NOW that you鈥檝e recovered from the new year bash, ask yourself a simple question. Why should it all happen on 1 January? Until AD 800, France preferred 1 March. For nearly two centuries after that it was 25 March, and from AD 996 until 1051 New Year鈥檚 Day coincided with Easter. The English had completely different ideas. Between the 7th century and 1338 they considered Christmas Day to be the start of the new year. But starting in 1339 New Year鈥檚 Day was moved to 25 March for civil purposes and to Easter for religious ones.
The history of the calendar is a long-running, planet-wide soap opera 鈥� a stream of brave attempts to put the seasons in their rightful places, accompanied by just as many chronological blunders. It is a wonderful example of one of humanity鈥檚 most endearing and infuriating traits: the inability to get the simplest and most basic things right, or even consistent.
We have still not sorted it out. Only in those countries that have adopted the Gregorian calendar does 1 January count as New Year鈥檚 Day. According to the Chinese calendar, it will be on 19 February this year, for the Burmese 15 April, in the Islamic world 19 May, and for the Jews it will not arrive until 14 September. And every calendar still needs to have a fudge factor thrown in 鈥� a few seconds or perhaps a day here and there 鈥� to stop day turning into night or summer into winter. In humanity鈥檚 defence, it must be said that the blame for our chronic calendric confusion lies ultimately with the bodies of the Solar System. Before you can even start the tricky task of predicting the date of the new year, you have to remember some of the fundamental facts about the heavens as perceived from the Earth.
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Our planet rotates on its axis once every 23.9345 hours, or 23 hours 56 minutes and 4 seconds. This is the time it takes for one rotation relative to the 鈥渇ixed鈥� stars. It is less than the 24 hours we know as a day, the interval between successive occasions when the Sun is overhead 鈥� that is, when it crosses a chosen meridian. While the Earth is rotating, it is also revolving around the Sun, and it takes that extra four minutes or so for the rotation to catch up with the Sun鈥檚 apparent slippage back across the sky. Even the 24-hour figure is only an average: the actual length varies, mainly because the Earth鈥檚 orbit is an ellipse rather than a perfect circle.
To ancient humanity, the next most obvious celestial cycle was the repeating sequence of phases of the Moon. Relative to the fixed stars, the Moon revolves around the Earth once every 27.32166 days. However, its phases are governed by the relative positions of the Sun and the Moon as observed from the Earth. Once more there is some slippage in the Sun鈥檚 position which has to be made up, leading to an average 鈥渟ynodic lunar month鈥� of 29.53058 days.
Tropical trouble
Finally, there is the year, the time that it takes the Earth to travel once round the Sun. This is the 鈥渟idereal year鈥� of 365.25636 days. For calendars, however, the more important period is the time between the start of a season in one year and its start in the next: this is called the 鈥渢ropical year鈥�. The Earth鈥檚 axis is tilted at an angle of 23.5掳 relative to the ecliptic, the plane of the Earth鈥檚 orbit. This causes variations throughout the year in the day/night division of the 24-hour day, giving long nights in winter and short ones in summer. There are two 鈥渆quinoxes鈥� at which the division is 50:50; these fall, near enough, on the first day of spring and the first day of autumn. But because the Earth bulges at the equator, its axis of spin slowly precesses like that of a spinning top: the direction in which the North or South Pole points revolves round a complete circle in the sky once every 25 800 years. One consequence of this is that the equinoxes come a little earlier each year and the tropical year is 365.24219 days long, slightly shorter than the sidereal year. Calendars have yet to deal with this effect, because it is smaller than other in-built errors.
The important point here as far as calendars are concerned is that neither the tropical year, nor the lunar month, are simple multiples of a day. The multiples are instead irrational numbers, which, like 鈭�2 or 蟺, cannot be represented as exact fractions. If the solar year were a rational multiple of a day then after some integral number of days, the year and the day would be back in step at exactly the same point. For example, if the tropical year were exactly 365.25 days, a ratio of 1461/4, then 4 years would be exactly equal to 1491 days, so that all the astronomical events related to the apparent position of the Sun would repeat precisely over that period. But both the tropical year and the lunar month contain an irrational number of days. Worse, the tropical year also contains an irrational number of lunar months. So nothing ever repeats exactly.
