A calendar is a system of reckoning time for the purpose of recording past events and planning for the future. A calendar is used to indicate a series of scheduled events such as court cases, bills before congress, appointments, holidays, etc.

In the new millennium, we take the calendar for granted. There are 12 months in a year and each year is numbered. For practical purposes, thats all we need to know. Our lives march from birth to death in a linear fashion, with both dates marking the beginning and ending of our existence. In between are the regularly occuring birthdays, anniversaries, holidays and other arbitrary events of our lives.

Weve mapped out our existance on the calendar of time. Every aspect of our lives has a mark on the calendar. We served in a war between 1944 and 1946. Usually we know the month. The most memorable events of our lives we remember down to the day and time of day. Our work histories are marked by start and end dates, interpolated with the years we went to college.

Just how in sync our made made calendars are with nature doesnt really seem to be much of a concern. Seasonal changes have little bearing on industry and business. Auto manufacturers pump out just as many cars in the winter as they do in the summer. Certain foods might be seasonal, but import/export is the way around that delimma. Factory farming, along with genetic engineering in the future, is continous throughout the year.

Indoor stadiums allow for sports events to take place without worry of rain or snow. And the fall season is more likely to be important in terms of TV programming schedules than when foods are ready for market.

Calendars got their start through ancient observations of the Sun, Moon, stars and planets. The earths rotation, orbit around the sun, phases of the moon and other celestial observations were eventually calculated in terms of season, year, month and day cycles. It took 365 cycles of day and nightgive or take an hour or secondfor the earth to orbit the sun (called the Solar year). A day is the time it takes for the earth to rotate once on its axis. A Lunar year is the total of the 12 times the moon passes through its phases (a Lunar month).

The sundialand eventually clocksfurther divided the day into hours, minutes and seconds. Now, with atomic clocks, the cycles of nature are calculated in terms of billionths and trillionths of nanoseconds (a nanosecond being one billioneth of a second).

Today, a worldwide resource for fundamental astronomical data is the Astronomical Almanac, a joint publication of the U.S Nautical Almanac Office and Her Majesty's Nautical Office in the UK. It contains technical and general astronomical data supplied by many scientists from around the world. Information includes: seasons, phases of the Moon, planet configuration, eclipses, time-scales and Coordinate systems, universal and sidereal times, detailed earth, sun, moon and other celestial sphere rotation and orbital positions, stars and steller systems, x-ray and gamma ray sources, quasars, pulsars, observatories, instrumentation, astronomical tables and data, and much more.

Exploring the history of ancient and modern calendars can be confusing and convoluted. How did we arrive at 365 days in a yearor is it 364 or 366give or take an hour or second? Why do some months have 31 days and some have 28? Why do we need a leap year? How do we compensate for the differences between the Solar Year and Lunar Year?

There are six principal calendars in current use: Gregorian, Hebrew, Islamic, Indian, Chinese, and Julian Calendars. Religion and politics have played important roles in determing how calendars came to be, regardless of astronomical obervations. Adjustments were made for sacred dates and extending the length of official times for rulers.

Mathematically, the fact that months and years cannot be divided exactly by days and that the years cannot be easily divided into months led to intercalation: the insertion of extra days or months into a calendar to make it more accurate. This meant making some months shorter and other months longer.

Solar years and Lunar years dont quite match up. Seasons and astronomical events do not repeat at an exact number of days, so a calendar that had the same number of days in each year would over time drift with respect to astronomical changes. A leap year (or intercalary year) is a year containing an extra day or month in order to keep the calendar year in sync with an astronomical or seasonal year.

The history of Egyptian, Roman, Greek, Chinese, Jewish, Mayan and other calendars is really a journey in how each calendar compensated for astronomical changes. Give or take a few leap seconds or leap years, its unlikely the astronomical changes that gave birth to calendars will change much in far into the future. It would take a castrophic event to knock the earth off its axis or change the speed of orbit around the sun. Are days getting longer and nights getting shorter? Will a meteor strike? Will continents divide and new oceans form? Will these changes have anything to do with how we get from one day to another?

Roman Calendar
The earliest versions of the Roman calendar had 10 months (using Roman names): March (31 days), April (29 days), May (31 days), June (29 days), Quintilis (31 days), Sextilis (29 days), September (29 days), October (31 days), November (29 days), and December (29 days). The Romans used intercalary days and the occassional month to maintain 365 days a year.

Julian Calendar
Prior to Julius Caesar, the Romans had so abused the calendar that January started in autumn. Julius Caesar, advised by astronomer Sosigenes, added 90 days to the year 46 B.C. (67 days between November and December, 23 at the end of February). This caused the spring of 45 B.C. to begin in March. He changed the length of other months as well, added a few days to other months, and changed Sextilis to July, named after himself.

In the Roman calendar three days in the month were used for counting the date. These three were the Kalends (1st day of the month), the Nones (the 7th day in March, May, July, and October, the 5th in the other months), and the Ides (the 15th day in March, May, July, and October, the 13th in the other months).

Gregorian Calendar
The Julian year is 365 days and 6 hoursa little too long. By the 16th century the accumulation of surplus time had displaced the vernal equinox to Mar. 11 from Mar. 21, the date set in the 4th century. To compensate, Pope Gregory XIII suppressed 10 days in the year 1582 and ordained the years ending in hundreds should not be leap years unless they were divisible by 400.

The year 1600 was a leap year under both systems, but 1700, 1800, and 1900 were leap years only in the unreformed calendar. The reform was gradually accepted throughout most Roman Catholic and Protestant countries, and in the Eastern Church the Julian calendar was used well into the 20th century. Slightly modified from the Julian calendar, the Gregorian calendar has become the internationally accepted civil calendar.

Solar Calendars
The Julian calendar introduced by Julius Caesar in 45 BCE added a simple leap year rule: insert an extra day every four years. The Julian calendar standardized March 21 as the date of the vernal equinox. However, the leap year rule did not precisely match the solar year. Over the centuries the date of the astronomical vernal equinox slowly drifted away from March 21. Ecclesiastical rules were used to compute the date of Easter matching March 21 as the date of the vernal equinox.

The leap year rule for the Gregorian calendar differs slightly from one for the Julian calendar. The Gregorian leap year rule states that every year divisble by four is a leap year, except for years that are exactly divisible by 100. The year 1900 is not a leap year; the year 2000 is a leap year. Years divisible by 400 are still leap years.

Lunar Calendars
The Islamic Calendar is a lunar calendar with months corresponding to the lunar phase cycle. The twelve months of the Islamic calendar systematically shift with respect to the months of the international civil calendar.

A lunar calendar bases each month on a full cycle of the Moon's phases (called a lunation or synodic month). Lunar calendars usually start each month with a New Moon or the first visible crescent moon after New Moon. Since the solar year does not contain an integral number of days or an integral number of lunar months, many calendars (called lunisolar calendars) adjust the length of their years and months. Without such adjustments the seasons will steadily drift through the months.

Lunisolar Calendars
The Hebrew Calendar is a lunisolar calendar based on calculation rather than observation and its current form dates from around 359 CE. It is the official calendar of Israel, although variations on this calendar exist. Passover dates are computed from a set of defined rules.

The National Calendar of India is a lunisolar calendar with leap years coinciding with those of the Gregorian calendar. Tabulations of the religious holidays are prepared by the India Meteorological Department and published annually in The Indian Astronomical Ephemeris.

The Chinese calendar is also a lunisolar calendar based on calculations of the positions of the Sun and Moon.

^ Top ^