The Julian Calendar and The Georgian Reforms
For many European institutions, we can thank the Romans (for good or for bad, though they too had their predecessors in the Greeks, Egyptians and Babylonians). So with the solar calendar currently in widespread use.

Originally the Romans numbered years ab urbe condita, that is, "from the founding of the city" (of Rome, where much of the character of the modern world had its beginnings). Had this old calendar remained in use, 1996-01-14 would have been New Year's Day in the year 2749 a.u.c.

Following his conquest of Egypt in 48 B.C. Julius Caesar consulted the Alexandrian astronomer Sosigenes about calendar reform (since the a.u.c. calendar then used by the Romans was completely inadequate to the needs of the emerging empire, which Caesar was poised to command, briefly as it turned out). The calendar which Julius Caesar adopted in the year 709 a.u.c. (what we now call 46 B.C.) was identical to the Alexandrian Aristarchus' calendar of 239 B.C., and consisted of a solar year of twelve months and of 365 days with an extra day every fourth year. It is unclear as to where or how Aristarchus arrived at this calendar, but one may speculate that Babylonian science was involved.

As we can read in the excellent article, "The Western Calendar and Calendar Reforms" in the Encyclopedia Brittanica, Sosigenes decided that the year known in modern times as 46 B.C. should have two intercalations. The first was the customary intercalation of 23 days following February 23, the second, "to bring the calendar in step with the equinoxes, was achieved by inserting two additional months between the end of November and the beginning of December. This insertion amounted to an addition of 67 days, making a year of no less than 445 days and causing the beginning of March, 45 B.C. in the Roman republican calendar, to fall on what is still called January 1 of the Julian Calendar."

It is said that Julius Caesar wanted to start the year on the Spring Equinox or the Winter Solstice, but the Senate, which traditionally took office on January 1st, the start of the Roman civil calendar year, wanted to keep January 1st as the start of the year, and Caesar yielded in a political compromise. Others opine that  January 1 of 45 B.C. was the date of a new moon and to change it would have been to invite bad luck according to the prevalent beliefs.

The Roman date-keepers initially misunderstood Caesar's instructions concerning the new calendar (according to Macrobius), and erroneously held every third year, rather than every fourth year, to be a leap year. There is some dispute as to exactly which years from 43 B.C. through to 8 A.D. were actually leap years, but a reconstruction which is consistent with the available evidence is that every third year following 43 B.C. (i.e. 40 B.C., 37 B.C., etc.) was a leap year, until 10 B.C., after which, according to this hypothesis, Augustus Caesar (Julius Caesar's successor) suspended leap years, reinstating them with the leap year of 4 A.D.

Another source of uncertainty regarding exact dating of days at this time derives from changes made by Augustus to the lengths of the months. According to some accounts, originally the month of February had 29 days and in leap years 30 days (unlike 28 and 29 now). It lost a day because at some point the fifth and six months of the old Roman calendar were renamed as Julius and Augustus respectively, in honor of their eponyms, and the number of days in August, previously 30, now became 31 (the same as the number of days in July), so that Augustus Caesar would not be regarded as inferior to Julius Caesar. The extra day needed for August was taken from the end of February. However there is still no certainty regarding these matters, so all dates prior to A.D. 4, when the Julian Calendar finally stabilized, are uncertain.

Subsequently the Julian Calendar became widespread as a result of its use throughout the Roman Empire and later by various Christian churches, which inherited many of the institutions of the Roman world.

The system of numbering years A.D. (for "Anno Domini") was instituted in about the year 527 A.D. by the Roman abbot Dionysius Exiguus, who reckoned that the Incarnation had occurred on March 25 in the year 754 a.u.c., with the birth of Jesus occurring nine months later. Thus the year 754 a.u.c. was designated by him as the year 1 A.D. It is generally thought that his estimate of the time of this event was off by a few years (and there is even uncertainty as to whether he identified 1 A.D. with 754 a.u.c. or 753 a.u.c.).


Georgian Reforms

Although the Julian calendar had been there for over a millennium and a half, a very basic controversy had not been sorted out. The average length of a year in the Julian Calendar is 365.25 days (one additional day being added every four years). This is significantly different from the "real" length of the solar year. The main competing values seem to be the "mean tropical year" of 365.2422 days ("mean solar days") and the "vernal equinox year" of 365.2424 days. The difference of the length of the Julian calendar year from the length of the real solar year is thus 0.0078 days (11.23 minutes) in the former case and 0.0076 days (10.94 minutes) in the latter case. Whatever, this error accumulates so that after about 131 years the calendar is out of sync with the equinoxes and solstices by one day. Thus as the centuries passed the Julian Calendar became increasingly inaccurate with respect to the seasons. This was especially troubling to the Roman Catholic Church because it affected the determination of the date of Easter, which, by the 16th Century, was well on the way to slipping into Summer.

Pope Paul III recruited several astronomers, principally the Jesuit Christopher Clavius (1537-1612), to come up with a solution. They built upon calendar reform proposals by the astronomer and physician Luigi Lilio (d. 1576). When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius.

On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform.

The Gregorian reform consisted of the following:

  • Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582.
  • The rule for leap years was changed. In the Julian Calendar a year is a leap year if it is divisible by 4. In the Gregorian Calendar a year is a leap year if either (i) it is divisible by 4 but not by 100 or (ii) it is divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.
  • New rules for the determination of the date of Easter were adopted.
  • The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th.

According to some, the term "leap year" derives from the fact that the day of the week on which certain festivals were held normally advanced by one day (since 365 = 7*52 + 1), but in years with an extra day the festivals would "leap" to the weekday following that. However, it may be derived from an old Norwegian word "hlaupÔr" which entered the English language at the time of the Viking invasions (8th - 10th Centuries).

The Gregorian Calendar as it came to be known as after the reforms was adopted immediately upon the promulgation of Pope Gregory's decree in the Catholic countries of Italy, Spain, Portugal and Poland, and shortly thereafter in France and Luxembourg. During the next year or two most Catholic regions of Germany, Belgium, Switzerland and the Netherlands came on board. Hungary followed in 1587. The rest of the Netherlands, Denmark, Germany and Switzerland made the change during 1699 to 1701. By the time the British were ready to go along with the rest of Europe, the old calendar had drifted off by one more day, requiring a correction of eleven days, rather than ten. The Gregorian Calendar was adopted in Britain (and in the British colonies) in 1752, with (Wednesday) September 2, 1752, being followed immediately by (Thursday) September 14, 1752.

In many countries the Julian Calendar was used by the general population long after the official introduction of the Gregorian Calendar. Thus events were recorded in the 16th to 18th Centuries with various dates, depending on which calendar was used. Dates recorded in the Julian Calendar were marked "O.S." for "Old Style", and those in the Gregorian Calendar were marked "N.S." for "New Style".

Sweden adopted the Gregorian Calendar in 1753, Japan in 1873, Egypt in 1875, Eastern Europe during 1912 to 1919 and Turkey in 1927. Following the Bolshevik Revolution in Russia it was decreed that thirteen days would be omitted from the calendar, the day following January 31, 1918, O.S. becoming February 14, 1918, N.S.

In 1923 the Eastern Orthodox Churches adopted a modified form of the Gregorian Calendar in an attempt to render the calendar more accurate (see below). October 1, 1923, in the Julian Calendar became October 14, 1923, in the Eastern Orthodox calendar. The date of Easter is determined by reference to modern lunar astronomy (in contrast to the more approximate lunar model of the Gregorian system).

The Gregorian Calendar is the calendar which is currently in use in all Western and Westernized countries, and Dionysius Exiguus's system of numbering years A.D. has endured to the present time.