The Full Moon that Determines Easter

April 15, 2017

Likely you have already heard this from a friend, or perhaps you’ve even made this observation yourself: “Easter is coming late this year.”

Have you ever wondered how the date of Easter is actually set? It is all based on the moon.

The day to be observed as Easter was fixed by a great council of Christian churches, called the First Council of Nicaea, which met at Nicaea (now İznik, in the province of Bursa, Turkey) in A.D. 325. Under the Nicaean rule, Easter is celebrated on the first Sunday following the fourteenth day of a particular new moon — the one that begins on or after the vernal equinox.

Put another way, Easter falls on the Sunday that follows the first full moon occurring on or the day after the March equinox. If the full moon occurs on a Sunday, however, then Easter is observed the following Sunday.

This year, the fourteenth day of this particular new moon was on April 11 (Eastern Daylight Time), which was a Tuesday. So the following Sunday, April 16, is designated as Easter.

When the Council of Nicaea met, Easter was already the most important festival of the church calendar, and it was the custom of thousands of Christians to make long pilgrimages to Jerusalem and other shrines to celebrate the Resurrection. The rule of the Council of Nicaea was established to make it certain that the pilgrims would always have the light of a full moon to guide them on their way at night.

Unfortunately, sometimes there is confusion about how to properly set the date for when Easter should fall in our current Gregorian calendar.

Calendar discontinuities

For astronomers, the moment of a “full” moon comes when our natural satellite is directly opposite from the sun in our sky. But there is also an “ecclesiastical” full moon. The latter, which originated with the Christian Church, was determined from ecclesiastical tables (epacts and “Golden Numbers”). And the church’s date does not necessarily coincide with the date of the “astronomical” full moon, which is solely based on astronomical calculations.

And then there is the problem involving the vernal equinox. Ecclesiastical rules also state that the vernal equinox always occurs March 21, even though from the years 2008 through 2101 it will actually occur no later than March 20 at European longitudes.

As a result, we occasionally end up with discrepancies. From the years 1583 to 2582, for example, there are 78 years in which the date of “astronomical Easter” differs from that of the traditional, “ecclesiastical Easter.”

Perhaps the most noteworthy is the dating for Easter in 2038. In that year, astronomically speaking, Easter should fall on March 28: The equinox falls on March 20, with a full moon the next day. However, according to the church’s rules, Easter will occur on its latest possible date — April 25.

Conversely, Easter can come as early as March 22. That last happened in 1818, but it will not happen again until the year 2285, although as recently as 2008, we came within a day of that extreme: Easter occurred in that year on March 23. So, there are 35 dates on which Easter can fall.

From the years 2000 to 2999, Easter falls on March 22 just five times. In the same 1,000-year time frame, the latest Easter date of April 25 comes up twice as many times: 10. During this same time period, the date that comes up the most (41 times) for celebrating Easter is April 16, which also just happens to be this year.

Since the beginning of the 20th century, people have been circulating a proposal to choose a fixed date for Easter, rather than having a movable one. In 1963, the Second Vatican Council said that it would agree if a consensus on that date were reached among Christian churches — and the second Sunday in April has been put forward as the most likely date. That happens to work out quite well this year.

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