Time is one of the
world's deepest mysteries. No one can say
exactly what it is. Yet, the ability to measure
time makes our way of life possible. Most human
activities involve groups of people acting
together in the same place at the same time.
People could not do this if they did not all
measure time in the same way.
One way of thinking
about time is to imagine a world without time.
This timeless world would be at a standstill.
But if some kind of change took place, that
timeless world would be different "now" than it
was "before." The period--no matter how
brief--between "before" and "now" indicates that
time must have passed. Thus, time and change are
related because the passing of
time
depends on changes taking
place. In the real
world, changes never stop happening. Some
changes seem to happen only once, like the
falling of a particular leaf. Other changes
happen over and over again, like the breaking of
waves against the shore.
Any change that takes
place again and again stands out from other
changes. The rising and setting of the sun are
examples of such change. The first people to
keep time probably counted such natural
repeating events and used them to keep track of
events that did not repeat. Later, people made
clocks to imitate the regularity of natural
events. When people began to count repeating
events, they began to measure time.
Units of time
measurement. For early peoples, the only changes
that were truly regular--that is, repeated
themselves evenly--were the motions of objects
in the sky. The most obvious of these changes
was the alternate daylight and darkness, caused
by the rising and setting of the sun. Each of
these cycles of the sun came to be called a day.
Another regular change in the sky was the change
in the visible shape of the moon. Each cycle of
the moon's changing shape takes about 29 1/2
days, or a month.
The cycle of the seasons
gave people an even longer unit of time. By
watching the stars just before dawn or after
sunset, people saw that the sun moved slowly
eastward among the stars. The sun made a full
circle around the sky in one cycle of the
seasons. This cycle takes about 365 1/4 days, or
a year.
Day
Week
Month
Year
Century
Calendar is a system of
measuring and recording the passage of time. A
major scientific advance occurred when people
realized that nature furnishes a regular
sequence of seasons. The seasons governed their
lives, determined their needs, and controlled
the supply of their natural foods. They needed a
calendar so they could prepare for the hardships
of winter.
Frequently Asked
Questions
About Calendars,
an extensive list of
questions and answers about several modern and
ancient calendars.
http://www.pip.dknet.dk/~c-t/cal/calendar20.html
Babylonian
calendar
Egyptian
calendar
Early Roman
calendar
Julian
calendar
Maya
calendar
Calendar reform would
simplify the present calendar. Two proposed
calendars have received considerable support. In
each, months and years would begin on the same
day of the week every year. All months would
contain the same or nearly the same number of
days. The Fixed Calendar, also called The
Thirteen-Month Calendar, would provide 13 months
exactly four weeks long. The extra month, Sol,
would come before July. A year day placed at the
end of the year would belong to no week or
month. Every four years, a leap-year day would
be added just before July 1. The World Calendar
would have 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.
The Aztec had a 260-day
religious calendar. Priests used the calendar to
determine luck days for such activities as
sowing crops, building houses, and going to war.
The Aztec also had a 365-day solar calendar. It
consisted of 18 months of 20 days each plus 5
extra days. Like the Maya before them, the Aztec
Indians of Mexico had a religious calendar of
260 days and a solar calendar of 365 days. Many
of the Aztecs' religious ceremonies, including
frequent human sacrifices, were performed at the
Great Temple (pictured), located in the center
of their capital city of
Tenochtitlan.
World Book illustration
by Robert AddisonEvery 52 years, the Aztec held
a great celebration called the Binding Up of the
Years or the New Fire Ceremony. Before the
celebration, people let their hearth fires go
out. At the start of the new 52-year cycle, the
priests lit a new fire on the chest of a
sacrificial victim. People pricked themselves to
add their blood to the sacrifice. Then they
relit their hearth fires from the new fire and
feasted.
Maya priests observed
the positions of the sun, moon, and stars. They
made tables predicting eclipses and the orbit of
the planet Venus.
The priests also used
mathematics and astronomy to develop two kinds
of calendars. One was a sacred almanac of 260
days. Each day was named with one of 20 day
names and a number from 1 to 13. Each of the 20
day names had a god or goddess associated with
it. The priests predicted good or bad luck by
studying the combinations of gods or goddesses
and numbers. The Maya also had a calendar of 365
days, based on the orbit of the earth around the
sun. These days were divided into 18 months of
20 days each, plus 5 days at the end of the
year. The Maya considered these last 5 days of
the year to be extremely
unlucky. During that
period they fasted, made many sacrifices, and
avoided unnecessary work.
The Romans apparently
borrowed parts of their earliest known calendar
from the Greeks. The calendar consisted of 10
months in a year of 304 days. The Romans seem to
have ignored the remaining 61 days, which fell
in the middle of winter. The 10 months were
named Martius, Aprilis, Maius, Junius,
Quintilis, Sextilis, September, October,
November, and December. The last six names were
taken from the words for five, six, seven,
eight, nine, and ten. Romulus, the legendary
first ruler of Rome, is supposed to have
introduced this calendar in the 700's B.C.
