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.