 5, for more westerly time zones). According to this general guideline, this makes April’s full Moon the Paschal Full Moon and therefore Easter will be observed on the first Sunday after April 6: Sunday, April 9!
How Late Can Easter Be?
For the western Christian churches and others that use the Gregorian calendar for their calculations, Easter can occur as early as March 22 and as late as April 25.
For the Eastern Orthodox churches and others that use the Julian calendar for their calculations, the observance can occur between April 4 and May 8 in the Gregorian calendar.