Calendar reform
Encyclopedia
A calendar reform is any significant revision of a calendar
system. The term sometimes is used instead for a proposal to switch to a different calendar.
Most calendars have several rules which could be altered by reform:
) and/or the synodic month in lunar
or lunisolar calendar
s. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar when it was modified into the Gregorian calendar.
The fundamental problem of the calendar is the imperfect divisibility of whole numbers into an irrational number (fitting whole days into a month; fitting whole days or whole months into a year). The physics of orbital mechanics does not phase-lock the rotation of the Earth (the day) to its revolution (the year), nor the rotation of the Earth (the day) to the revolution of the Moon (the month). Therefore any attempt to divide a month into days or a year into days will leave a fractional remainder of a partial day. Likewise, any attempt to divide a year into months will leave a fractional remainder of a partial month. Such remainders accumulate from one period to the next thereby driving the cycles out of synch.
A typical solution for forcing synchronization is called 'intercalation
'. This is an artificial harmonization that, after the fractional remainders have sufficiently accumulated, adds a whole day (or month) into the cycle. An alternative solution is to ignore the mismatch and simply let the cycles continue to drift apart. The general strategies include:
An obvious disadvantage of the lunisolar method of inserting a whole extra month is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar may tend to be seen as less advantageous at larger latitudes where seasonal effects are experienced more strongly. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or a device to track solar day-to-day progression, such as that established at places like Stonehenge
. After centuries of empirical observations, the theoretical aspects of calendar construction could become more refined, enabling predictions that identified the need for reform.
over 2500 years, most of which were intended to better fit the calendar months to astronomical lunations and to more accurately add the extra month so that the regular months maintain their proper seasonal positions, even though each seasonal marker can occur anywhere within its month.
There have been at least four similar reforms of the lunisolar version of the Hindu calendar
, all intended to make the month a better match to the lunation and to make the year a better fit to the sidereal year. There have been reforms of the 'solar' version of the Hindu calendar which changed the distribution of the days in each month to better match the length of time that the Sun spends in each sidereal zodiacal sign. The same applies to the Buddhist calendar
.
The first millennium reform of the Hebrew calendar
changed it from an observational calendar into a calculated calendar.
The Islamic calendar
was a reform of the preceding lunisolar calendar which utterly divorced it from the solar year.
took power in Rome, the Roman calendar
had ceased to reflect the year accurately. The provision of adding an intercalary month to the year when needed had not been applied consistently, because it affected the length of terms of office.
The Julian reform
lengthened seven months and replaced the intercalary month with an intercalary day to be added every four years to February. This produced a noticeably more accurate calendar, but it was based on the calculation of a year as 365 days and 6 hours (365.25 d). In fact, the tropical year
is 11 minutes and 14 seconds less than that. This had the effect of adding three-quarters of an hour every four years, and the effect accumulated. By the sixteenth century, the northern vernal equinox fell on March 10 or 11.
Under Pope Gregory XIII
the leap rule was altered: century years which are divisible by four, would not be leap years unless they are also divisible by 400. Thus 1600 and 2000 were leap years, and 2400 will be, but 1700, 1800, and 1900 were not, and 2100, 2200 and 2300 will not be either. This rule makes the mean year 365.2425 days long. While this does not synchronize the years entirely, it would require a few thousand years to accumulate a day.
This new calendar was synchronized with the traditional seasons again and was not applied to dates in the past, which caused a leap of at least ten days from the final day the Julian calendar was in effect. This reform slowly spread through the nations that used the Julian calendar, although the Russian church year still uses the Julian calendar. The times varied so widely that some countries had to drop more than ten days: Great Britain, for instance, dropped eleven.
When noting dates occurring within the period, "Old Style" and "New Style" are used to distinguish which calendar was used by the person who recorded the date.
In 1923, Milutin Milanković
proposed to a synod
of some Eastern Orthodox Church
es at Constantinople
that only those centennial years (those ending in '00) that leave a remainder of 200 or 600 upon division by 900 would be leap years, decreasing the average year length to 365.242222 days. These remainders were chosen to delay the first year (after 1923) that this calendar would disagree with the Gregorian calendar as much as possible, until 2800. It was adopted by some Eastern Orthodox Churches under the names Revised Julian calendar
or New calendar, but was rejected by others.
is currently used by most of the world. There is also an international standard describing the calendar, ISO 8601
, with some differences to traditional conceptions in many cultures.
