Worldbook on Time &Calendars

(© Worldbook , Worldbook is a multimedia encyplopedia)


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.



•  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.



by L. E. Doggett, a detailed essay about calendars and their history, reprinted from the Explanatory Supplement to the Scientific Almanac.


The Calendar Zone,

a Web site offering a variety of information about calendars.


Calendar Conversions,

a Web page that allows you to convert dates among several calendar systems.


Encyclopedia Britannica: Calendars.  


See also calendar links of the time-directory of the Linking Library.



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