According to tradition,
the Roman ruler Numa Pompilius added January and
February to the calendar. This made the Roman
year 355 days long. To make the calendar
correspond approximately to the solar year, Numa
also ordered the addition every other year of a
month called Mercedinus. Mercedinus was inserted
after February 23 or 24, and the last days of
February were moved to the end of Mercedinus. In
years when it was inserted, Mercedinus added 22
or 23 days to the year.
By the time of Julius
Caesar, the accumulated error caused by the
incorrect length of the Roman year--and by the
occasional failure to add extra days at the
proper times--had made the calendar about three
months ahead of the seasons. Winter occurred in
September, and autumn came in the month now
called July.
In 46 B.C., Caesar asked
the astronomer Sosigenes to review the calendar
and suggest ways for improving it. Acting on
Sosigenes's suggestions, Caesar ordered the
Romans to disregard the moon in calculating
their calendars. He divided the year into 12
months of 31 and 30 days, except for February,
which had only 29 days. Every fourth year, it
would have 30 days. To realign the calendar with
the seasons, Caesar ruled that the year we know
as 46 B.C. should have 445 days. The Romans
called it the year of confusion.
The Romans renamed
Quintilis to honor Julius Caesar, giving us
July. Sextilis was renamed August by the Roman
Senate to honor the Emperor Augustus. According
to tradition, Augustus moved a day from February
to August to make August as long as July.
The Julian calendar was
widely used for more than 1,500 years. A Julian
year lasted 365 1/4 days. But it was actually
about 11 minutes and 14 seconds longer than the
solar year. This difference led to a gradual
change in the dates on which the seasons began.
By A.D. 1580, the spring equinox fell 10 days
earlier on the Julian calendar than its
appointed date.
Two years later, in
1582, Pope Gregory XIII corrected the calendar
with the newly developed Gregorian calendar.
The Babylonians, who
lived in what is now Iraq, added an extra month
to their years at irregular intervals. Their
calendar, composed of alternate 29-day and
30-day months, kept roughly in step with the
lunar year. To balance the calendar with the
solar year, the early Babylonians calculated
that they needed to add an extra month three
times every eight years. But this system still
did not accurately make up for the accumulated
differences between the solar year and the lunar
year. Whenever the king felt that the calendar
had slipped too far out of step with the
seasons, he ordered another extra month.
However, the Babylonian calendar was quite
confused until the 300's B.C., when the
Babylonians began to use a more reliable system.
The Egyptians were
probably the first to adopt a mainly solar
calendar. They noted that the Dog Star, Sirius,
reappeared in the eastern sky just before
sunrise after several months of invisibility.
They also observed that the annual flood of the
Nile River came soon after Sirius reappeared.
They used this combination of events to fix
their calendar and came to recognize a year of
365 days, made up of 12 months each 30 days
long, and an extra five days added at the end.
But they did not allow for the extra fourth of a
day, and their calendar drifted into error.
According to the famed Egyptologist J. H.
Breasted, the earliest date known in the
Egyptian calendar corresponds to 4236 B.C. in
terms of the Gregorian calendar.
Before the invention of
the clock, people watched the sun, the moon, and
the stars to tell time. The daily rising of the
sun provided a short unit of time, the solar
day. The cycle of seasons roughly indicated a
longer unit of time, the solar year. But early
people did not know that the earth's revolution
around the sun caused the different seasons. The
changing position and shape of the moon was
easier for them to observe. As a result, the
early calendars used the interval between the
successive full moons, called the lunar month,
as an intermediate unit of time.
We now know that the
lunar month lasts about 29 1/2 days. Twelve such
months amount to about 354 days. This interval
is almost 11 days shorter than the true solar
year, which has 365 days, 5 hours, 48 minutes,
and 46 seconds. But a year of 13 lunar months
would amount to about 383 1/2 days and would be
more than 18 days longer than the solar year.
The solar year, therefore, does not equal any
whole number of lunar months.
The discrepancy between
whole lunar months and days in a solar year
explains the confusion over calendar keeping
during thousands of years. A calendar based on
12 lunar months becomes out of step with the
seasons. Some people who used lunar calendars
kept them roughly in step with the seasons by
making some years 12 months long and other years
13 months long.
Early calendars usually
represented some sort of compromise between the
lunar and solar years. Some years lasted 12
months, and others lasted 13 months.
