Learn what other calendars have been used besides the Gregorian one.
Besides the Gregorian calendar the other calendars that have been used are as follows:
A lunar calendar composed of alternating 29-day and 30-day months to equal roughly 354 lunar days. When the calendar became too misaligned with astronomical events, an extra month was added. In addition, three extra months were added every eight years to coordinate this calendar with the solar year.
A lunar month calendar of 12 periods having either 29 or 30 days (to compensate for the 29.5 days from new moon to new moon). The new year begins on the first new moon over China after the sun enters Aquarius (between January 21 and February 19). Each year has both a number and a name. For example, the year 1992 or 4629 in the Chinese era is the year of the monkey. The calendar is synchronized with the solar year by the addition of extra months at fixed intervals.
A lunar, 12-month calendar with 30 and 29 days alternating every month is equal to 354 days. The calendar has a 30-year cycle with designated leap years of 355 days (one day added to last month) in the 30-year period. The Islamic year does not attempt to relate to the solar year (relate to the season). The dating of the beginning of the calendar is 622 C.E. (the date of Mohammed's flight from Mecca to Medina).
A blend of the solar and lunar calendar, this calendar adds an extra month (Adar Sheni or the second Adar or Veadar) to keep the lunar and solar years in alignment. This occurs seven times during the 19-year cycle. When the extra 29-day month is inserted, the month Adar has 30 days instead of 29. In a usual year the 12 months alternate in a 30 then 29 day repetition.
The ancient Egyptians were the first to use a solar calendar (about 4236 B.C.E. or 4242 B.C.E.), but their year started with the rising of Sirius, the brightest star in the sky. The year, composed of 365 days, was 0.25 days short of the true solar year, so eventually the Egyptian calendar did not coincide with the seasons. It used 12, 30-day months with five-day weeks and five dates of festival.
Still used in areas of Egypt and Ethiopia, it has a similar cycle to the Egyptian calendar—12 months of 30 days followed by five complementary days. When a leap year occurs, usually preceding the Julian calendar leap year, the complementary days increase to six.
Borrowing from the ancient Greek calendar, which had a four-year cycle based on the Olympic Games, the earliest Roman calendar (about 738 B.C.E.) had 304 days with 10 months. Every second year a short month of 22 or 23 days was added to coincide with the solar year. Eventually two more months were added at the end of the year (Januarius and Februarius) to increase the year to 354 days.
The Roman republican calendar replaced this calendar during the reign of Tarquinius Priscus (616-579 B.C.E.). This new lunar calendar had 355 days with the month of February having 28 days. The other month had either 29 or 31 days. To keep the calendar aligned with the seasons, an extra month was added every two years. By the time the Julian calendar replaced this one, the calendar was three months ahead of the season schedule.
Julius Caesar (100 B.C.E.-44 B.C.E.) in 46 B.C.E., wishing to have one calendar in use for all the empire, had the astronomer Sosigenes develop a uniform solar calendar with the year being 365 days with one day (leap day) added every fourth year to compensate for the true solar year of 365.25 days. The year had 12 months with 30 or 31 days except for February, which had 28 days (or 29 days in a leap year). The first of the year was moved from March 1 to January 1.
Pope Gregory XIII (1502-1585) instituted calendrical reform in 1582 to realign the church celebration of Easter with the vernal equinox (the first day of spring). To better align this solar calendar with the seasons, the new calendar would not have a leap year in the century years that were not divisible by 400. Because the solar year is shortening, today a one-second adjustment (usually on December 31 at midnight) is made when appropriate to compensate for this effect.
It has the same structure as the Gregorian calendar in years, months and weeks. But the years are enumerated in terms of the reigns of emperors as epochs. The last epoch (for Emperor Akihito) is Epoch Heisei, starting January 8, 1989.
The principal Indian calendars reckon their epochs from historical events, such as rulers accessions or death dates, or a religious founder's dates. The Vikrama era (originally from northern India and still used in western India) dates from February 23, 57 B.C.E. in the Gregorian calendar. The Sake era dates from March 3, 78 C.E. in the Gregorian calendar and is based on the solar year with 12 months of 365 days and 366 days in leap years.
The first five months have 31 days; the last seven have 30 days. In leap years, the first six months have 31 days and the last six have 30. The Saka era is the national calendar of India (as of 1957). The Buddhist era starts with 543 B.C.E. (believed date of Buddha's death).
Three other secular calendars of note are the Julian Day calendar (a calendar astronomers use that counts days within a 7,980-year period and must be used with a table), the perpetual calendar (which gives the days of the week for the Julian and Gregorian calendars as well), and the World calendar, which is similar to the perpetual calendar, having 12 months of 30 or 31 days a year day at the end of each year and a leap-year day before July 1 every four years.
There have been attempts to reform and simplify the calendar. One such example is the Thirteen-Month or International Fixed calendar that would have 13 months of four weeks each. The month Sol would come before July; there would be a year-day at the end of each year and a leap-year-day every four years just before July 1.
A radical reform was done in France when the French republican calendar (1793-1806) replaced the Gregorian calendar after the French Revolution. It had 12 months of 30 days and five supplementary days at the end of the year (six in a leap-year) and weeks were replaced with 10-day decades.