We inherited our calendar from the Romans (the Greeks and Egyptians had their own, different calendars). [We inherited our base-60 time and measurement systems (minutes, seconds, degrees) from the Babylonians].

Evolution of The Roman Calendar

Prior to 46 B.C. the Roman calendar originally started the year with the Vernal Equinox (the day in Spring when the amount of daylight was the same as night - 12 hours each) 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 our calendar 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).

Since the Roman calendar had a year of 304 days, this was obviously 61 days short of the real length of a year and these "winter days" as they were called (between December and March) were not assigned to any month.

Numa Pompilius, Rome's king after Romulus, tried to even out the difference by adding two new months to the calendar: Janus (January) and Februarius (February). His version of the calendar now had 355 days, still 10 days short of the actual length of a year, so he created a new month called "Mercedinus" that would come after February every other year in order to keep the calendar in sync with the seasons.

Why 355 days? The word "Month" is derived from "Moon". Most early calendars were based on the lunar cycle. A month was how long it took the Moon to orbit Earth - about 29.5 days. Astronomers knew that the year was actually 365.25 days long. So, 365.25 / 29.5 = 12 (rounded down). So that's how there came to be 12 months. Since months had to have an integer number of days, some months were rounded down to 29 days and some up to 30 days. However, 29.5 x 12 is only 355. So those missing 10 days from the year, Numa Pompilius assigned to the month of "Mercedinus".

Then, in 46 B.C., along comes Julius Caesar who attempted to clean up the calendar. He removed Mercidinius and reallocated its 10 days to some of the other months so that all months had 30 or 31 days, except February which had 29 days. Every 4 years, February would have 30 days to account for the quarter day in the Earth's 365.25 day-long year. Quntilis (later renamed to "July" in Julius' honour) had 31 days. It's not known if this was due to Julius Caesar's vanity or if their calendar simply worked better this way. The month of "Sextilus" which came after July had 30 days.

Month days Etymology
January 31 Janus, two-headed god of doorways and gates
February 28/29 Februarius, the month of expiation
March 31 Mars, god of war
April 30 derived from Latin verb meaning "to open"
May 31 Maia, goddess of Spring and growth
June 30 Juno, goddess of wisdom and marriage
July 31 Julius Caesar
August 31 Augustus Caesar
September 30 7 in Latin
October 31 8 in Latin
November 30 9 in Latin
December 31 10 in Latin

The Julian calendar consisted of cycles of three 365-day years followed by a 366-day leap year.

When Julius Caesar decided to put his Julian calendar into practice, he had to wait for the perfect time for the seasons to match the calendar date and as a result, the year 46 BC was only 455 days long!

A couple of decades later, Augustus was Emperor and the senate voted that that Sextilus should be renamed in his honour and so it became Augustus (later August). However, the senate didn't like the fact that Augustus's month had 30 days and Julius's has 31 so they changed Augustus to 31 days, taking the extra day from February, dropping it to having 28 days. This threw the organised way the months changed from 30 and 31 days into disarray.

Not satisfied, they tinkered more with altering the number of days in the months according to what they thought was important and because they did not want three consecutive months with 31 days. Two successive months with 31 days only occurs twice in our calendar - July/August and December/January as a result.

Roll on a few hundred years and the Venerable Bede, an English scholar, noticed in the early 8th century that the Vernal Equinox was occurring 3 days earlier than it should. This is because the length of the year is actually 365.2422 days and not 365.25. That 0.0078 difference was starting to move the actual seasons out of sync with the Julian calendar.

Nevertheless, the calendar remained in use until the late 16th century when the calendar and seasons were now 11 days out of sync.

Introduction of the Gregorian Calendar

German astronomer Christoph Clavius was the motivating force behind the needed revision of the Julian calendar and Pope Gregory XIII implemented the change in how the calendar was calculated. It's known as the Gregorian calendar and is the one we still use today. This introduced the new rules for leap year calculation where years divisible by 4 or 400 would be leap years whereas those divisible by by both 4 and 100 were not. These modifications were sufficient to match almost precisely the length of the "tropical year" (365.2422 days).

In order to get the calendar back on track, Pope Gregory decreed that the day after October 4, 1582 would be October 15, 1582. People at the time were not happy that they were robbed of 11 days of their lives (as they saw it).

