The word calendar is derived from the latin word "kalendae"; literally, "the day on which accounts are due".
Primitive man did not have a civil calendar as would be familiar to us all today. Instead, a less sophisticated system was used based upon easily observed natural phenomenon, such as the alternation between night and day and the phases of the moon. The phases of the moon bore particular significance to primitive man, as the moon's cycle approximated the menstrual cycle of women, and thus dictated mating to a degree. Less "accurate" timekeeping methodology relied upon the natural seasonal phenomena, such as winter snows, spring leaves and autumnal deciduation.
Amongst the earliest civil calendars were the Roman Republican calendar and the Greek calendar, which were broadly similar.
The Roman Republican calendar, however, was derived from the lunar calendar; it had 10 periods based upon the lunar cycle and then a 2 period winter "intermission" to provide equilibrium.
The twelve periods of the calendar were not equal, however, with four of the twelve months having two additional days and one being a day shorter: already the derivation of the modern western calendar was taking shape.
The Roman Republican calendar aggregated to 355 days, thus being 10.25 days shorter than the solar year.
The twelve months were:
Martius (31 days)
Aprilis (29 days)
Maius (31 days)
Iunius (29 days)
Quintilis (31 days)
Sextilis (29 days)
September (29 days)
October (31 days)
November (29 days)
December (29 days)
Ianuarius (29 days)
Februarius (28 days)
Already the derivation of most of the western calendar month names is also clear.
This first Roman Republican calendar is attributed to Numa Pompilius (c 716-673BC), who was the second King of Rome; however, there are good grounds to believe that the twelve month year concept owes substantially to the influence of the Etruscans of Tuscany. This theory is supported by the naming of the fourth month, which bears close resemblance to the Etruscan spelling of the goddess "Uno", namely "Iunonius".
The years began with the month of March, until 153BC when it was recognised as beginning in Ianuarius (January); this month's name is derived from the name "Ianus", the god of "first beginnings".
The Roman Republican calendar was regulated by priests and pontifices; after the Capitol was dedicated in 509BC, this became the centre of calendar calculation.
Based upon the Roman Republican calendar, the Julian calendar came into being during the reign of Julius Caesar; not surprisingly it bears his name.
In 46BC, Caesar consulted the Alexandrian astronomer Sosigenes. By this date, the RR calendar was widely out of step with the actual fall of the seasons. Sosigenes initial reformation was to add 67 days between November and December of that year. This 445 day year brought the calendar cycle back into alignment with the astronomical cycle. Caesar then abolished the lunar calendar, on the advice of Sosigenes, and installed a solar calendar. Sosigenes was the first to propose the solar calendar as we now know it: 365.25 days; however, because this was impractical, a four yearly cycle, which still exists today, was introduced.
In 45BC, the new solar calendar came into effect, and in 44BC, the seventh month, "Quintilis" was renamed as July in Caesar's honour.
The day names used in the modern western calendar owe much to the heritage of their respective dialects.
The English names for the days of the week can be traced back largely to Saxon Britain and from Viking Gods.
The Saxon names for the days were:
Sun's day
Moon's day
Tiw's day
Woden's day
Thor's day
Frigg's day
Seterne's day
Why there were seven days in the Saxon week is not clear: there appears to be no derivation from ancient calendars. The Inca calendar had a ten-day week; the Romans had eight days; the Assyrians used six.
Most of the "romantic" languages (e.g.: French, Spanish), derived their day names from Latin.
The Latin names for the days of the week were: (the French equivalents are in parentheses)
Dies Solis (dimanche)
Dies Lunae (lundi)
Dies Martis (mardi)
Dies Mercurii (mercredi)
Dies Jovis (jeudi)
Dies Veneris (vendredi)
Dies Saturni (samedi)
The modern calendar as used today in much of the western world, the Gregorian calendar is derived largely from the Julian Calendar, but illustrates the greater accuracy of time calculation that has evolved during man's history.
Although 365.25 days is much more accurate than the 355 days of the original RR calendar, it is still not entirely accurate. Precision calculations tell us that the solar year is actually 365.2422 days in length. Thus, the error, which amounts to 11 minutes 14 seconds a year, led to the gradual misalignment of the Julian Calendar.
