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Counting the days;History

What date is it today? Are you sure? Aren't we celebrating the new millennium in the wrong year? Ann Low-Beer checks the calendar

Think of calendars when you are thinking about the millennium. Calendars put it into perspective: after all, it is only a millennium because of the ways in which we have constructed the measurement of time. The waxing and waning of the Moon, the rotation of the Earth around the Sun, the movement of the stars in the sky, the changing seasons of each year, were all observed by ancient peoples. These basic cycles will continue, unaffected by what we call the millennium.

You know of several different New Years - Chinese, Islamic, Jewish, Hindu - each based on different calendars. There are others still in use, which you or your pupils may know of.

The start of the next New Year is only the millennium on the calculations of the Gregorian calendar, sometimes called the Christian calendar, though not used by Orthodox Christians. Years are reckoned from the supposed date of the birth of Jesus Christ. This calendar is awkwardly divided into BC (before Christ) and AD (Anno Domini, the year of our Lord), now often signified by BCE (before the Common Era) and CE (the Common Era).

The Islamic calendar is calculated from the Hejira (or flight) of the Prophet from Mecca to Medina, so 1999 sees the start of the year 1420. The Hebrew calendar is reckoned from the supposed start of the world so, in 1999, the year 5760 begins. Both of these overlap into 2000. The Chinese calendar, meanwhile, has a different name for each New Year, and these repeat in a 60-year cycle, which has been used for more than 4,000 years.

So when does the New Year really start? For Hebrews, each day begins at sunset, not midnight. In India, the official New Year begins at the spring equinox, as it does in the Persian calendar. In England, until 1752, the New Year began on March 25. The tax year still runs from April 5 because by then the old calendar was 11 days out. In the early middle ages, the New Year began on December 25.

Calculating the first year of a new millennium is confusing because the first century began in AD 1. The second century, therefore, started 100 years later, in AD 101, which means the 21st century must start 2000 years after the first - in 2001. Have we got something wrong?

There have been many attempts to record and measure the different natural cycles of the year, but they do not neatly coincide, and the mathematics are complex. A year measures the earth's motion around the sun, which takes 365 days, five hours and 48 minutes. The idea of a month is based on the Moon's cycles, 12 in a year. The Islamic calendar follows the lunar cycle, each month starting at the crescent moon, which appears on the flags of Muslim nations. This gives a year of 354 or 355 days, so that the fast of Ramadan for instance, is about 11 days earlier each year against the solar calendar.

The Gregorian calendar is based on the solar year and the months do not follow the cycles of the moon.

The Jewish calendar attempts to combine both, putting in an extra month on leap years, so the year is about 354 days but 384 in a leap year. The ancient Egyptians also developed a solar calendar.

The stones of Stonehenge were erected in about 2000 BCE, so that at the Summer solstice, the rays of the rising sun fall directly down the main avenue. The people who built Stonehenge were astronomically sophisticated, although it may have been built primarily for religious purposes that are not understood.

Co-ordinating the observance of religious festivals was one of the main reasons for the development of calendars, but there were other reasons. The state was interested in the orderly collection of taxes. Trading meant calculating times for the return of debts and final payments, and predicting and agreeing dates when payment was due.

Astronomers and scholars were intrigued by the puzzles set by their observations. Bede (672-735), who never left his Northumbrian monastery, thought the calendar of his day was inaccurate from recordings of his sundial and observation of the coastal tides. And in many countries people looked for astrological signs that would tell them about lucky and unlucky days when planning important occasions.

A small miscalculation adds up over years and centuries, and calendars have needed adjustment. Julius Caesar listened to astronomers who knew the work of the Egyptians, and decreed that there should be a new calendar, starting on January 1, 45 BCE. The Julian calendar had 365 and a quarter days each year and used leap years to adjust awkward numbers. It also had 12 months rather than the previous 10, but the names were not changed: September, October, November and December were originally the seventh, eighth, ninth and tenth months. Two more were inserted; July and August were named after Julius Caesar and the emperor Augustus. Later, the emperor Constantine ordered a seven-day week and borrowed the idea of a rest day from the Jews, but the Christians insisted on making it a different day.

All calculations used Roman numerals without the concept of zero. Zero was developed in India but came to Europe with Arabic numerals in the 12th century. By 1582, Pope Gregory XIII decreed a new calendar was needed because little inaccuracies in the Julian calendar had added up. Ten days were removed, so October 5 was followed by October 14, which caused confusion to shipping times and debt collectors. The leap year was regularised, so every year divisible by four is a leap year, but not if it is divisible by 100, unless it is divisible by 400, which means that 2000 is, after all, a leap year!

This new style calendar was adopted by most Catholic countries in Europe. Protestant countries resisted. Elizabeth Iof England favoured change; her archbishop did not. Britain was one of the last countries in Europe to adopt the new calendar in 1752, when these dates came into the American colonies too. Russia moved to the new calendar in 1917, Greece not till 1924. In Asia, the Japanese moved to this calendar in 1873, the Chinese when it was proclaimed by Mao Zedong in 1949. People writing letters from one country to another got used to using both new- and old-style dates.

Timetables required for modern communications, such as rail and air travel, have led to the global use of this calendar. There have been several schemes to simplify it and make it more repetitive, rather than changing each year. If leap days were regarded as extras, not attached to any month, the year and the months could be more regular, starting always on the same day of the week. The United Nations discussed adopting the World Calendar in 1961, but any agreement is difficult. We are used to our present system and many people still use two calendars, one for official or international affairs and another for religious festivals, national events, or finding auspicious days.

So, what about Fridays? On the Gregorian calendar, the 13th of any month occurs most frequently on a Friday. No calendar gets it all right!

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