A Tale of Two Calendars

Christians throughout the globe believe in the same God and the resurrection of Jesus Christ, yet Easter, one of the two most important Christian holidays, is often celebrated on different dates. Orthodox Christians in some parts of Europe, Africa, and the Middle East celebrate Easter later than in the western world. It’s all about the calendar. Christian churches in Western countries use the Gregorian calendar; their fellow believers in Eastern countries follow the Julian calendar.

Ancient Match Sets the Dates of Easter

If it all seems confusing, blame it on the moon—ecclesiastical moons, paschal full moons, the astronomical equinox, and the fixed equinox—are at the root of the complicated dates set for Easter.

A complicated mixture of astronomy, mathematics, and theology that develops an algorithm or recipe fixes Easter’s date following the sun and moon motions. Easter is the first Sunday after the paschal moon, the first full moon on or after the vernal equinox. Here’s where it begins to get more confusing. 

The “ecclesiastical” full moon is an idealized event that’s defined mathematically and may be slightly earlier or later than the astronomical event. When the ecclesiastical equinox is fixed on March 21, it doesn’t always concur with the sun crossing the equator, further complicating the formula.

The Orthodox Christian Easter always happens after the Jewish Passover celebrations because the crucifixion of Jesus took place after he came to Jerusalem for Passover some 2,000 years ago. On the other hand, the Western Easter is sometimes scheduled before Passover, which, historically, does not seem to make a lot of sense.

What Causes the Mismatch of Dates? 

There are two answers to this question. 

First, there’s a fundamental misalignment between the Christian and the Jewish festival calendars. Both holidays are supposed to fall on, or near, a full moon in the spring. Passover always begins on the 15th day of the Hebrew month of Nisan. Because the Hebrew months are directly aligned to the lunar cycle, the 15th day of Nisan is always a full moon.

Early Christians used the Jewish calendar as a reference, celebrating Easter on the first Sunday after Nisan 15. But at the First Council of Nicaea in a.d. 325, the Church decided to set its own date for Easter, independent of the Jewish calendar. Today most Christian communities celebrate Easter on the first Sunday after the first full moon after March 21. But sometimes, this full moon isn’t the same as the Jewish one.

The second reason the lunar mismatch occurs is that both calendars must contend with the same underlying problem: A lunar year is not the same length as a full solar year.  Nothing is exactly the same length as a solar year because not all solar years are the same. 

The Hebrew Calendar Solar Year Challenge

The Hebrew calendar uses lunar months, which are about 29 or 30 days each. Twelve of those months add up to 354 days. A solar year is about 365.2425 days, which leaves the Hebrew calendar approximately 11 days short. Not addressing the discrepancy would cause the Hebrew calendar to quickly fall out of sync with the solar calendar, violating the biblical commandment to celebrate Passover during the spring. 

The Hebrew calendar periodically resolves the problem by adding an extra month to the calendar. Two thousand years ago, during the month of Adar (the Hebrew month before the Passover month of Nisan), the ancient rabbinical court would base their decision on whether or not to add another month to the calendar based on the weather. If spring seemed to be on track, Nisan could occur. When the weather wasn’t warm enough outside yet, the rabbis would tack on another month of Adar. They called this leap month Adar II.

The weather-based observational system was replaced with a fixed calendar around the third century (known as the Common Era). The Hebrew calendar now adds a leap month seven years out of every 19. (Or, now Adar II is added in the third, sixth, eighth, 11th, 14th, 17th, and 19th years of the cycle.)

The new system doesn’t completely resolve the issues. Now, over 19 years, it comes out almost to the length of the solar years.  The Jewish calendar drifts about one day later every 200 years, and there isn’t any mechanism in place yet to correct that.

The Gregorian Calendar Solar Year Challenge

The same problem—that 12 lunar months do not add up to a full solar year—also creates an issue with the Gregorian calendar, the calendar system now used by most of the world. First proclaimed by Pope Gregory XIII, it uses a standard 365-day year most of the time. But about 24 times a century, it adds a leap day on February 29. The leap year addition nearly, but not totally, brings the calendar year in sync with the solar year.

Today Christians in the western world celebrate Easter after March 21 on the Gregorian calendar. This year, Easter is on Sunday, April 4. But the Eastern Orthodox Church uses the older version of that calendar, known as the Julian, to determine the date of Easter and other festivals. This year, Orthodox Easter is Sunday, May 2.

Yet, the Gregorian calendar still requires regular tweaks by hand. Today, there’s still a high rabbinical court (of sorts) that adjudicates the Gregorian calendar every year, deciding whether it should be adjusted to match reality better. The difference is that the court is now staffed by physicists, not by rabbis. 

Remember, no solar year is the same length: Earth’s orbit determines solar years, so some years are a second or two longer or shorter than others. Every year, the International Earth Rotation and Reference Systems Service announces whether to add a leap second to align Earth time with solar time. (The United States officially opposes this practice.)

We invite you to join CMEP’s devotions on the theme of “Beacons of Hope: Journeying in Faith for Peace and Justice in the Middle East during the season of Lent.

Have a joyful Easter!