The ancient Egyptians began numbering their years when the star Sirius rose at the same place as the Sun. The Egyptian calendar was the first solar calendar and contained 365 days. These were divided into 12 30-day months and five days of festival (Neugebauer 1969). From astronomical calculations, Sirius and the Sun coincided in 4241 and 2773 BC, so either of these could have served as Egyptian Year 1.

The calendar used by the ancient Greeks was based on the Moon, and is known as the Metonic calendar. This calendar was based on the observations of Meton of Athens (ca. 440 BC), which showed that 235 lunar months made up almost exactly 19 solar years. This 19-year cycle became known as the Metonic cycle. However, given a nominal twelve-month year, an additional lunar months needed to be added to synchronize the cycle. These were added in years 3, 5, 8, 11, 13, 16, and 19 of the cycle. Around 325 BC, Callippus modified the calendar by noting that 4 19-year Metonic cycles with 940 months were very close to 27,759 days. This is called the Callipic cycle. Hipparchus noted that an even more accurate cycle (now called the Hipparchic cycle) consisted of four Callipic cycles less a day, in which days were very nearly 3760 months. However, neither system was widely used. A lunar-based calendar is still used by some religious sects to determine holidays. Easter, for instance, generally occurs on the first Sunday following the first full moon after the vernal equinox, although the actual scheme is a bit more complicated still (Montes).

Prior to 46 BC, the Roman calendar, or what has been reconstructed of it, is described as a "mess." The Romans calendar originally started the year with the vernal equinox and consisted of 10 months (Martius, Aprilis, Maius, Junius, Quntilis, Sextilis, September, October, November, and December) having a total of 304 days. The numbers still embedded in the last four months of the year are the fossil of this (September, October, November, and December, contain the Latin roots for the numerals seven, eight, nine, and ten, but now fall on the ninth, tenth, eleventh and twelfth months of the year). The 304 days were followed by an unnamed, unnumbered period in winter. The Roman emperor Numa Pompilius (715-673 BC) introduced February and January between January and March, increasing the length of the year to 354 or 355 days. Then in 450 BC, February was moved to its current position.

In the year 46 BC, the Greek Sosigenes convinced Julius Caesar to reform the calendar to a more manageable form. The Julian calendar consisted of cycles of three 365-day years followed by a 366-day leap year.

Although a great improvement over the Metonic calendar, the Julian calendar was still not quite in synchronization with the seasons. The Venerable Bede, an English scholar who lived from 673-735, noted that the vernal equinox had slipped three days earlier than the traditional March 21. The Julian calendar remained in use, however, until replaced by the Gregorian calendar in the late sixteenth century. Although the Roman abbot Dionysius Exiguus proposed that the years be numbered from the birth of Christ in about 524 (Boyer 1968, p. 272), Bede was the first to actually date events from the birth of Christ. This system gives rise to the familiar classification of dates as BC or AD (also sometimes denoted BCE and CE). Interestingly enough, probably because the concept for zero was not widely used in Europe at the time, this method of dating omits the year zero, so that the year 1 BC is followed immediately by the year 1 AD. In any case, whoever zeroed the calendar made an error, since the Bible says Jesus was alive in Herod's time, but Roman records showed that Herod died in what turns out to be 4 BC.

The German astronomer Christoph Clavius (1537-1612) was the motivating force behind the needed revision of the Julian calendar. The reform brought the calendar back in synchronization with the seasons (which now occurred 11 days earlier that their traditional dates), and altered the rules under which leap years occurred. By the new rules, the years that were divisible by 400 were leap years, while other century years were not. These modifications were sufficient to match almost precisely the length of the tropical year.

The reform was first adopted by Pope Gregory XIII, who decreed that the day after October 4, 1582 would be October 15, 1582. This decree was followed by the Catholic countries of France, Spain, Portugal, and Italy. Various Catholic German countries (Germany was not yet unified), Belgium, the Netherlands, and Switzerland followed suit within a year or two, and Hungary followed in 1587. Because of the Pope's decree, the reform of the Julian calendar came to be known as the Gregorian calendar. The rest of Europe did not follow suit for more than a century. The Protestant German countries adopted the Gregorian reform in 1700. By this time, the calendar trailed the seasons by twelve days. England finally followed suit in 1752, declaring that Wednesday, September 2, 1752 was immediately followed by Thursday, September 14, 1752 as shown in the below calendar. The English calendar was also used in America.

English Calendar:

Sweden followed England's lead in 1753. Russia, however, did not follow suit until 1918, when January 31, 1918 was immediately followed by February 14th. In fact, Russia is not on the Gregorian calendar, but on a more accurate one of their own devising. The Russian calendar is designed to more closely approximate the true length of the tropical year, thus has one additional rule for when a year is a leap year. It will remain in synchronization with the Gregorian calendar for thousands more years, by which time one or both will have probably fallen into disuse. Similarly, Iranian calendar is also a more accurate version of the Gregorian calendar (Ross).

The names of the days of the week were derived from gods, "planets," and--in some languages--metals. These name were later carried over to almost all modern European languages, though the names may sound different. In English, Wednesday is derived from a form of the Norse god Odin and Thursday from the Norse God Thor.

During the French Revolution, the French invented and put into use a new French revolutionary calendar. The Revolutionary calendar was established in October 1793, but Year I was made effective on September 22, 1792 (the autumnal equinox). The Revolutionary calendar had 12 months of 30 days, plus 5 or 6 leap days (with a rule for leap years). The French Revolutionary calendar was abolished when Napoleon re-instituted the Gregorian calendar on December 31, 1805.

The Julian calendar still remains in some use, since it is the basis of the system of the Julian date, devised by Clavius' contemporary Julian Scaliger(1540-1609). (In addition, some religious sects still calculate holidays based on the Julian calendar.) The name for this system, incidentally, was from Julius Scaliger, not Julius Caesar. In it, Scaliger defined Day One was as a day when three cycles converged on it. The first cycle was the 28 year period over which the Julian calendar repeats. (After 28 years, all the dates fall on the same days of the week, so you need only buy 28 calendars. Note that since the Gregorian calendar was adopted the calendar now takes 400 years to repeat.) The second was the 19 year Metonic cycle, over which phases of the moon almost land on the same dates of the year. The third cycle was the 15 year ancient Roman tax cycle. Scaliger picked January 1, 4713 BC on the Julian calendar as Day One (Seidelmann 1992, p. 55). I don't know the significance for picking this date as opposed to any other "triple convergence" date.

After Julian date One, subsequent Julian dates are sequential. Therefore, midnight before January 1, 1982 is Julian Date 2,444,970.5. The modified Julian date system, defined as the Julian date minus 2,400,000.5, is also occasionally used by astronomers, but not so frequently in recent years. The Julian and Gregorian calendars differ by 13 days in the 20th and 21st Centuries. They would have been in synchronization during the 3rd Century.

The following table gives the dates corresponding to January 1, 1989 in the Gregorian calendar for various other calendar systems (Astronomical Almanac).

*begins at sunset