Question: Why and when did the Church change to the Gregorian calendar? Why can't the calendar just follow the seasons? Why do the Greeks celebrate Easter later than we do? [M.R.]
Answer: Mankind has ever been concerned with marking the passage of time. The most primitive man realized that night reliably gave way to day and then to night again. With a little more reflection he recognized the regular waxing and waning of the moon. Even as a nomadic gatherer he came to recognize the longer cycle of the seasons, and when he became a farmer he had a compelling interest in predicting when to expect the cold and the rain and the heat and the snow. Even rudimentary commerce needed some way of reckoning time in order to collect rents fairly, and to mark the beginning and ending of contracts.
The task of drawing up a calendar falls to the priests in many cultures, for the passage of time often has a religious significance. Both Judaism and Christianity commemorate the events of salvation history by observing holy days each year. Among pagans, the solstices, the equinoxes, the full, and new moons may be taken as manifestations of divinity to be worshipped in themselves, or dedicated to a pagan deity.
The task of putting together a calendar is not as simple as it might seem. The obvious units of time do not fit together especially well: the hours of the day vary with the season; the moon becomes full again in a fractional number of days (29.53) that cannot be divided into so many weeks of uniform days; 365 day-and-night days does not bring us back to the earth's position in orbit around the sun it had a year ago. Even 364-1/4 doesn't work with long term precision. And modern science has given us more than one way to measure the day and the year, each slightly different.
The practical difficulties of this stellar imprecision are enormous. In commerce (for example) two parties might agree to rent a piece of property by the month: "I'll pay you the rent each "moonth" at the new moon." But successful business require longer terms to properly develop -- how, then, do we extrapolate the monthly rent to a five year term? Some years would have twelve "moonths," while others would have thirteen. If the first year began on the day of the new moon, the other four would not. Long before the advent of Christianity, it was clear that some measure of artificiality had to be injected into the calendar. There has to be some way to make some number of days add up to a month, and some number of months add up to a year.
Some early calendars dealt with the problem of the lunar month by an extra month to the year when astronomers determined that to be necessary. The difficulties for commerce are obvious, but the phases of the moon played an important part in ancient religions, preserving the influence of the lunar calendar. The Jewish calendar contains twelve months of 29 or 30 days, and adds an extra month (Adar II) seven years out of nineteen.
The ancient Greeks employed a 354 day year, 12 months of 29.5 days, but which required the addition of three 30 day months every eight years to bring the year back into sync with the sun.
The early Roman calendar had only 304 days of 10 months, and was subject to constant refinement over the centuries until the reign of Julius Caesar, who in 45 B.C. gave us the 365 day year with an extra day in February every fourth year, and the vernal equinox (the day beginning the season of spring when day and night are both 12 hours long) at March 25th. Errors made by Caesars's successors required a fresh reform by Augustus Caesar, who took the opportunity to name the eighth month after himself, lengthening it to 31 days with a day stolen from February.
The Julian calendar worked fairly well, but its reckoning of the year was about 11-1/4 minutes longer than the actual solar year. With the passage of centuries those extra minutes became perceptible as the vernal equinox (and, consequently the feast of Easter and the season of spring) came earlier and earlier in the calendar year. Left unchecked, Easter would one day have eventually come before Christmas!
In 1582, acting on the advice of his astronomers, Pope Gregory XIII decreed the first calendar revision since Augustus: 1) Century years would not be leap years unless divisible by 400 (not just 4). 2) The vernal equinox would be reckoned as March 21 (as it was during 325 A.D. when the Council of Nicea set the rule for determining the date of Easter), with the recognition that Gregory's new tables for determining the date of Easter might occasionally differ from the vernal equinox determined by astronomers. 3) In order to set the calendar vernal equinox to the actual vernal equinox, ten days were dropped from the year 1582, with October 4th followed by October 15th.
The precision of Pope Gregory's calendar is remarkable -- we won't have to make an adjustment until 4905 A.D, when a single day will have to be added. Catholic countries were quick to adopt the Gregorian calendar, but Protestant and Orthodox countries generally were not. England adopted the calendar in 1752, and the Soviet Union in 1917. The Byzantine Rite Churches, both Orthodox and Catholic have been more resistant to the change than their political counterparts. As a young man, the present writer attend a Ukrainian Rite Catholic Church that still used the Julian calendar in the 1960s -- Christmas, for example, was observed on December 25th on the Julian Calendar, which equated to January 7th of the next year on the Gregorian calendar.
The Easter of the Orthodox is required to follow Passover, while that of the Catholics and Protestants does not. Since the Gregorian tables work somewhat differently from the Jewish system of adding extra months to the years, the Orthodox often celebrate Easter a week or more later than we. In both the East and the West, Easter is the Sunday following the full moon after the vernal equinox. Both East and West put the equinox at March 21, but the Julian calendar observes March 21 thirteen days later than the Gregorian--only when the full moon intervening before Easter provides a long enough "buffer" to negate those days will both East and West observe a common Easter.
In summary: Days, month, and years do not work out to be simple multiples of each other, and even the best calendars have small inaccuracies that grow large over the centuries. The Gregorian calendar was introduced to keep the calendar and the feasts linked to the Easter cycle in step with the seasons of the year.
One other thing, while we are on the subject of the calendar: The new millennium will begin on January 1st, 2001 -- not 2000. The calendar is based on years "anno Domini," A.D. -- "years of our Lord. It assumes that our Lord was born in the year 1 A.D. (not zero A.D.). The hundredth year after our Lord's birth date was 101 A.D. A thousand years was 1001 A.D., and two thousand years will be 2001 A.D. Of course the merchants and the travel agents would like to separate you from any money you plan to spend in observing the new millennium a year early -- and the people who bring us the nightly "news" probably don't know any better.
However, what is improperly being called the "millennium bug," or, more correctly, the Y2K (year 2000) problem will manifest itself in the year 2000 in calculating systems that keep track of the year by only the last two digits. When 1999 gives way to 2000, such systems will go from '99 to '00, and some of them will interpret this as the year 1900. Most computer users have already set their "clocks" ahead to see what effect the last year of the millennium will have on their systems. A greater problem may occur with computer chips (perhaps unknown to their owners) that are imbedded in industrial process control systems. For example, the elevator that reads its imbedded clock before taking passengers to another floor may refuse to run if it thinks it has received no maintenance since 1900.
New Year's Day 2000, a year before the millennium, might be a good day to be at home with plenty of blankets, candles, and a supply of food and water. Think of it as winter hurricane preparedness.