History of the Calendar
The purpose of the calendar is to reckon past or future time, to show how many days until a certain event takes place—the harvest or a religious festival—or how long since something important happened. The earliest calendars must have been strongly influenced by the geographical location of the people who made them. In colder countries, the concept of the year was determined by the seasons, specifically by the end of winter. But in warmer countries, where the seasons are less pronounced, the Moon became the basic unit for time reckoning; an old Jewish book says that “the Moon was created for the counting of the days.”
Most of the oldest calendars were lunar calendars, based on the time interval from one new moon to the next—a so-called lunation. But even in a warm climate there are annual events that pay no attention to the phases of the Moon. In some areas it was a rainy season; in Egypt it was the annual flooding of the Nile River. The calendar had to account for these yearly events as well.
The Egyptian Calendar
The ancient Egyptians used a calendar with 12 months of 30 days each, for a total of 360 days per year. About 4000 B.C. they added five extra days at the end of every year to bring it more into line with the solar year.1 These five days became a festival because it was thought to be unlucky to work during that time.
The Egyptians had calculated that the solar year was actually closer to 3651/4 days, but instead of having a single leap day every four years to account for the fractional day (the way we do now), they let the one-quarter day accumulate. After 1,460 solar years, or four periods of 365 years, 1,461 Egyptian years had passed. This means that as the years passed, the Egyptian months fell out of sync with the seasons, so that the summer months eventually fell during winter. Only once every 1,460 years did their calendar year coincide precisely with the solar year.
In addition to the civic calendar, the Egyptians also had a religious calendar that was based on the 291/2-day lunar cycle and was more closely linked with agricultural cycles and the movements of the stars.
1. The correct figures are lunation: 29 d, 12 h, 44 min, 2.8 sec (29.530585 d); solar year: 365 d, 5 h, 48 min, 46 sec (365.242216 d); 12 lunations: 354 d, 8 h, 48 min, 34 sec (354.3671 d).
During antiquity the lunar calendar that best approximated a solar-year calendar was based on a 19-year period, with 7 of these 19 years having 13 months. In all, the period contained 235 months. Still using the lunation value of 291/2 days, this made a total of 6,9321/2 days, while 19 solar years added up to 6,939.7 days, a difference of just one week per period and about five weeks per century.
Even the 19-year period required adjustment, but it became the basis of the calendars of the ancient Chinese, Babylonians, Greeks, and Jews. This same calendar was also used by the Arabs, but Muhammad later forbade shifting from 12 months to 13 months, so that the Islamic calendar now has a lunar year of about 354 days. As a result, the months of the Islamic calendar, as well as the Islamic religious festivals, migrate through all the seasons of the year.
The Roman Calendar
When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence their months were 29 or 31 days long, with the exception of February, which had 28 days. However, four months of 31 days, seven months of 29 days, and one month of 28 days added up to only 355 days. Therefore the Romans invented an extra month called Mercedonius of 22 or 23 days. It was added every second year.
Even with Mercedonius, the Roman calendar eventually became so far off that Julius Caesar, advised by the astronomer Sosigenes, ordered a sweeping reform. 46 B.C. was made 445 days long by imperial decree, bringing the calendar back in step with the seasons. Then the solar year (with the value of 365 days and 6 hours) was made the basis of the calendar. The months were 30 or 31 days in length, and to take care of the 6 hours, every fourth year was made a 366-day year. Moreover, Caesar decreed the year began with the first of January, not with the vernal equinox in late March.
This calendar was named the Julian calendar, after Julius Caesar, and it continues to be used by Eastern Orthodox churches for holiday calculations to this day. However, despite the correction, the Julian calendar is still 111/2 minutes longer than the actual solar year, and after a number of centuries, even 111/2 minutes adds up.
The Gregorian Reform
By the 15th century the Julian calendar had drifted behind the solar calendar by about a week, so that the vernal equinox was falling around March 12 instead of around March 20. Pope Sixtus IV (who reigned from 1471 to 1484) decided that another reform was needed and called the German astronomer Regiomontanus to Rome to advise him. Regiomontanus arrived in 1475, but unfortunately he died shortly afterward, and the pope's plans for reform died with him.
Then in 1545, the Council of Trent authorized Pope Paul III to reform the calendar once more. Most of the mathematical and astronomical work was done by Father Christopher Clavius, S.J. The immediate correction, advised by Father Clavius and ordered by Pope Gregory XIII, was that Thursday, Oct. 4, 1582, was to be the last day of the Julian calendar. The next day would be Friday, Oct. 15. For long-range accuracy, a formula suggested by the Vatican librarian Aloysius Giglio was adopted: every fourth year is a leap year unless it is a century year like 1700 or 1800. Century years can be leap years only when they are divisible by 400 (e.g., 1600 and 2000). This rule eliminates three leap years in four centuries, making the calendar sufficiently accurate.
In spite of the revised leap year rule, an average calendar year is still about 26 seconds longer than the Earth's orbital period. But this discrepancy will need 3,323 years to build up to a single day.
Reform Adopted Gradually
The Gregorian reform was not adopted throughout the West immediately. Most Catholic countries quickly changed to the pope's new calendar in 1582. But Europe's Protestant princes chose to ignore the papal bull and continued with the Julian calendar. It was not until 1700 that the Protestant rulers of Germany and the Netherlands changed to the new calendar. In Great Britain (and its colonies) the shift did not take place until 1752, and in Russia a revolution was needed to introduce the Gregorian calendar in 1918. In Turkey, the Islamic calendar was used until 1926.
A Better Calendar?
Despite its widespread use, the Gregorian calendar has a number of weaknesses. It cannot be divided into equal halves or quarters; the number of days per month is haphazard; and months and years may begin on any day of the week. Holidays pegged to specific dates may also fall on any day of the week, and few Americans can predict when Thanksgiving will occur next year. Since Gregory XIII, many other proposals for calendar reform have been made, but none has been permanently adopted. In the meantime, the Gregorian calendar keeps the calendar dates in reasonable unison with astronomical events.
This bit from Wikipedia describes the calendars used in the world today:
Currently used calendars
Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes, including in the People's Republic of China and India (along with the Indian national calendar). Due to the Gregorian calendar's obvious connotations of Western Christianity, non-Christians and even some Christians sometimes justify its use by replacing the traditional era notations "AD" and "BC" ("Anno Domini" and "Before Christ") with "CE" and "BCE" ("Common Era" and "Before Common Era"). The Hindu calendars are some of the most ancient calendars of the world. Eastern Christians of eastern Europe and western Asia used for a long time the Julian Calendar, that of the old Orthodox church, in countries like Russia. For over 1500 years, Westerners used the Julian Calendar also.
While the Gregorian calendar is widely used in Israel's business and day-to-day affairs, the Hebrew calendar, used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays) and can be used there for business dealings (such as for the dating of checks).
The Iranian (Persian) calendar is used in Iran and Afghanistan. The Islamic calendar is used by most non-Iranian Muslims worldwide. The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and/or social purposes. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea. In Thailand, where the Thai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditional Buddhist calendar.
Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar or the astronomical year numbering system.
Mayan Calendar Complex presented on this blog in October.
The Earth Calendar. An interactive calendar featuring all the countries of the world and the religions.
The Jewish Calendar. It is described as a lunisolar calendar, thus incorporating the best of both cycles.
Calendar as described in Wikipedia.
"History of the Calendar." Infoplease.© 2000–2007 Pearson Education, publishing as Infoplease. Accessed: 31 Dec. 2009. http://www.infoplease.com/ipa/A0002061.html.