Since the last papal reform, several proposals have been offered to make the Western calendar more useful or regular. Very few reforms have gained official acceptance. The rather different decimal
French Republican Calendar
was one such official reform, but was repealed shortly after its introduction. After World War II the newly-formed United Nations continued efforts of its predecessor, the League of Nations
, to establish a new calendar
but postponed the issue after a veto from the USA, which was mainly based upon concerns of religious groups about the proposed days that would be out of the seven day week cycle and thus disrupt having a sabbath occur every seven days. Independently the World Council of Churches
still tries to find a common rule for the date of Easter, which might be eased by a new common calendar.
Reformers cite several problems with the Gregorian calendar:
It is impossible to solve all these issues in just one calendar.
Most plans evolve around the solar year of little more than 365 days. This number does not divide well by seven or twelve, which are the traditional numbers of days per week and months per year respectively. The nearby numbers 360, 364 and 366 are divisible in better ways. There are also lunar centric proposals.
. These reforms would make it easy to work out the day of week
of a particular date
, and would make changing calendars each year unnecessary.
There are, roughly speaking, two options to achieve this goal: leap week calendar
s and intercalary day
s. Both make it easier to work out the day of week by having exactly 52 weeks in each year. The former add a whole 53rd week every five or six years, the latter have an extra day not belonging to any week and two such in leap years. Proposals mainly differ in their selection of a leap rule, placing of the leap item (usually middle or end of the year), in the start day of the week and year, in the number (12 or 13) and size of months and in connected naming; some are compatible to the week date of ISO 8601.
The World Calendar
, favored by the UN in the 1950s, and the International Fixed Calendar
, quite popular among economists between the World Wars, are proposals that start each year on a Sunday. The remaining 364 days then form 52 weeks of 7 days. The World Calendar has every quarter beginning on the same day of week.
In the World Calendar, the 365th and 366th day are considered holidays and named Worlds Day and Leap Year Day. These "off-calendar" days stand outside the seven-day week and caused some religious groups to strongly oppose adoption of The World Calendar. Such concerns helped prevent the World Calendar from being adopted. Supporters of the World Calendar, however, argue that the religious groups' opposition overlooked every individual's right to celebrate these holidays as extra days of worship, or Sabbaths. This option, they reason, maintained the seven-day worship cycle for those who share that concern, while allowing benefits of a perpetual calendar to be shared by all.
Leap week calendar
s add a leap week of seven days to the calendar every five or six years to keep the calendar roughly in step with the tropical year
. They have years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week.
Some calendars have quarters of regularly patterned uneven months e.g. a 35 day (five week) month and a pair of 28 day (four week) months, with a leap week appended to the final month when needed. The Common Civil Calendar and Time calendar has months of 30 and 31 days, but inserts a leap week in the middle of the year, when needed.
The 53-week calendar, used in government and in business for fiscal years, is a variant of this concept. Each year of this calendar can be up to 371 days long.
Still other proposals abandon attempts to make the calendar perpetual, instead opting for eleven 30-day months and one long month at the end of 35 (or 36) days.
An early 13-month proposal was the 1849 Positivist calendar
, created by Auguste Comte
. It was based on a 364-day year which included one or two “blank” days. Each of the 13 months had 28 days and exactly four weeks, and each started on a Monday. The International Fixed Calendar
is a more modern descendant of this calendar.
Some proposals add one or two days to the calendar each year to account for the annual solar cycle, while others keep these days off the calendar entirely, to make the calendar perpetual.
Around 1930 Colligan invented the Pax Calendar
, which avoids off-calendar days by adding a 7-day leap week to the perpetual 364-day year for 71 out of 400 years.
In 1987, Jose Arguelles
proposed the Dreamspell
calendar.
Dividing the years into 52 weeks creates 13 months of 28 days (4 weeks), and 4 quarters of 91 days (13 weeks). This creates months and quarters of fixed, even duration, but not quarters containing a whole number of months.
Lunisolar calendars usually have 12 or 13 months of 29 or 30 days.
Some propose to improve leap rules of existing calendars, such as the Hebrew calendar
. The Rectified Hebrew calendar uses a more accurate leap cycle of 4366 months per 353-year cycle, with 130 leap years per cycle, and a progressively shorter molad
interval, intended to replace the 19-year leap cycle and the constant molad interval of the traditional fixed arithmetic Hebrew calendar
, respectively.
Proposals to change the traditional month and weekday names are less frequent. The Gregorian calendar obtains its names mostly from gods of historical religions (e.g. Thursday from Nordic Thor
or March from Roman Mars) or leaders of vanished empires (July and August from the first Cæsars), or ordinals that got out of synchronization (September through December, originally seventh through tenth, now ninth through twelfth).
Calendar reformers, therefore, seek to correct what they see as deficiencies by focusing on more homogeneous sets of individuals, who usually share common traits.
