	Some other calendars Jewish, Islamic, Julian, Gregorian
download and description program tour and use a short guide other calendars other countries Easter day web links bibliography glossary red letter days roman dates the English experience	There are three natural primary cycles, each associated with the largest (from an earthly point of view) astronomical objects: the earth, the sun and the moon. It would be natural for man to seek convenient seasonal divisions to measure out time - the day (one earthly rotation), the month (a revolution of the moon around the earth) and the year (a revolution of the earth about the sun). Unfortunately none of these is a simple multiple of the others. Moon - the natural lunar cycle is the length of time between one phase of the moon and its next occurrence. Although these periods vary slightly in length, they average to 29½ days (29.531). This lunar 'month' does not have a convenient whole number of days and neither does it fit a whole number of times into a year. A calendar using lunar months could either: Alternate months of 29 and 30 days, in order to maintain synchrony between monthly and lunar cycles. Use months of either 29 or 30 days continuously and tolerate months starting earlier or later year after year. Sun - the natural solar cycle is the length of time taken by the Earth to move in orbit around the Sun, which approximates to 365¼ days (365.2425) or 12.4 lunar months. Years of 365 days will result in the seasons drifting, spring will occur a little later each year. After only a century midsummer will be in the middle of July - a noticeable drift. The option chosen at the time of Julius Caesar - having 3 years of 365 days, followed by a leap year of 366. This would maintain synchronicity between seasons and calendar, although only approximately. The history of calendars could be described as the history of our bodged attempts to fit these mismatched periods of time together. Four are described below: Islamic calendar - is a lunar calendar. From it you can predict the cycles of the moon, all months in Islam start at the new moon. The solar cycle is ignored - you cannot predict the seasons of the year by reference to this calendar. When using Islamic dates, equinoxes and solstices happen about 11 days later each year - alternatively the start of Ramadan in the Gregorian calendar occurs 11 days earlier each year. The 12 Islamic months alternate in length between 29 and 30 days. There is a leap year which occurs 11 times during a period of 30 years. This maintains the alignment of the new moon with the first day of the month. Jewish calendar - is a lunisolar calendar. It makes a good attempt at satisfying both the cycles of the moon and the sun. As well as following the cycles of the moon, all Jewish months start with the new moon, the Jewish calendar follows the solar cycle with the start of the civil year (Rosh Hashana) falling within 2 weeks of the autumn equinox. Alignment is maintained by having one month (Adar) which varies in lengths of 29 and 30 days - this enable alignment of moon and calendar. In order to maintain the alignment of the seasons with the calendar an additional month (Adar II) is inserted into appropriate years. Additional adjustments have to be made because, for other reasons, the year cannot start on a Wednesday, Friday or Sunday. Julian calendar - is a solar calendar. It has a cycle of four years, three of 365 days and one of 366 days. This produces an average year of 365.25 days, only very slightly longer than the correct value of 365.2425 days. Seasonal alignment can be maintained for many years, however the error slowly accumulates with the seasons occurring eleven minutes earlier each year - by three days in every 400 years. The Julian calendar ignores the lunar month which slips forward by approximately eleven days each year. Britain used the Julian calendar until September 1752. Rule every year which is wholly divisible by four is a leap year of 366 days. Gregorian calendar - is a solar calendar (1 year = 365.2425 days). From it you can predict the seasons of the year; the solstices and equinoxes occur at approximately the same time each year. Like the Julian calendar, the Gregorian calendar ignores the lunar month. Britain has used the Gregorian calendar since September 1752. Rule every year which is wholly divisible by four is a leap year of 366 days, except that years which are divisible by 100 are normal years, with the exception of those divisible by 400. ie 1700, 1800 1900 and 2100 are not leap years but 1600 and 2000 are leap years.

Some other calendars

Jewish, Islamic, Julian, Gregorian

There are three natural primary cycles, each associated with the largest (from an earthly point of view) astronomical objects: the earth, the sun and the moon. It would be natural for man to seek convenient seasonal divisions to measure out time - the day (one earthly rotation), the month (a revolution of the moon around the earth) and the year (a revolution of the earth about the sun). Unfortunately none of these is a simple multiple of the others.

Moon - the natural lunar cycle is the length of time between one phase of the moon and its next occurrence. Although these periods vary slightly in length, they average to 29½ days (29.531). This lunar 'month' does not have a convenient whole number of days and neither does it fit a whole number of times into a year. A calendar using lunar months could either:

Alternate months of 29 and 30 days, in order to maintain synchrony between monthly and lunar cycles.

Use months of either 29 or 30 days continuously and tolerate months starting earlier or later year after year.

Sun - the natural solar cycle is the length of time taken by the Earth to move in orbit around the Sun, which approximates to 365¼ days (365.2425) or 12.4 lunar months.

Years of 365 days will result in the seasons drifting, spring will occur a little later each year. After only a century midsummer will be in the middle of July - a noticeable drift.

The option chosen at the time of Julius Caesar - having 3 years of 365 days, followed by a leap year of 366. This would maintain synchronicity between seasons and calendar, although only approximately.

The history of calendars could be described as the history of our bodged attempts to fit these mismatched periods of time together. Four are described below:

Islamic calendar - is a lunar calendar. From it you can predict the cycles of the moon, all months in Islam start at the new moon. The solar cycle is ignored - you cannot predict the seasons of the year by reference to this calendar. When using Islamic dates, equinoxes and solstices happen about 11 days later each year - alternatively the start of Ramadan in the Gregorian calendar occurs 11 days earlier each year. The 12 Islamic months alternate in length between 29 and 30 days. There is a leap year which occurs 11 times during a period of 30 years. This maintains the alignment of the new moon with the first day of the month.

Jewish calendar - is a lunisolar calendar. It makes a good attempt at satisfying both the cycles of the moon and the sun. As well as following the cycles of the moon, all Jewish months start with the new moon, the Jewish calendar follows the solar cycle with the start of the civil year (Rosh Hashana) falling within 2 weeks of the autumn equinox. Alignment is maintained by having one month (Adar) which varies in lengths of 29 and 30 days - this enable alignment of moon and calendar. In order to maintain the alignment of the seasons with the calendar an additional month (Adar II) is inserted into appropriate years. Additional adjustments have to be made because, for other reasons, the year cannot start on a Wednesday, Friday or Sunday.

Julian calendar - is a solar calendar. It has a cycle of four years, three of 365 days and one of 366 days. This produces an average year of 365.25 days, only very slightly longer than the correct value of 365.2425 days. Seasonal alignment can be maintained for many years, however the error slowly accumulates with the seasons occurring eleven minutes earlier each year - by three days in every 400 years. The Julian calendar ignores the lunar month which slips forward by approximately eleven days each year. Britain used the Julian calendar until September 1752. Rule every year which is wholly divisible by four is a leap year of 366 days.

Gregorian calendar - is a solar calendar (1 year = 365.2425 days). From it you can predict the seasons of the year; the solstices and equinoxes occur at approximately the same time each year. Like the Julian calendar, the Gregorian calendar ignores the lunar month. Britain has used the Gregorian calendar since September 1752. Rule every year which is wholly divisible by four is a leap year of 366 days, except that years which are divisible by 100 are normal years, with the exception of those divisible by 400. ie 1700, 1800 1900 and 2100 are not leap years but 1600 and 2000 are leap years.

last edited 1-Dec-2003