Easter, Astronomy, and Slippery Dates

“Easter is early this year.” Have you heard that recently? In 2016 Easter occurs on March 27.) In our culture most holidays have either a set calendar date, such as December 25, or fixed relative date, such as the second Sunday in May. Easter stands out for its wide swings of time. Growing up I always expected Easter to mean spring: short sleeved dresses, sunshine, and flowers. It was just as likely to require winter coats, have snow on the ground with nary a daffodil in sight. It can come as early as March 22 and as late as April 25. At least it can for Western churches, in most places. Let me explain.


Mosaic, Cathedral Basilica of St. Louis Missouri

Easter is, of course, essentially a religious celebration and on the Christian calendar, a number of other dates (including Pentecost, Ascension Thursday, Corpus Christi, and Ash Wednesday) slide along with it. While not precisely religious, Mardi Gras takes its date from Easter also. The reason for this odd phenomenon is complicated astronomically, historically, and to some extent politically.

Because Easter celebrates the heart of Christian belief, it is the most important day of the year to the churches. It probably shouldn’t be a surprise that the date was mired in controversy in the early church.

No one wrote down the day Christ rose, but it was associated with Passover in the scriptures, and spring seemed to suit the theology of new life. Local churches followed their own methods for determining the date. Usage of multiple calendars  in the ancient world exacerbated confusion. Romans followed a solar calendar, the Julian calendar. Others used older, lunar calendars, such as that traditional in Jewish practice. Some communities held that Easter should always be celebrated on the night of the fourteenth Nisan, when Jews celebrated Passover. Others believed it should always be on the first day of the week. Synods and councils began arguing the issue by the early second century.


Council of Nicaea 325, Sistine Chapel Fresco, 1590

The Council of Nicea settled the matter once and for all (or so it thought) in the fourth century. They decided to use the solar calendar in part to avoid too close an association with Passover. Given the close association of Passover and the resurrection in the Gospels, that is ironic, in my opinion. Be that as it may, the council also noted that one solar date could be clearly identified: the vernal equinox. The equinox is the spring day on which the sun crosses the sun’s equator and day and night are roughly equal. They still needed a way to ensure Easter would always be a Sunday, however, and so they decided Easter would always be celebrated on the first Sunday after the first full moon after the vernal equinox. We still use that formula.

If that sounds complex, it gets worse when you ask, “According to what calendar?” Calendars are man-made artificial constructs. The sun and the moon do not lend themselves to neat measurements. The Gregorian calendar, which in turn has undergone reforms, but which is the generally accepted civic calendar in our era, has superseded the Julian calendar in use at the time of Nicea. The Julian assumed a solar year to be 365.250 days. In truth it is approximately (but not precisely) 365.242 days. The history of the solar calendar alone would take more room than we have here.

Overlay all that with the fact that the Nicean rules oversimplify lunar motion, that ecclesiastical rules don’t always match the astronomical rules, and that local custom tends to be persistent, and you see the problem.

The U.S. Naval Observatory points out the variations:

There are three major differences between the ecclesiastical and the astronomical systems.

  • The times of the ecclesiastical full moon are not necessarily identical to the times of astronomical full moons. The ecclesiastical tables do not account for the full complexity of the lunar motion.
  • The astronomical definition of the vernal equinox is the instant when the Sun, as seen from the Earth, has a zero apparent ecliptic longitude. (Yes, the Sun’s ecliptic longitude, not its declination, is used for the astronomical definition.) This instant shifts slightly from year to year within the civil calendar. In the ecclesiastical system the vernal equinox does not shift. It is fixed on March 21 regardless of the actual position of the Sun.
  • The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. The astronomical vernal equinox occurs at same instant everywhere on the Earth.

(USNO Astronomical Applications Department, retrieved on March 17, 2016, http://aa.usno.navy.mil/faq/docs/easter.php )

You might guess that controversy didn’t go away. In the Middle Ages when religion ruled civic life, these

A page from Historia Ecclesiastica Gentis Anglorum by Bede

A page from Historia Ecclesiastica Gentis Anglorum by Bede

were not small matters. When Pope Gregory the Great sent missionaries into England, his people found that the Celtic missionaries, brought into northern England from Ireland by Saint Columba, used a different tradition for calculating the date using a lunar calendar. The two began to rub against each other particularly when Oswiu, king of Northumbria, married a woman who practiced the Roman traditions. It was possible for his Easter to occur during her Lenten fast. Well, they couldn’t have that!

At this point I might pause and ask why none of these people took the advice of Saint Ambrose of Milan who, when asked whether the people in Rome who observed a different fast were wrong and what his people should do when they traveled there, is reputed to have said, “When in Rome, fast as the Romans do.”

In Northumbria, there was no room for such compromise. With another Lenten fast/ Easter conflict looming in 665, the Synod of Whitby was called in 664. It took place at the monastery of the abbess Saint Hilda in Whitby. Saint Wilfred, who had been educated at the Celtic monastery at Lindisfarne and in the Roman province of Gaul, and who had traveled extensively, gave a speech that turned the vote to the Roman method. In his speech he quoted the scripture, “’Thou art Peter, and upon this rock I will build my church. . .” That reputedly made the king nervous. Worried for his own salvation, he asked if it was true that Peter held the keys to heaven. At least that is how Bede recorded it in his history. Whatever the case, England standardized on the Roman date.

Whitby Abbey

Whitby Abbey

The Synod of Whitby, it should be noted, has spawned a number of fictional works and provides a subplot in others. It tends to capture the imagination of authors of medieval historical fiction.

In our own era most Western Christian churches use the Gregorian calendar and the Nicean formula. The Easter Orthodox uses the same formula but the Julian calendar, which is why Easter rarely if ever falls on the same date.

If nothing else, chasing a date for Easter facilitated the study of astronomy at various times. A modern student of astronomy has commented, “So when is Easter? Hard to say. But I do know this. You’d have to be very, very good at astronomy to know.”

(Richard Beck, “On Easter and Astronomy,” on Experimental Theology, retrieved March 17, 2016, http://experimentaltheology.blogspot.com/2008/03/on-easter-and-astronomy.html )

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Caroline Warfield, a regular contributor to this blog, writes historical romance. She has not yet written a book that features Easter, but she may. You can find her work here:


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