, for example, proposed naming the 13 months in his calendar after figures from religion, literature, philosophy and science.
Similarly, the Hermetic Lunar Week Calendar uses 12 or 13 lunar months named after 13 contributors to research on psychoactive plants and chemicals.
The Simple Lunisolar Calendar names its months after the letters of the Greek alphabet
.
Such as Karl Palmen have suggested, to reuse an existing 13 × 4 naming system.
The one found is playing cards. Thus either months are numbered Ace, Two through Ten, Jack, Queen and King with four weeks each, named after the four suit
s (♠♣♥♦); or the roles are reversed if the calendar has four quarters with thirteen weeks each. Leap days or weeks are assigned the Joker. This system has internationalisation problems, though, because even where the 52-card deck is known, the order of suits may vary.
Also the contemporary roman alphabet has 26 letters, which could be used, together with a further binary indicator, as keys for 52 weeks.
The following count one or more days outside the standard seven-day week
:
The following are leap week calendar
s:
The following track the moon as well as the sun:
There have also been proposals to revise the way years are numbered:
Calendar
A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time, typically days, weeks, months, and years. The name given to each day is known as a date. Periods in a calendar are usually, though not...
system. The term sometimes is used instead for a proposal to switch to a different calendar.
Most calendars have several rules which could be altered by reform:
- If and how days are grouped into subdivisions such as monthMonthA month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...
s and weekWeekA week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...
s, and days outside those subdivisions, if any. - Which years are leap yearLeap yearA leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...
s and common yearCommon yearA common year is a common type of calendar year. It has exactly 365 days and so is not a leap year. More generally, it is a calendar year without intercalation....
s and how they differ. - Numbering of years, selection of the epochEpoch (reference date)In the fields of chronology and periodization, an epoch is an instance in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured...
, and the issue of year zeroYear zero"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1...
. - Start of the year (such as winter solsticeSolsticeA solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...
, January 1, March 1, spring equinoxEquinoxAn equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
, Easter). - If a week is retained, the start, length, and names of its days.
- Start of the day (midnight, sunrise, noon, or sunset).
- If months are retained, number, lengths, and names of months,
- Special days and periods (such as leap day or intercalary day).
- Alignment with social cycles.
- Alignment with astronomic cycles.
- Alignment with biological cycles.
- Literal notation of dates.
Historical reforms
Historically, most calendar reforms have been made in order to synchronize the calendar in use with the astronomical year (either solar or siderealSidereal year
A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence it is also the time taken for the Sun to return to the same position with respect to the fixed stars after apparently travelling once around the ecliptic. It was equal to at noon 1 January...
) and/or the synodic month in lunar
Lunar calendar
A lunar calendar is a calendar that is based on cycles of the lunar phase. A common purely lunar calendar is the Islamic calendar or Hijri calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to...
or lunisolar calendar
Lunisolar calendar
A lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the calendar will...
s. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar when it was modified into the Gregorian calendar.
The fundamental problem of the calendar is the imperfect divisibility of whole numbers into an irrational number (fitting whole days into a month; fitting whole days or whole months into a year). The physics of orbital mechanics does not phase-lock the rotation of the Earth (the day) to its revolution (the year), nor the rotation of the Earth (the day) to the revolution of the Moon (the month). Therefore any attempt to divide a month into days or a year into days will leave a fractional remainder of a partial day. Likewise, any attempt to divide a year into months will leave a fractional remainder of a partial month. Such remainders accumulate from one period to the next thereby driving the cycles out of synch.
A typical solution for forcing synchronization is called 'intercalation
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...
'. This is an artificial harmonization that, after the fractional remainders have sufficiently accumulated, adds a whole day (or month) into the cycle. An alternative solution is to ignore the mismatch and simply let the cycles continue to drift apart. The general strategies include:
- The lunar calendar solution, which fits days into the lunar cycle month, adding an extra day when needed, while ignoring the annual solar cycle of the seasons.
- The solar calendar solution, which fits artificial months into the year, adding an extra day into one month when needed, while ignoring the lunar cycle of new/full moons.
- The lunisolar calendar solution, which keeps both the lunar and solar cycles, adding an extra month into the year when needed.
An obvious disadvantage of the lunisolar method of inserting a whole extra month is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar may tend to be seen as less advantageous at larger latitudes where seasonal effects are experienced more strongly. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or a device to track solar day-to-day progression, such as that established at places like Stonehenge
Stonehenge
Stonehenge is a prehistoric monument located in the English county of Wiltshire, about west of Amesbury and north of Salisbury. One of the most famous sites in the world, Stonehenge is composed of a circular setting of large standing stones set within earthworks...
