GREGORIAN CALENDAR

Calendar introduced by pope Gregorius 13th in the year 1582, to rectify the difference between the calendar year and the solar year that had accumulated in the Julian Calendar.

The essential difference between the Julian Calendar and the Gregorian Calendar, is that the Gregorian Calendar only have 97 leap-years in every 400 year period, whereas the Julian Calendar had 100 leap-years in the same period.

The Gregorian Calendar was introduced in Britain and it's Colonies and Dominions in 1752, where the calendar jumped directly from September 2nd to September 14th. In Denmark-Norway and the Protestant States of Germany the Gregorian Calendar was introduced in the year 1700, where the days between February 18th and March 1st were omitted. In Russia the Julian Calendar was introduced in 1700 to replace the old Russian time starting with "Creation of the World".

In other countries the Gregorian Calendar was introduced at different times, see the table below for specifics:

JULIAN PERIOD

Period of 7980 years, the product of three shorter periods on 28, 19, and 15 years, with the names The Sunday Letter Circle, The Meton Circle, and The Indiktion Circle. Using a Julian Calendar, the first two circles determines the return of the same date of weekdays and the phase of the moon, while the indiktion circle is solely a year number used in the middle ages.

Amongst the more prominent people who made use of the Julian period for chronological calculation is counted the Frenchman Joseph Scaliger (1540-1609).

JULIAN CALENDAR

Calendar that was made by Julius Caesar in 47 BC and introduced in 45 BC (706 AUB) with a solar year of 365.25 days as basis, as three consecutive year were 365 days long and the fourth, a leap year 366 days long. The Julian Calendar maintained the partition of the year into 12 months, that followed the four seasons, the 7 days week, but moved new year to January 1, instead of March 1.

The Julian Calendar thus assume that a year is exactly 365.25 days, however the average solar year is 365.2422, which means that a Julian year is approximately 11 minutes and 14 seconds too long, an error that accumulates to one day in 133 years. Thus 400 Julian years is approximately 3 days too long. This was corrected with the Gregorian Calendar.

To bring order in older and more complicated calendars and get the month to be aligned with the seasons, Caesar had to introduce three leap months in the year 46 BC, which became 455 days long and was remembered by the romans as The year of confusion

In the first five decades after the introduction, uncertainty ruled as to which calendar was the official, however the Julian Calendar was finally accepted in 8 AD or 761 AUC.

In the year 1582 pope Gregorius XIII, improved the system in the Gregorian Calendar, which corrected the accumulated deviation since the time of Caesar.

CALENDAR (latin)

Division of time into years, months, weeks and days; An index of the days of the year as well as other annual publications and handbooks such as The Farmer's Almanac.

In the different systems for time reckoning the apparent movement of the Sun has been observed (days and years), or the Moon's phases (months and weeks), and in most systems it has been rather difficult to combine these two ways of calculation: There isn't an integral number of Moon-months in the Solar-year, and even worse there is not an integral number of days in neither the Moon-month or the Solar-year. In addition to this it has been attempted of consideration to businesses to make the division of time as regular as possible without considering the complex mathematics of celestial objects too much.

Invention of Calendars

The calendar was introduced by the babylonians approximately 6000 BC, by the Egyptians approx. 4000 BC, by the Chinese approx. 2000 BC. The now widely used Gregorian Calendar is based upon the Julian Calendar which in turn was based upon the Roman Calendar.

The Roman Calendar

In the Roman Calendar the year was a lunar year with the length of 355.25 days and new year March 1, Julius Caesar introduced the Julian Calendar 45 BC, with the length 365.25, divided into 12 months that followed the four seasons, 7 days week and New Year January 1.

Pope Gregorius XIII, improved the Julian Calendar into the Gregorian Calendar, and the since Caesar accumulated deviation from the Solar Year, by removing ten days, so that the day after October 4, 1582 became October 15, 1582, at the same time spring equinox was fixed at March 21. The years divisible by 4 is leap years, but only the 100 years divisible by 400 is leap years; thus 1900 wasn't a leap year, 2000 was a leap year, 2100 will not be a leap year.

The Jewish Calendar

The Jewish calendar counts the years from the creation of the world, which has been set arbitrarily to the year 3761 BC. In the Jewish calendar the year consists of 12 months, from new moon to new moon, approximately 354 days, but with an inserted 13th month with 30 days in 7 out of every 19 years, to give the year the correct average length.

Previously the Jewish calendar started the moon when the new moon had been observed; but now it follows rules based on the 19-year lunar period.

The Islamic Calendar

The Islamic Calendar, counts from the exodus to Hedjrea, 622 AD, as year 1, and have a lunar year with 354 days with months of 29 or 30 days, as well as 11 leap years in a 30 year period, each with 355 days. In this calendar, the months are not following the seasons. The month begins when new moon has been observed.

The French Revolution Calendar

The Revolution Calendar in France started year 1 on September 22, 1792, and divided the year into 12 months of 3 decades of 10 days and 5 or 6 celebration days at the end of the year.