Calendars therefore have to make compromises, and it is the history of those compromises in different cultures that has led to a plethora of calendar systems. Depending on which one you choose, there will be at least 26 different New Year鈥檚 Days in any given year 鈥� an average of one a fortnight. Admittedly, some of these have been obsolete for a few millennia, but there are around 16 still in widespread use. These fall into two basic types: lunar and solar. In one, primary attention is paid to the apparent motion of the Moon; in the other the Sun has pride of place. Nevertheless, most lunar calendars include clever solar-related jiggles to keep them roughly in tune with the seasons, and most solar calendars at least pay lip service to the movement of the Moon.
Our own calendar is solar, and goes back to ancient Rome. In 46 BC Julius Caesar reformed the previously erratic Roman calendar, a lunar one. On the advice of the astronomer Sosigenes he took the length of the tropical year to be 365.25 days, and set up a cycle of 1461 days consisting of one leap year of 366 days and three common years of 365. He added 90 days to the year 46 BC to get spring back to its traditional date of mid-March, and he also decided that the year would start on 1 January, which proves what a sensible chap he was.
Julius decreed that from that point on there would be 12 months, relics of the lunar system but no longer linked to the lunar month. Their lengths were not quite those we have now; in particular February had 29 days in a common year and 30 in a leap year. Our current lengths seem to have been introduced by Julius鈥檚 successor Augustus Caesar, and an apocryphal story has it that he pinched a day from February to make August (renamed after him from its original, Sextilis) exactly the same length as July (previously renamed from Quintilis in honour of Julius).
As a result of a monumental cockup by Roman priests, the calendar had to be reformed again around 10 BC. The priests had been instructed to insert a leap year every four years. But to count the years they used the common Roman procedure of beginning a new count with the end of the previous one. This meant that a leap year occurred every third year instead of every fourth. So to get the calendar back on track, leap years were omitted until AD 4.
The Julian year exceeds the true tropical year by 0.00781 days, so by 1582 the spring equinox had slipped back from 21 March to 11 March. To prevent further slippage, Pope Gregory XIII reformed the calendar once more. Leap years thenceforth were to be omitted in years ending 00, unless that year happened to be a multiple of 400. This resulted in the omission of 3 days from every 400-year cycle, and reduced the 鈥渢heoretical鈥� length assumed by the structure of the calendar to 365.2425 days, much closer to the true value of 365.24219. To bring the equinoxes back into alignment, ten days of 1582 were removed, 5 October becoming 15 October. The new year, as in the Julian calendar, began on 1 January.
Taxing changes
The adoption of the Gregorian calendar was rapid in Catholic countries or provinces, but slower in Protestant ones. Italy and France made the switch in 1582, the Germans followed at various times between 1583 and 1700, while Finland left it as late as 1918. In England the Gregorian calendar was adopted in 1750 and put into practice in 1752, by which time the year 1600 had come and gone, so equinoxes had slipped by a further day. This meant catching up 11 days, and 3 September to 14 September were the ones that were omitted. A consequence of this move that still haunts us is the date of 6 April as the start of the financial year. It was originally 25 March 鈥� an inaccurate approximation to the spring equinox, but one consistent with three-monthly accounting periods, including the all-important 25 December. With the addition of 11 days it became 5 April. In 1800 it became 6 April because a Julian leap year was omitted, but this anachronistic modification was not applied again in 1900.
A good example of a lunar calendar tuned to accommodate the solar year is the Jewish one. The calendar uses a lunar cycle of 19 years combined with a solar cycle of 28 years. To approximate the lunar month, the 12 Jewish calendar months contain either 29 days (when they are called 鈥渄efective鈥�) or 30 (鈥渇ull鈥�). Most months have fixed lengths but two are variable, depending on the solar cycle. There is also a 30-day intercalary month, which occurs only in years 3, 6, 8, 11, 14, 17 and 19 of the lunar cycle. The structure of the Jewish calendar is complicated by the need to avoid certain events falling on certain days of the week. For instance, the year cannot begin on a Sunday, Wednesday or Friday. New year wanders erratically through September and early October in our Gregorian calendar. For anyone who feels like an extra celebration, the next five Jewish new years fall on the following dates: 14 September 1996, 2 October 1997, 21 September 1998, 11 September 1999, and 30 September 2000.
