Isaac Asimov would have been 104 on January 2, 2024. Or maybe not. To explain that, I’m going to quote from and summarize one my favorite science essays that he wrote for F&SF for the month of August, 1964. It was based on a talk that he had given on Leap Day of that year, so the subject was calendars and it was entitled “The Days of Our Years”.
The earliest unit of time-telling, he began, was the day, and since the Sun governs the day, naturally the next most prominent heavenly body, the Moon, offered another unit with its regularly occurring phases, so there was the “lunar month”. Once humans began farming, it became necessary to sow at the proper time, so farmers would rely on experts, i.e. the priests, to tell them when it was time to sow their seeds. The cycle of seasons is made up of twelve lunar months and therefore is a “lunar year”.
Today the Islamic calendar is still based on the lunar year. One problem with it is that twelve lunar months only comes to 354.37 days, so extra days (known as “intercalary days”) need to be inserted into it.
Another method of keeping the calendar from falling behind the Sun was used by the Babylonians. They added an intercalary month every few years. Both the Hebrews and the Greeks adopted this model, and the Jewish calendar still uses it to this day.
The individual dates in the Jewish calendar are allowed to fall slightly behind the Sun until the intercalary month is added, when they suddenly shoot slightly ahead of the Sun. That is why holidays like Passover and Yom Kippur occur on different days of the civil calendar (kept strictly even with the Sun) each year. These holidays occur on the same day of the year each year in the Jewish calendar.
The early Christians continued to use the Jewish calendar for three centuries, and established the day of Easter on that basis. As the centuries passed, matters grew somewhat complicated, for the Romans (who were becoming Christian in swelling numbers) were no longer used to a lunar-solar calendar and were puzzled at the erratic jumping about of Easter. Some formula had to be found by which the correct date for Easter could be calculated in advance, using the Roman calendar.
It was decided at the Council of Nicaea, in A.D. 325 (by which time Rome had become officially Christian), that Easter was to fall on the Sunday after the first full Moon after the vernal equinox, the date of the vernal equinox being established as March 21. However, the full Moon referred to is not the actual full Moon, but a fictitious one called the “Paschal Full Moon” (“Paschal” being derived from Pesach, which is the Hebrew word for Passover). The date of the Paschal Full Moon is calculated according to a formula involving Golden Numbers and Dominical Letters, which I won’t go into.
The result is that Easter still jumps about the days of the civil year and can fall as early as March 22 and as late as April 25. Many other church holidays are tied to Easter and likewise move about from year to year.
Moreover, all Christians have not always agreed on the exact formula by which the date of Easter was to be calculated. Disagreement on this detail was one of the reasons for the schism between the Catholic Church of the West and the Orthodox Church of the East. In the early Middle Ages there was a strong Celtic Church which had its own formula.
Our calendar comes from Egypt which was based on the flooding of the Nile River which occurred (on average) every 365 days. So no Moon was needed and a “solar calendar” based on the “tropical year” with 365 days was adopted. To keep things simple, there were twelve months of 30 days each, and five holidays were added to fill out the 365 days.
The problem arises because the tropical year does not have an even 365 days; it’s more like 365¼ days. So the calendar still kept falling behind the Sun.
The Greeks learned about that extra quarter day in 380 B.C., but they were unable to get the ultra-conservative Egyptians to incorporate it into their calendar.
Meanwhile, things were getting confusing in Rome, and by 46 B.C. the Roman calendar was 80 days behind the Sun! This was because the Roman priests were elected officials and when someone from their party was in power, they added a month to make it a long year. When someone from another party was in power they kept the year short. Clearly, we don’t see those kinds of shenanigans these days, do we?
Enter Julius Caesar.
Julius Caesar was in power then and decided to put an end to this nonsense. He had just returned from Egypt where he had observed the convenience and simplicity of a solar year, and imported an Egyptian astronomer, Sosigenes, to help him. Together, they let 46 B.C. continue for 445 days so that it was later known as “The Year of Confusion.” However, this brought the calendar even with the Sun so that 46 B.C. was the last year of confusion.
With 45 B.C. the Romans adopted a modified Egyptian calendar in which the five extra days at the end of the year were distributed throughout the year, giving us our months of uneven length. Ideally, we should have seven 30-day months and five 31-day months. Unfortunately, the Romans considered February an unlucky month and shortened it, so that we ended with a silly arrangement of seven 31-day months, four 30-day months, and one 28-day month.
In order to take care of that extra ¼ day, Caesar and Sosigenes established every fourth year with a length of 366 days. (Under the numbering of the years of the Christian era, every year divisible by 4 has the intercalary day—set as February 29. Since 1964 divided by 4 is 491, without a remainder, there is a February 29 in 1964.)
This is the “Julian year,” after Julius Caesar. At the Council of Nicaea, the Christian Church adopted the Julian calendar. Christmas was finally accepted as a Church holiday after the Council of Nicaea, and given a date in the Julian year. It does not, therefore, bounce about from year to year as Easter does.
