Time constant; calendars aren't


As the sun sets tonight around the globe, Jews worldwide will begin the celebration of their new year, Rosh Hashana, which, according to the Jewish calendar, ushers in the 5,761st year since the creation of the world.

Rosh Hashana serves as a reminder that while much of the world marks time by the Gregorian calendar, established in the 16th century by the Roman Catholic Church, other cultures and religions mark their days, months, years and eras differently. And it also shows that any calendar is essentially arbitrary, as those who devise it must select some fixed date as a starting point.

So when does time begin?

Hebrew calendar

For Jews, time begins 5,761 years ago, a date that corresponds to 3761 B.C.

The Hebrew calendar has 12 lunar months, following the cycle of the moon around the earth. Because each lunar orbit is approximately 29 1/2 days, the 12 months total 354 days, about 11 1/4 days short of the solar year of 365 1/4 days. If there were no modifications, the months would gradually drift through the solar year, and a feast such as Passover, which must be celebrated in the spring, would slip to the winter, fall and summer before returning to the springtime.

In making the calendar more stable, the ancient Israelites received help from the Babylonians, albeit through a tragic experience. In 586 B.C., the Babylonian army invaded Israel, destroyed the Temple and took many of the country's inhabitants captive.

The Babylonian Exile was a bitter period for Israel but resulted in the adaptation of elements of the Babylonian calendar to the Hebrew measure of time. The Israelites adopted the Babylonian names for the 12 months and a method for solving their calendar conundrum of drifting months.

To compensate for the extra 11 or so days each year, an extra month of 30 days was inserted at irregular intervals.

For religious purposes, the day begins at sunset and ends the next sunset. So Rosh Hashana, a two-day holiday, begins at sunset tonight and ends at sunset Sunday.

The days of the week are numbered, Day One beginning at sunset Saturday and concluding at sunset Sunday. Only the seventh day, the Sabbath, has a proper name.

Islamic calendar

Muslims mark their time from the date when the prophet Muhammad emigrated from Mecca to Medina, referred to as the Era of Hegira, which many scholars calculate as occurring on July 16, 622.

The Islamic calendar is lunar, with no adjustments to accommodate the solar year. This is set forth by the prophet in the Koran: "The postponing [of a Sacred Month] is indeed an addition to disbelief: thereby the disbelievers are led astray, for they make it lawful one year and forbid it another year in order to adjust the number of months forbidden by Allah, and make such forbidden ones lawful. The evil of their deeds seems pleasing to them. And Allah guides not the people, who disbelieve."

Therefore, the 12 months drift through the seasons over a cycle of about 33 years. Ramadan, the month of fasting set to begin this year in November, starts earlier in succeeding years, eventually occurring in the summer, then spring, then winter. For a devout Muslim who fasts during the daytime hours of Ramadan, the ninth month of the Islamic calendar, this makes a big difference -- a fast during the relatively mild temperatures and shorter days of winter is easier than a summer fast, during which even water is forbidden.

The day begins with sunset, the month with the sighting of the crescent moon after the new moon. For that reason, the number of days in a month is not fixed, making it difficult to determine exact dates far in the future.

There is a seven-day week, beginning with sunset Saturday, with numbers for the days. Day five -- Jum'a, beginning at sunset Thursday and concluding at sunset Friday -- is the day set aside for congregational prayer. Unlike the Christian and Jewish Sabbath, this is not a day of rest.

Hindu calendar

Until the mid-1950s, when the Indian government instituted the Calendar Reform Committee, Indians relied on about 30 calendars to set religious festivals for Hindus, Muslims, Buddhists and Jains. The reform instituted a civil calendar, which has leap years that coincide with the Gregorian calendar but has 12 solar months with the traditional Indian names and do not coincide with the start of the months on the Western calendar.

Secular calendars there measure time from the Saka Era, which begins with the vernal equinox of the year 79, corresponding to the accession to the throne of a Hindu ruler, King Salivahana. But Hindu religious authorities date events according to the Kali Era, which begins with the death of Krishna in 3012 B.C.; and the Vikram Era, corresponding to the coronation of a King Vikramaditya in approximately 58 B.C.

Religious holidays are based on a lunar calendar of 12 months of 29 days each. As in any lunar calendar, the year comes up short by about 11 days, so an extra month is added about every three years. Each lunar month has a designated fortnight of the waxing moon, the bright fortnight, which is considered auspicious; and the waning moon, the dark fortnight, which is considered inauspicious.

Gregorian calendar

The Western calendar has its roots in the Roman calendar, which was based on the lunar cycles.

At Julius Caesar's behest, an Alexandrian astronomer, Sosigenes suggested adopting a solar year of 365.25 days, similar to that used by the Egyptians. To compensate for the extra quarter day each year, Julius Caesar ordered that an extra day be inserted every four years. Rome adopted the Julian Calendar in 46 B.C.

The new calendar had a problem: A solar year is 364.2422 days, 11 minutes and 14 seconds less than a Julian year. Over the centuries, this error compounded. After 131 years, the calendar had moved ahead of the vernal equinox, used in determining the date of Easter, by about a day. By 1545, the vernal equinox date in the Julian calendar had moved by 10 days.

To correct the discrepancy, Pope Gregory XIII issued a papal bull ordering that 10 days be omitted from the calendar, decreeing that Oct. 5, 1582, be followed by Oct. 15, 1582. To ensure future synchronicity, he decreed the length of the year to be 365.2422 days.

He also modified the rules for establishing a leap year. In the Julian calendar, a year is a leap year if it is divisible by 4; in the Gregorian reform, a leap year is one that is divisible by 4 but not by 100, or one that is divisible by 400. Thus, 2000 is a leap year, but 1900 and 2100 are not. The Gregorian bull also established new rules for determining the date of Easter.

Today is

Sept. 29, 2000 (Gregorian calendar)

The 29th day of Elul, in the year 5760, until sundown (Jewish calendar)

The 1st day of Tishri, in the year 5761, after sundown (Jewish calendar)

1 Rajab 1421 (Islamic calendar)


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