The Muslim calendar is unique in being totally lunar. The year consists of 12 lunar months or 354 days, so significant dates and festivals drift relative to the seasons. The calendar starts counting from the Hijrah, the prophet Mohammed鈥檚 flight from Mecca to Medina to escape religious persecution. Most Muslims consider his arrival time in Medina to be sunset on 16 July 622, but a few who count days from midnight to midnight (rather than the normal sunset to sunset) employ 15 July as their starting date. The calendar has 12 months, which are alternately 30 and 29 days long. However, the 12th month, which is usually 29 days long, acquires an extra day in intercalary years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29 of a repetitive 30-year cycle. Those who start the year on 15 July add the extra day in the 16th year, not the 15th.
For the Chinese, our brand-new 1996 will be the Year of the Rat (prophesying a British general election, perhaps). The Chinese calendar, known as the yin-yang-li, goes back to 2953 BC, making it older than any other in current use 鈥� although since 1911 China has followed the Gregorian calendar for official business. However, it has seldom been the only calendar in use in China: one authority estimates that at least 102 different types of calendar system have been used there at one time or another. The Japanese and the Koreans also use the yin-yang-li, with some minor modifications. It is based on a 60-year cycle which combines a cycle of 10 constellations with a zodiacal cycle of 12 animals, and it allocates the timing of the new year by the phase of the Moon.
The constellations (for which there is no sensible translation) are kiah, yih, ping, ting, wu, ki, kang, sin, jin and kwei.
The animals are tse (rat), chau (ox), yin (tiger), mau (hare), shin (dragon), se (snake), wu (horse), wi (sheep), shin (monkey), yu (rooster), siuh (dog) and hai (pig). The combined cycle starts with kiah-tse and then moves one step along each list to give yih-chau, ping-yin, and so on. Each list wraps round to its start, so that after kwei-yu comes kiah-siuh. Exactly half of the 120 possible combinations occur, because 10 and 12 have the common factor 2. The new year is defined independently of this cycle: it falls on the new Moon nearest to the time when the Sun is at a certain fixed point in the constellation Aquarius. This always turns out to be within 15 days of 4 February. For example in 1996 (ping-tse in the cycle) it will fall on 19 February; in the following years it will be on 7 February 1997 (ting-chau), 28 January 1998 (wu-yin), 15 February 1999 (ki-mau), and 4 February 2000 (kang-shin).
For any party animals who are interested, the two tables (left and opposite) give a selection of new years to celebrate. The first shows new years during 1996 鈥� for calendars that are still in use. For those with a historical turn of mind, the second table lists the different New Year鈥檚 Days from past times, extrapolated into 1996for instance, by choice of Easter where appropriate. The table lists only the first recorded occurrence of each date for the new year. In total, the tables give 27 excuses to celebrate.
Calendar chaos
What of the future? It鈥檚 certainly not getting any simpler. All of the various astronomical cycles are slowly changing their lengths: the day, the tropical year and the lunar month are all lengthening because of tidal gravitational forces. There are other difficulties in the pipeline, such as irregularities in the precession of the equinoxes. These are caused by occasional glitches, called Milankovitch shifts, in the tilt of the Earth鈥檚 axis. And thanks to the work of astronomers such as Jack Wisdom of Massachusetts Institute of Technology in Cambridge and Jacques Laskar of the Bureau des Longitudes in Paris, we have known since 1993 that the motion of objects in the Solar System is chaotic. No matter what scheme you settle on and how carefully you have accounted for all the variables, the 鈥渂utterfly effect鈥� will cause an unpredictable drift away from whatever calendar you calculated in advance.
So it would be best to institute an interactive method of tinkering with the calendar, as is already done to keep the length of the year in tune with a gradually slowing Earth. However, this means that science fiction titles such as A. E. van Vogt鈥檚 200 000 000 AD need to be taken with a pinch of salt 鈥� especially if you want to see in the new year on the right day. Look far enough ahead, and even that is not predictable.