The Church Calendar

The Church Calendar uses the solar year for the fixed holidays and the other holidays are moving according to the phases of the moon.

The currently used Gregorian Calendar has the disadvatages that months, seasons and half-years has a differing number of days and that months and weeks doesn't follow each other. Improvements have been suggested; common to all of these is that they use a celebration day outside of the month or season at the end of the year. One suggestion The World Calendar uses 4 seasons of 91 dats (31+30+30) and another uses 13 months of 28 days.

THE YEAR

A calendar year is 365 days long, in leap years 366 days long, grouped into 12 months each of 28 to 31 days long

A calendar year is a period of almost the same length as the astronomic solar year; it takes the Earth on the average 365.2422 days to complete a revolution around the Sun. The difference between the two periods length can seem insignificant, but is however large enough that it throughout the centuries would cause a displacement of the seasons though the calendar year, unless corrected. In our calendar this takes place by introducing a leap day in some calendar years, these years thus have a length of 366 days.

In it's original form (the Julian calendar) the calendar added an extra day every fourth year; this introduced a slight over correction, so that the calendar in 400 years gained approximately 3 days on the solar year

A revised version (the Gregorian calendar), reduces the number of leap days from 100 to 97 in a 400 year period, and thus reduces the effect of the difference between the solar and the calendar year. In the Gregorian the difference between the calendar year and the solar year grows to 3 days in 10,000 years, and is compensated by introducing leap seconds whenever the calendar time differs from the solar time by ±0.9 seconds.

Leap years are years whose numbers are divisible by 4 (1988, 1992, 1996, etc.); the centuries (1600, 1700, 1800, etc.) only if they are divisible by 400 (1600, 2000, etc.). The leap day is the 55th day in the year, the day after February 23.

THE MONTH

The Julian and the Gregorian calendar is based upon the solar year. The corresponding calendar year's 365 or 366 days are divided into 12 calendar months; periods of 28 to 31 days, each with their latin-derived name: January 31 days, February 28 or 29 days, March 31 days, April 30 days, May 31 days, June 30 days, July 31 days, August 31 days, September 30 days, October 31 days, November 30 days, December 31 days.

This way of division and naming to a certain extend reflects the system in the old Roman calendar, that was replaced by the Julian calendar in the year 45 BC.

The pre-Julian calendar tried with different tricks to synchronize the solar year with a bit above 365 days, with a lunar year with just below 355 days (12 periods from full moon to full moon, each a bit above 29.5 days). In the calendar rge lunar year was divided into 12 periods of 28 to 31 days each totalling 355 days: March 31 days, April 29 days, May 31 days, June 29 days, quintilis 29 days, sextilis 31 days, September 29 days, October 31 days, November 29 days, December 29 days, January 29 days, february 28 days. This reflected crudely the phases of the moon; coordinated with the solar year by every other year to insert an extra month Mercedonius with 22 or 23 days after February 23.

The Julian calendar didn't try to fit the lunar phases into the calendar, but chose to structure the calendar year into a system with 12 periods of approximately the same length and with the same names as the Roman calendar's lunar months. Of these names six of them just gave the placement in the year starting with March: quintilis ('the fifth'), sextilis ('the sixth'), september ('the seventh'), october ('the eighth'), november ('the ninth'), december ('the tenth'); and in a not too transparent way the other six seems to be connected to Gods or religious rites: March (cf. the God Mars), April (cf. latin aperire 'to open'), May (cf. the Goddess Maia), June (cf. the God Juno), January (cf. the God Janus), February (cf. latin februare 'to clean'). One of the months in the new calendar was renamed in remembrance of the creator of the new calendar, Julius Caesar, who got the month of July named after him, another was later renamed after his successor Augustus, who after some years of decay got the Julian calendar's usage restored.

THE WEEK

Our calendar divides the year into 12 months seemlessly, in addition to this time is divided into shorter periods of 7 days. The seven day week does not as the calendar periods years and months have any direct connection to movement of celestial objects, but is based in Jewish and other ancient near-oriental social/religious based calendar tradition. In the European calendar tradition it was introduced in the beginning of the 4th century, when Christianity was established as an official religion in the Roman Empire

The weeks of the year doesn't have a name like the months. According to an international agreement from 1971 they are identified with a successive number from 1 to 52. The first week (starting with Monday as the first day) that contains at least four days of the new year is considered week number 1.

THE WEEKDAYS

Because of the displaced rhythm between the caledar year (365/366 days) and the period covered by 52 weeks (52 × 7 = 364), the placement of weekdays in the calendar year is being displaced a bit every year, this as well as the moving holidays are the main reason for making new calendars and almanacs every year.

Older calendars was meant to be reused year after year, the problem with the displacement of weekdays was solved by not dividing the year into weeks and weekdays, but give every day one of the letters between A to G, the Sunday Letter gave the placement of the Sunday, if the Sunday Letter of the year was 'C', then January the 3rd, 10th, 17th, and 31st would be Sundays, and finding the other days from that would be trivial.

