Easter, you may have noticed, is not a fixed day in the calendar. While Christmas, in contrast, occurs reliably every year on Dec. 25, Easter wanders around on a given Sunday in late March or April. This year Easter will be celebrated on March 31, but in 2014 it will occur on April 20. The reason has to do with the mismatch between the periodicity of the sun and the moon and the long history of human efforts to create a reliable and consistent calendar.
The Earth takes approximately 365.25 days to complete its orbit around the sun. The moon, on the other hand, completes its monthly cycle in approximately 28 days. Twenty-eight is neatly divided into a month consisting of four seven-day weeks (and now you know why there are seven days in a week instead of six or 10 or...). If the annual solar cycle were evenly divisible by the "monthly" lunar cycle, creating an accurate calendar would be easy. Instead, 365.25 solar days divided by 28 lunar days equals 13.0446 months.
Calculating the seasons was vitally important in the growing agricultural societies of the past and to the religious-political hierarchies that presided over these expanding civilizations. Without this perplexing mismatch between the periodicity of the sun and the moon to challenge their imaginations, observations and calculations, our ancestors might never have developed advanced mathematics and the astronomical sciences.
Most of the early calendars created by humans were based on the lunar cycle (13 months of 28 days, each month divided into four seven-day weeks). Religious Muslims and Jews today still use modified lunar calendars with an occasional leap month thrown in to prevent the months from wandering too far away from their solar seasons.
In 46 A.D., Julius Caesar instituted the Julian calendar throughout the Roman Empire. It would be the predominant calendar used throughout Europe for 1,500 years. The Julian calendar is a solar calendar consisting of 12 months of varying lengths (28 days, 30 days, and 31 days) with a leap day added to February every fourth year (i.e., 29 days instead of 28 days).
The problem is that the annual periodicity of the sun is not exactly 365.25 days, as the Greek astronomer Hipparchus had calculated back in the second century B.C. The actual solar year is 365 days, five hours, 55 minutes and 12 seconds. Hipparchus' calculations were off by only a few minutes, which is pretty impressive for 2,200 years ago. But this tiny discrepancy in the sun's actual periodicity resulted in a gain of about three days every four centuries under the Julian calendar as compared with the observed equinoxes and solstices.
The First Council of Nicaea in 325 A.D. established that Easter should occur on the first Sunday after the full moon that follows the vernal equinox. This year, the spring equinox occurred on March 20 with the full moon on March 27, ergo Sunday, March 31 is Easter. This formulation also linked Easter in proximity to the Jewish Passover, from which the Christian holiday is symbolically derived. But there was another problem: Over many centuries under the Julian calendar, in which a Julian year differed slightly from an actual year, the vernal equinox wandered 10 days earlier to around March 10.
The Gregorian calendar was instituted in 1582 by the Catholic Church to correct these accumulated discrepancies. Pope Gregory's bull began by skipping 10 calendar days to restore Easter to its "proper" place in the calendar -- late March or April.
Imagine the ruckus that would be created today if, by decree, we simply skipped 10 days in March. In 1582, Europe was already hot in the midst of the Reformation. Protestants couldn't care less for the "Popish" bull on esoteric astronomy. A Euro crisis ensued, not over a common currency, but over the lack of a common calendar. This crisis lasted for centuries. The Catholic countries had lost 10 days in the "New Style" Gregorian reform, but the Protestant countries continued to follow the "Old Style" Julian calendar.
The Gregorian calendar had one major improvement over the earlier Julian calendar. It modified leap years to fine-tune the actual solar periodicity. Every year divisible by four is still a leap year (i.e., 29 days in February instead of 28) with the exception of centurial years that are not divisible by 400. Thus, the years 1700, 1800, 1900 and 2100 are not leap years, but 1600, 2000 and 2400 are. This latter adjustment made the calendar more consistent and it could hold up for millennia in keeping with the solar year.
The Gregorian calendar is now the Civil Calendar of the world, but the road to its acceptance was not easy. The British Empire finally adopted the "New Style" Gregorian calendar in 1752, by which time the discrepancy had grown from 10 days to 11 days. By decree, London eliminated these 11 days, and resynchronized their dates with Catholicism and the sun. President George Washington, for instance, was born on Feb. 11, 1732 in the Virginia colony under the "Old Style" calendar, which would be reckoned as Feb. 22, 1732 under the "New Style" calendar.
Today, there remains a 13-day difference between the Gregorian calendar and the Julian calendar still used by Eastern Christianity. Thus, Orthodox Christianity celebrates Christmas on Jan. 7, 13 days after Dec. 25. As for Easter -- the moveable feast with no fixed date -- this year, Orthodox Christians will celebrate the holiday on May 5. Curiously, Western and Eastern Christianity will celebrate Easter together in 2014 on Sunday, April 20. I have no idea how often that happy confluence occurs.
Today, the relative movements of the Earth, moon and sun can be measured more precisely than our ancestors were capable of doing or knowing. We also know these periodicities to be more uneven. A lunar month, for instance, is actually quite variable and is on average 29 days, 12 hours and 44 minutes (not 28 days).
Time in our techno-scientific, global civilization is even more precious and precise, such that we measure it reliably in scales from nanoseconds to light-years. Someday our descendants will no doubt have to adjust their calendars again, but something of the rising and setting sun and the phases of the moon will always govern our existential understanding of the passing of time in ways that matters most in our human-scale experience.
The history of telling time is an education in the education of our species on a finely tuned, slightly quirky and fantastically blessed planet teeming with life and consciousness. It is a planet that wobbles on its axis in elliptical orbit around the sun even as it is pushed and pulled by the gravitation of the moon. It is a planet governed by the periodicity of the sun and the moon, but also cycles of birth, death and renewal of life. It is a planet governed by the Great Eucharistic law -- eat and be eaten. Earth itself, we now understand, is literally a moving feast.
Chag Sameach and Happy Easter.