Gregorian calendar,
pronounced gruh GAWR ee uhn, is the calendar
that is used in almost all the world today. All
modern business uses its dates. Pope Gregory
XIII established it in 1582 to correct the
Julian calendar, which Julius Caesar put into
effect in 46 B.C. The Julian calendar year was
11 minutes and 14 seconds longer than the solar
year. By A.D. 1580, this difference had
accumulated to 10 days. Pope Gregory dropped 10
days from October to make the calendar year
correspond more closely to the solar year. He
also decreed that each fourth year would be a
leap year, when February would have an extra
day. Years marking the century would not be leap
years unless divisible by 400. For example, 1600
was a leap year, but 1700, 1800, and 1900 were
not. At present, the average Gregorian year is
about 26.3 seconds longer than the solar
year
The Gregorian calendar
is based on the year of Jesus Christ's birth,
according to some calculations. Many people
refer to dates before that year as B.C., or
before Christ. They use A.D., or anno Domini (in
the year of our Lord), for dates after that
year. Some people--especially
non-Christians--write B.C.E. for before common
era or before Christian era and C.E. for common
era or Christian era instead of B.C. and
A.D.
The Christian church
calendar is regulated partly by the sun and
partly by the moon. Immovable feasts include
Christmas and such feasts as the Nativity of the
Blessed Virgin. They are based on the solar
year. Such days as Ash Wednesday, Palm Sunday,
and Easter are called movable feasts, because
their dates vary from year to year, according to
the phases of the moon.
The Hebrew calendar
begins with an estimated moment of the world's
creation. Hebrew tradition has placed this
moment at 3,760 years and 3 months before the
birth of Jesus Christ. To find a year in the
Hebrew calendar, we must add 3,760 to the date
in the Gregorian calendar. For example, 2000 in
the Gregorian calendar is 5760 in the Hebrew
calendar. But this system will not work to the
exact month, because the Hebrew year begins in
September or October in the Gregorian calendar.
By November 2000, for instance, the Hebrew year
will have become 5761.
The Hebrew year is based
on the moon and normally consists of 12 months.
The months are Tishri, Heshvan, Kislev, Tebet,
Shebat, Adar, Nisan, Iyar, Sivan, Tammuz, Ab,
and Elul. They are alternately 30 and 29 days
long. Seven times during every 19-year period,
an embolismic or extra 29-day month, called
Veadar, is inserted between Adar and Nisan. At
the same time, Adar is given 30 days instead of
29. These additions keep the Hebrew calendar
The Chinese calendar
begins at 2637 B.C., the year in which the
legendary Emperor Huangdi is said to have
invented it. This calendar counts years in
cycles of 60. For example, the year 2000 in the
Gregorian calendar is the 17th year in the 78th
cycle. The years within each cycle are broken
down into repeating 12-year cycles. In these
cycles, each year is named after 10 Chinese
constellations and 12 animals. The animals are
the rat, ox, tiger, hare, dragon, snake, horse,
sheep, monkey, rooster, dog, and pig. The year
2000 is the year of the dragon.
The Chinese year is
based on the moon and generally consists of 12
months. Each month begins at new moon and has 29
or 30 days. A month is repeated seven times
during each 19-year period, so that the calendar
stays approximately in line with the seasons.
The year starts at the second new moon after the
beginning of winter in the Northern Hemisphere.
Thus, the Chinese New Year occurs no earlier
than January 20 and no later than February
20.
The Islamic calendar
begins with Muhammad's flight from Mecca to
Medina. This flight, called the Hegira, took
place in A.D. 622 by the Gregorian calendar.
The Islamic year is
based on the moon, and has 12 months,
alternately 30 and 29 days long. These months
are Muharram, Safar, Rabi I, Rabi II, Jumada I,
Jumada II, Rajab, Shaban, Ramadan, Shawwal,
Zulkadah, and Zulhijjah.
The Islamic year is much
shorter than the solar year, with only 354 days.
As a result, the Islamic New Year moves backward
through the seasons. It moves completely
backward in a course of 32 1/2 years. The
Islamic calendar divides time into cycles 30
years long. During each cycle, 19 years have the
regular 354 days, and 11 years have an extra day
each. This method of counting time makes the
Islamic year nearly as accurate in measuring the
lunar year as the Gregorian year is in measuring
the solar year. The Islamic calendar would be
only about one day off every 2,570 years with
respect to the moon. The Gregorian calendar
would be only a little more accurate with
respect to the sun.
"Calendars"
by L. E. Doggett, a
detailed essay about calendars and their
history, reprinted from the Explanatory
Supplement to the Scientific Almanac.
http://astro.nmsu.edu/~lhuber/leaphist.html
The Calendar Zone,
a Web site offering a
variety of information about
calendars.
http://www.calendarzone.com/
Calendar Conversions,
a Web page that allows
you to convert dates among several calendar
systems.
http://genealogy.org/~scottlee/calconvert.cgi
Encyclopedia Britannica:
Calendars.
See also
calendar
links of the time-directory of the Linking
Library.