The system was inherited by the oldest almanacs, and it took some time before the editors of the almanacs realised that it was unnecessary to make the readers do the calculations, when the calendar of the almanac was updated every year. In Denmark the first almanac to put the weekdays into the calendar was the 1685 edition (the first regular university almanac).

The names of the weekdays reflects the origin of the the week in near-oriental calendar tradition. The Babylonian astronomers assigned each of the days of the week to one of the seven planets or planetary Gods (including the Sun and the Moon), and this system was even in pre-Christian times copied by the Greeks and the Romans. In the contact zone between Roman and Germanic culture a system of finding equivalents to the Roman and Germanic Gods etc. was established most probable in the 3rd or 4th century AD.

For the God Saturnus no equivalent was found in the Germanic culture, to denominate the 7th day of the week, the Roman name was used some places (cf. Saturday in English) in the Nordic countries probably in continuation of an existing tradition as 'laundry day' the seventh day became Lørdag (first part of the name comes from the Nordic word laug or løgh meaning bath).

SOLAR CIRCLE AND THE SUNDAY LETTER

The Solar Circle and the Sunday Letter is being used to determine placement of the the Sunday in the year. A normal year has 52 weeks and one day, and such a year will end on the same day as it started. A leap year has 52 weeks and two days and will thus end with the day after the one it started with. This order in which the weekdays fall on a certain day in a period of 28 years, is the same as the preceeding 28 years, and is called the Solar Circle, and its numeral value denominated the placement of the year in this period.

To denominate the days in the year each day is assigned a letter from 'A' to 'G', so that January 1 is given the letter 'A', January 2 the letter 'B' etc. When 'G' is reached the sequence is restarted with 'A'. The Sunday Letter for a given year is thus the letter found at Sundays. In leap years the leap day (February 24) is given the same letter as February 23th, thus in leap years two Sunday Letters are used one before and one after the leap day.

These numbers can be calculated, since the Solar Circle grows with one each year, and that to each Solar Circle number a Sunday Letter corresponds. See table below.

GREENWICH MEAN TIME (GMT), UTC AND UT1

Since 1884 the Royal British Observatory at Greenwich, which was placed in the 0-meridian, has been responsible for the official measuring of time and the place from which all time zones of the world originated. The average time at the Greenwich observatory was known as GMT (Greenwich Mean (or Meridian) Time) and was determined as an average of astronomical measurements. Before 1925 the time 00:00:00 GMT was observed at noon when the Sun was highest in the sky (12:00 local time) but after 1925 GMT starts at Midnight and follows the local time (except for Daylight Savings Time which doesn't affect GMT).

GMT is still widely used as a reference time in many places, but is no longer the official reference for time, as atomic clocks are now used to measure time, instead of using observed time. The measured time is called Universal Time Coordinated or UTC and for most practical purposes identical to GMT, why the two terms are often used indiscriminantly, however today the most correct term is UTC.

Since the day is defined as an astronomical unit, the time it takes Earth to complete one revolution around the Sun, and this isn't exactly 86,400 seconds, a discrepancy arises between the solar time (called: UT1) and the measured time (UTC), to correct this UTC is adjusted with leap seconds when the difference between UTC and UT1 exceeds ±0.9 seconds. The adjustments usually takes place in the end of June or December, and as the solar time on the average is slightly longer than 86,400 seconds, leap seconds are usually added, but provisions for negative leap seconds exist (but haven't been used yet). When a leap second is inserted it will appear as 23:59:60 (the minute between 23:59:00 and 00:00:00 thus contains 61 seconds).

Web pages for further Information

• Greenwich Mean Time
• GMT - Explained

WHEN DOES THE NEW DECADE, CENTURY OR MILLENIUM START?

QUESTION: When did the new millenium start, at New Year 2000 or New Year 2001? When does a new decade, century or millenium start?

ANSWER:This question reappears every so often, and often there is some confusion about the issue, and at the turn of the 3rd millenium the confusion was so great that most people thought that the turning from 1999 to 2000 marked the entry into the new millenium, and not the turning from 2000 to 2001. And it's not easy, because the numbers are deceiving, why aren't the 1990s just the years 1990 to 1999, instead of 1991 to 2000 and why aren't the new millenia not just the years starting with '2', except that it just isn't so....

At the start of year 1 the first year started, the first decade, century and millenium. When a year lasts a year, a decade ten years, a century 100 years and a millenium 1000 years, then the first year must last until the end of the year 1, the first decade until the end of year 10, the first century until the end of year 100, and finaly the first milenium until then end of year 1000. Therefore the second year has to start with New Year year 2, the second decade New Year year 11, the second century New Year 101 and the second millenium New Year 1001.

Thus the correct answer is that we entered the 3rd millenium at New Year 2000-2001, even though the whole world celebrated the even one year too early!