Although this error amounted to only one day every 128 years, by the 13th Century, this had accrued seven days. At this time, a re-alignment was suggested, but no action came from this.
By 1472, during the realm of Pope Sixtus IV, the astronomer Johann Müller was invited to adjust the error, which had now reached nine days; however, he was assassinated in 1476 before he was able to complete his calculations.
It was not until the 16th Century, in the era of Pope Gregory XIII that action was finally taken. The now ten-day misalignment was corrected by the issuing of a new calendar in March 1582. Like Caesar before him, Gregory saw that the calendar bore his name.
The Gregorian Calendar was the work of two scientests: the Neopolitan astronomer Aloysius Lilius commenced work, which remained incomplete upon his death in 1576; the German mathematician Christopher Clarius completed it. In 1603, it was published as "Romanii Calenarii a Gregorio XIII PM restituti Explicatio".
The fundamental change that occurred in the Gregorian calendar was the attempt to account for the extra day every 128 years. It was decided that the discrepancy could be overcome by removal of three days every 400 years, which bore close approximation to the actual loss (three days every 384 years). To achieve this, Gregory XII decreed that no century year that was divisible by 400 should be considered a leap year (i.e. with an extra day in February).
To adjust for the ten days that had already been lost, Gregory further ordered that the day following the Feast of St Francis (October 5th) should become October 15th. Thus, the difference between the Julian and Gregorian calendars became ten days overnight. In 1700, which was not a leap year in the Gregorian Calendar, this became 11 days.
Not all states adopted the change in 1582; Portugal, Spain and France did, but the change did not occur in Great Britain until 1752, when September 2nd was followed by September 14th. This fact led to some curious anomolies, such as both Shakespeare and Cervantes dying on April 23rd 1616, but actually dying ten days apart due to the different calendar references.
Of course, whilst the Gregorian Calendar is used in large parts of the modern world, and was the basis for the Year 2000 celebrations, many cultures still have their own calendars. Principle amongst these is the Chinese Calendar.
Based upon a sixty day cycle, the day names are actually derived from two particulates: a celestial "stem" and a terrestial "ending".
There are ten celestial "stems", which are repeated six times to form a sixty day cycle; however, there are twelve terrestial "endings", which are repeated five times.
The ten celestial "stems" are:
Chia
I
Ping
Ting
Wu
Chi
Keng
Hsin
Jen
Kuei
The twelve terrestial "endings" are:
tzu
ch'ou
yin
mao
ch'en
szu
wu
wei
shen
yu
hsü
hai
The cycle begins with "Chai-tzu", and continues with "I-ch'ou", "Ping-yin", etc. The 60th day is "Kuei-hai".
Since the 7th Century, a twelve year cycle has been adopted. This was based upon the position of the "year star" Sui-hsing (Jupiter in Western astronomy) in one of the twelve constellations; however, as Jupiter orbits the sun in approximately 11.86 years, the Chinese Calendar is also subject to a "leap year" scheme. This is called "Ch'ao-ch'en" or "leaped over one constellation".
Twelve constellations are used in the Chinese cycle:
Hsing-chi (Capricorn)
Hsüan-hsiao (Aquarius)
Ch'ü-tzu (Pisces)
Chiang-lü (Aries)
Ta-liang (Taurus)
Shih-ch'eng (Gemini)
Chun-shou (Cancer)
Chun-huo (Leo)
Chun-wei (Virgo)
Shou-hsing (Libra)
Ta-huo (Scorpio)
Hsi-mo (Sagittarius)
Although the Year 2000 was supposed to signify the 2000th anniversary of the birth of Christ in the Gregorian Calendar, in many other calendars it bore no significance.
January 1st, 2000 in the Gregorian Calendar was as follows in other calendars:
Jewish calendar: 23 Tebet, 5760.
Moslem calendar: 24 Ramadan, 1420.
Alexandrian calendar: 22 Tahsas, 1992.
French Republican calendar: 11 Nivôse, 208.
It was also the 5,119th year of the Mayan great cycle.