It was on this day in 1582 that the Gregorian calendar—the most widely used calendar in the world—was introduced by Pope Gregory XIII in his bull Inter gravissimas. The new calendar system replaced the Julian calendar's year of 365.25 days with a year of 365.2425 days, meaning that the number of leap years went from 100 in 400 years to 97 in 400 years. What a difference 0.0075 of a day makes. The Gregorian calendar is quite accurate, accumulating an error of only one day in about 3,300 years but that it also has some weaknesses: it cannot be divided into equal halves or quarters, and years and months may begin on any day of the week.

This decree was to be 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. The rest of Europe did not update their calendars for more than a century and the Protestant German countries adopted the Gregorian reform in 1700 (their calendar was behind by 12 days at this time).

England finally joined the reform in 1752, declaring that Wednesday, September 2, 1752 was immediately to be followed by Thursday, September 14, 1752. This calendar was also used in Ireland and America. Sweden followed England's lead in 1753.

Russia Joins The Calendrical Reformation

The calendrical mess with different countries using different systems persisted into the 20th century. Russia only implemented the Gregorian calendar in 1918, when January 31, 1918 was immediately followed by February 14th.

In fact, Russia is not actually 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 (365.2422 days), so it has one additional rule for when a year is a leap year. It will remain in sync with the Gregorian calendar for thousands more years, by which time one or both will have probably fallen into disuse. Interestingly, the Iranian calendar is also a more accurate version of the Gregorian calendar.

What about the Chinese Calendar?

The Chinese calendar is lunisolar. It is based on exact astronomical observations of the sun's longitude and the moon's phases. It attempts to have its years coincide with the tropical year and shares some similarities with the Jewish calendar. The similarities are that an ordinary year has 12 months and a leap year has 13 months; and an ordinary year has 353-355 days while a leap year has 383-385 days.

The Chinese calendar

Although the Chinese calendar originated in China, the Gregorian calendar is used there for civil purposes in modern times. However, the Chinese calendar is still used among various Chinese communities around the world to determine festival dates, such as Chinese New Year, as well as auspicious dates, such as wedding dates. It is also used to determine moon phases because it follows the moon.

The Jewish Calendar

The Jewish calendar is another that's still in use today for calculating or determining when holidays and dates of note occur. It doesn't use the BC/AD denomination of years. The year number on the Jewish calendar represents the number of years since creation, as calculated by adding up the ages of people in the Bible back to the time of creation.

The calendar is primarily lunar, with each month beginning on the new moon, when the first sliver of moon becomes visible after the dark of the moon. In ancient times, the new months used to be determined by observation. When people observed the new moon, they would notify the Sanhedrin. When he heard testimony from two independent, reliable eyewitnesses that the new moon occurred on a certain date, they would declare the rosh chodesh (first of the month) and send out messengers to tell people when the month began.

The Problem With Lunar Calendars

The problem with strictly lunar calendars is that there are approximately 12.4 lunar months in every solar year, so a 12-month lunar calendar loses about 11 days every year and a 13-month lunar calendar gains about 19 days every year. The months on such a calendar "drift" relative to the solar year. On a 12 month calendar, the month of Nissan, which is supposed to occur in the Spring, occurs 11 days earlier each year, eventually occurring in the Winter, the Autumn, the Summer, and then the Spring again. To compensate for this drift, an extra month was occasionally added: a second month of Adar. The month of Nissan would occur 11 days earlier for two or three years, and then would jump forward 29 or 30 days, balancing out the drift.

In the fourth century, Hillel II established a fixed calendar based on mathematical and astronomical calculations. This calendar, still in use, standardized the length of months and the addition of months over the course of a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar II is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle. The new year that began Monday, September 25, 1995 (Jewish calendar year 5756) was the 18th year of the cycle. Jewish year 5758 (beginning October 2, 1997) was the first year of the next cycle.

The Islamic Calendar

The Islamic calendar, Muslim calendar or Hijri calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days (like the early Roman calendar).

It is used to date events in many Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper days on which to observe the annual fast (Ramadan), to attend Hajj, and to celebrate other Islamic holidays and festivals.

The first year was the Islamic year beginning in AD 622 during which the emigration of the Islamic Prophet Muhammad from Mecca to Medina, known as the Hijra, occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra).

The current Islamic year is 1434 AH. In the Gregorian calendar 1434 AH runs from approximately 14 November 2012 (evening) to 4 November 2013 (evening).


And that's only a handful of the various calendars that have existed or still exist and are in use today.

Who said calendars were easy??

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