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.
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 291/2 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 3831/2 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.
Some calendars today
Most people in the Western world use the Gregorian calendar, worked out in the 1580’s by Pope Gregory XIII. It has 12 months, 11 with 30 or 31 days. The other month, February, normally has 28 days. Every fourth year, called a leap year, it has 29 days. However, century years that cannot be divided evenly by 400 lose the extra day, though they are leap years. For example, February had 28 days in 1900 but 29 days in 2000.
The Gregorian calendar is based on the year of Jesus Christ’s birth, according to a dating system started in 532 by the monk Dionysius Exiguus. In this system, the year of Christ’s birth was A.D. 1., and the year before that was 1 B.C. The abbreviation A.D. stands for anno Domini (in the year of our Lord), and B.C. means before Christ. But modern scholars believe Christ was born no later than 1 B.C. He was born during the lifetime of Herod the Great, who died in either 4 B.C. or 1 B.C. An alternative system uses Gregorian numbering, but does not refer to Christ. In that system, C.E. (common era) replaces A.D., and B.C.E. (before the common era) replaces B.C. See A.D. ; B.C. .
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 mid-October 2000, for instance, the Hebrew year had 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 and holidays in agreement with the seasons of the solar year.
The Islamic calendar
begins with Muhammad’s flight from Mecca to Medina. This flight, called the Hijra, also spelled Hijrah or 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 Dhūl-Hijja.
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.
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 21 and no later than February 20.
History
Early calendars
usually represented some sort of compromise between the lunar and solar years. Some years lasted 12 months, and others lasted 13 months.
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.
The Maya
of Central America used a combination of calendars. The Tzolk’in << tzohl KEEN >> was a 260-day almanac of sacred events. Each day was named for 1 of 20 gods and given a number between 1 and 13. The combination of number and name predicted good or bad luck on that day.
The Maya also had a 365-day calendar based on the solar year. This calendar was called the Haab << hahb >> . It included 18 months of 20 days each. The remaining 5 days, called the wayeb << wah YAYB >> , were considered unlucky. The Maya fasted, made sacrifices, and avoided unnecessary work during this time. The Maya adapted the haab from the calendar of an earlier civilization called the Olmec. Combining the Tzolk’in and Haab gave a 52-year cycle called the Calendar Round.
The longest Maya calendar, called the Long Count, was based on a 5,128-year cycle. It was used to record notable events. The Aztec of Mexico adapted the Maya calendars for their own use.
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.
The Julian calendar.
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. 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 calendar year was divided into 12 months of 31 and 30 days, except for February, which ended up with 28 days. Every fourth year, February would have 29 days.
The Romans renamed Quintilis to honor Julius Caesar, giving us July. Later, Sextilis was renamed August by the Roman Senate to honor Augustus, who ruled the Roman Empire as its first emperor from 27 B.C. to A.D. 14.
The Julian calendar was widely used for more than 1,500 years. A Julian year lasted 3651/4 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.
The Gregorian calendar
was designed to correct the errors of the Julian calendar. In 1582, on the advice of astronomers, Pope Gregory XIII corrected the difference between sun and calendar by ordering 10 days dropped from October, the month with the fewest Roman Catholic holy days. The day that would have been Oct. 5, 1582, became October 15. This procedure restored the next equinox to its proper date. To correct the Julian calendar’s error regularly, the pope decreed that February would have an extra day in century years that could be divided evenly by 400, such as 1600 and 2000, but not in others, such as 1700, 1800, and 1900.
The Gregorian calendar is so accurate that the difference between the calendar and solar years is now only about 26 seconds. This difference will increase by 0.53 second every hundred years, because the solar year is gradually becoming shorter.
The Roman Catholic nations of Europe adopted the Gregorian calendar almost immediately after Gregory XIII devised it. Various German states kept the Julian calendar until 1700. The United Kingdom and the American Colonies changed to the Gregorian calendar in 1752. Russia and Turkey did not adopt the Gregorian calendar until the early 1900’s.
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.