The 365-day year is just 52 weeks and 1 day long. This means that if February 6, for instance, is on a Sunday in one year, it is on a Monday the next year, on a Tuesday the year after, and so on. If there were only 365-day years, then any given date would move through the days of the week in steady progression. If a 366-day year is involved, however, that year is 52 weeks and 2 days long, and if February 6 is on Tuesday that year, it is on Thursday the year after. The day has leaped over Wednesday. It is for that reason that the 366-day year is called “leap year” and February 29 is “leap day.”
There’s still a problem because the tropical year is actually 365 days, 5 hours, 48 minutes, 46 seconds, or 365.24220 days long.
By 1582 the Julian calendar had gained ten days and the vernal equinox was falling on March 11 rather than on March 21.
Pope Gregory XIII finally took action. First, he dropped ten days, changing October 5, 1582 to October 15, 1582. That brought the calendar even with the Sun and the vernal equinox in 1583 fell on March 21 as the Council of Nicaea had decided it should.
The next step was to prevent the calendar from getting out of step again. Since the Julian year gains a full day every 128 years, it gains three full days in 384 years or, to approximate slightly, three full days in four centuries. That means that every 400 years, three leap years (according to the Julian system) ought to be omitted.
Consider the century years—1500, 1600, 1700, and so on. In the Julian year, all century years are divisible by 4 and are therefore leap years. Every 400 years there are 4 such century years, so why not keep 3 of them ordinary years, and allow only one of them (the one that is divisible by 400) to be a leap year? This arrangement will match the year more closely to the Sun and give us the “Gregorian calendar.”
To summarize: Every 400 years, the Julian calendar allows 100 leap years for a total of 146,100 days. In that same 400 years, the Gregorian calendar allows only 97 leap years for a total of 146,097 days. Compare these lengths with that of 400 tropical years, which comes to 146,096.88. Whereas, in that stretch of time, the Julian year had gained 3.12 days on the Sun, the Gregorian year had gained only 0.12 days.
Still, 0.12 days is nearly 3 hours, and this means that in 3400 years the Gregorian calendar will have gained a full day on the Sun. Around A.D. 5000 we will have to consider dropping out one extra leap year.
But the Church had waited a little too long to take action. Had it done the job a century earlier, all western Europe would have changed calendars without trouble. By A.D. 1582, however, much of northern Europe had turned Protestant. These nations would far sooner remain out of step with the Sun in accordance with the dictates of the pagan Caesar, than consent to be corrected by the Pope. Therefore they kept the Julian year.
The year 1600 introduced no crisis. It was a century year but one that was divisible by 400. Therefore, it was a leap year by both the Julian and Gregorian calendars. But 1700 was a different matter. The Julian calendar had it as a leap year and the Gregorian did not. By March 1, 1700, the Julian calendar was going to be an additional day ahead of the Sun (eleven days altogether). Denmark, the Netherlands, and Protestant Germany gave in and adopted the Gregorian calendar.
Great Britain and the American colonies held out until 1752 before giving in. Because of the additional day gained in 1700, they had to drop eleven days and changed September 2, 1752 to September 13, 1752. There were riots all over England as a result, for many people came quickly to the conclusion that they had suddenly been made eleven days older by legislation.
“Give us back our eleven days!” they cried in despair.
(A more rational objection was the fact that although the third quarter of 1752 was short eleven days, landlords calmly charged a full quarter’s rent.)
As a result of this, it turns out that Washington was not born on “Washington’s birthday.” He was born on February 22, 1732 on the Gregorian calendar, to be sure, but the date recorded in the family Bible had to be the Julian date, February 11, 1732. When the changeover took place, Washington—a remarkably sensible man—changed the date of his birthday and thus preserved the actual day.
The Eastern Orthodox nations of Europe were more stubborn than the Protestant nations. The years 1800 and 1900 went by. Both were leap years by the Julian calendar, but not by the Gregorian calendar. By 1900, then, the Julian vernal equinox was on March 8 and the Julian calendar was 13 days ahead of the Sun. It was not until after World War I that the Soviet Union, for instance, adopted the Gregorian calendar. (In doing so, the Soviets made a slight modification of the leap year pattern which made matters even more accurate. The Soviet calendar will not gain a day on the Sun until fully 35,000 years pass.) The Orthodox churches themselves, however, still cling to the Julian year, which is why the Orthodox Christmas falls on January 6 on our calendar. It is still December 25 by their calendar.
In fact, a horrible thought occurs to me—
I was myself born at a time when the Julian calendar was still in force in the—ahem—old country. [WeII, the Soviet Union, if you must know. I came here at the age of 3.] Unlike George Washington, I never changed the birthdate and, as a result, each year I celebrate my birthday 13 days earlier than I should, making myself 13 days older than I have to be.
And this 13-day older me is in all the records and I can’t ever change it back.
Give me back my 13 days! Give me back my 13 days! Give me back…