This Julian calendar greatly improved the situation, but there was
still a small error, where the calendar would get about 3 days off
for each 400 years. As a result, the actual occurrence of the
equinoxes and solstices slowly drifted away from
their assigned calendar dates. As the date of the spring
equinox determines that of Easter, the church was
concerned, and Pope Gregory XIII, with the help of an
astronomer, Christopher Clavius (1537-1612), introduced
what is now called the Gregorian calendar. Wednesday, Oct.
4, 1582 (Julian), which was followed by Thursday, Oct. 15, 1582
(Gregorian); leap years occur in years exactly divisible
by four, except that years ending in 00 must be divisible
by 400 to be leap years. Thus, 1600, 1984, and 2000 are
leap years, but 1800 and 1900 are not.
Many different societies have used their own special calendar
during recorded history. Most have been based on the apparent
motion in the sky of the Sun or Moon. Early in the Roman Empire,
around what we would now call 400 BC, a calendar with a year of
365 days was instituted. Over time, the calendar got out of step
with the seasons, and the Emperor Julius Caesar declared every fourth year
to be a 'leap year' (with an extra day) and, to solve the past problems,
the year we would call 46 BC was made 445 days long!
The Gregorian civil calendar is a solar calendar, calculated
without reference to the Moon. However, the Gregorian
calendar also includes rules for determining the date of
Easter and other religious holidays, which are based on
both the Sun and the Moon. The Gregorian calendar was
quickly adopted by Roman Catholic countries. Other
countries adopted it later, sometimes choosing only the
civil part. It was not adopted by the Soviet Union until
1918; Turkey did not adopt it until 1927.
The year used to begin at different times in different
localities. The Roman year originally began in March; December,
whose name is derived from the Latin word for "ten," was
the tenth month of the year. (Similarly, September was the
seventh month, October, the eighth, and November, the ninth.)
In 153 BC, Roman consuls
began taking office on January 1, which became the
beginning of the year. This practice was retained in the
Julian and Gregorian calendars, although other starting
dates continued to be used; England and its colonies, for
example, used March 25 and the Julian reckoning until 1752.
Thus, George Washington was officially born on Feb. 11,
1731, Old Style (O.S.); this is Feb. 22, 1732, Gregorian,
or New Style (N.S.).
The Babylonians used a nonastronomical, 7-day interval, the
week, which was adopted by the Jews. The seventh day, the
Sabbath, was given a religious significance.
Independently, the Romans associated a cycle of 7 days with
the Sun, the Moon, and the five known planets. Their names
became attached to the days of the week: Sunday (dies solis, "Sun's
day"), Monday (dies lunae, "Moon's day"), and
Saturday (dies Saturni, "Saturn's day") retain their names
derived directly from the Roman culture, and Tuesday ("Tiw's
day"), Wednesday ("Woden's day"), Thursday ("Thor's
day"), and Friday ("Frigg's day") are derived from the
Germanic equivalents of Mars, Mercury, Jupiter, and Venus,
In ancient calendars, years were generally numbered
according to the year of a ruler's reign. About AD 525, a
monk named Dionysius Exiguus suggested that years be
counted from the birth of Christ, which was designated AD
(anno Domini, "the year of the Lord") 1. This proposal
came to be adopted throughout Christendom during the next 500 years.
The year before AD 1 is designated 1 BC (before Christ).
(There was no "year zero!")
Dionysius had referred the year of Christ's birth to other
eras. Modern chronology, however, suggests that Dionysius had
been off in his calculations that now firmly places the event
of Jesus' Birth at about 4 BC.
The 1st century of the Christian Era began in AD 1, the 2d
in AD 101; the 21st began in 2001.
The Hebrew Calendar
The Hebrew calendar in use today begins at the Creation,
which is calculated to have occurred 3,760 years before the
The week consists of 7 days, beginning with
Saturday, the Sabbath.
The year consists of 12 lunar
months -- Tishri, Heshvan, Kislav, Tebet, Shebat, Adar,
Nisan, Iyar, Sivan, Tammuz, Ab, and Elul -- which are
alternately 29 and 30 days long. Because a year is some 11
days longer than 12 lunar months, a 13th month ve-Adar, is
added seven times during every 19-year cycle.
Therefore, the Hebrew calendar stays fairly synchronized with
The Islamic Calendar
Muslims begin their calendar at the day and year (July 16,
622, by the Gregorian calendar) when Muhammad fled from
Mecca to Medina. (The Hegira actually occurred around two
months after their beginning of that official lunar year.)
There are 12 lunar months of alternate 30
and 29 days, making the year 354 days long. Because of the
shortness of the year, the months move backward through all
the seasons, completing a cycle every 32 1/2 years.
The months are Muharram, Safar, Rabi I, Rabi II, Jumada I,
Jumada II, Rajab, Shaban, Ramadan, Shawwal, Zulkadah, and
(The result of all this is that the Holy month of Ramadan occurs
in different seasons in different years.)
Archer, Peter, The Christian Calendar and the
Gregorian Reform (1941); Asimov, Isaac, The Clock We Live
On (1963); Keane, Jerryl, Book of Calendars (1981);
Michels, A. K., The Calendar of the Roman Republic (1967;
repr. 1978); Monaco, James, The French Revolutionary
Perpetual Calendar (1982); Philips, Alexander, The
Calendar: Its History, Structure, and Improvement (1921);
Schocken, W. A., The Calendar of the Mayas (1986);
Watkins, Harold, Time Counts: The Story of the Calendar (1954).
The individual articles presented here were generally first published
in the early 1980s. This subject presentation was first placed
on the Internet in December 1997.
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