. After centuries of empirical observations, the theoretical aspects of calendar construction could become more refined, enabling predictions that identified the need for reform.
Reform of lunar and lunisolar calendars
There have been 50 to 100 reforms of the traditional Chinese calendarChinese calendar
The Chinese calendar is a lunisolar calendar, incorporating elements of a lunar calendar with those of a solar calendar. It is not exclusive to China, but followed by many other Asian cultures as well...
over 2500 years, most of which were intended to better fit the calendar months to astronomical lunations and to more accurately add the extra month so that the regular months maintain their proper seasonal positions, even though each seasonal marker can occur anywhere within its month.
There have been at least four similar reforms of the lunisolar version of the Hindu calendar
Hindu calendar
The hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar. Nepali calendar, Bengali calendar, Malayalam calendar, Tamil calendar, Telugu calendar, Kannada...
, all intended to make the month a better match to the lunation and to make the year a better fit to the sidereal year. There have been reforms of the 'solar' version of the Hindu calendar which changed the distribution of the days in each month to better match the length of time that the Sun spends in each sidereal zodiacal sign. The same applies to the Buddhist calendar
Buddhist calendar
The Buddhist calendar is used on mainland Southeast Asia in the countries of Thailand, Cambodia, Laos, Burma and Sri Lanka in several related forms. It is a lunisolar calendar having months that are alternately 29 and 30 days, with an intercalated day and a 30-day month added at regular intervals...
.
The first millennium reform of the Hebrew calendar
Hebrew calendar
The Hebrew calendar , or Jewish calendar, is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits , and daily Psalm reading, among many ceremonial uses...
changed it from an observational calendar into a calculated calendar.
The Islamic calendar
Islamic calendar
The Hijri calendar , also known as the Muslim calendar or Islamic calendar , is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in many Muslim countries , and used by Muslims everywhere to determine the proper day on which to celebrate Islamic...
was a reform of the preceding lunisolar calendar which utterly divorced it from the solar year.
Julian/Gregorian reforms
At the time when Julius CaesarJulius Caesar
Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....
took power in Rome, the Roman calendar
Roman calendar
The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...
had ceased to reflect the year accurately. The provision of adding an intercalary month to the year when needed had not been applied consistently, because it affected the length of terms of office.
The Julian reform
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
lengthened seven months and replaced the intercalary month with an intercalary day to be added every four years to February. This produced a noticeably more accurate calendar, but it was based on the calculation of a year as 365 days and 6 hours (365.25 d). In fact, the tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...
is 11 minutes and 14 seconds less than that. This had the effect of adding three-quarters of an hour every four years, and the effect accumulated. By the sixteenth century, the northern vernal equinox fell on March 10 or 11.
Pope Gregory XIII
Pope Gregory XIII , born Ugo Boncompagni, was Pope from 1572 to 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally-accepted civil calendar to this date.-Youth:He was born the son of Cristoforo Boncompagni and wife Angela...
the leap rule was altered: century years which are divisible by four, would not be leap years unless they are also divisible by 400. Thus 1600 and 2000 were leap years, and 2400 will be, but 1700, 1800, and 1900 were not, and 2100, 2200 and 2300 will not be either. This rule makes the mean year 365.2425 days long. While this does not synchronize the years entirely, it would require a few thousand years to accumulate a day.
This new calendar was synchronized with the traditional seasons again and was not applied to dates in the past, which caused a leap of at least ten days from the final day the Julian calendar was in effect. This reform slowly spread through the nations that used the Julian calendar, although the Russian church year still uses the Julian calendar. The times varied so widely that some countries had to drop more than ten days: Great Britain, for instance, dropped eleven.
When noting dates occurring within the period, "Old Style" and "New Style" are used to distinguish which calendar was used by the person who recorded the date.
In 1923, Milutin Milanković
Milutin Milankovic
Milutin Milanković was a Serbian geophysicist and civil engineer, best known for his theory of ice ages, suggesting a relationship between Earth's long-term climate changes and periodic changes in its orbit, now known as Milankovitch cycles. Milanković gave two fundamental contributions to global...
proposed to a synod
Synod
A synod historically is a council of a church, usually convened to decide an issue of doctrine, administration or application. In modern usage, the word often refers to the governing body of a particular church, whether its members are meeting or not...
of some Eastern Orthodox Church
Eastern Orthodox Church
The Orthodox Church, officially called the Orthodox Catholic Church and commonly referred to as the Eastern Orthodox Church, is the second largest Christian denomination in the world, with an estimated 300 million adherents mainly in the countries of Belarus, Bulgaria, Cyprus, Georgia, Greece,...
es at Constantinople
Constantinople
Constantinople was the capital of the Roman, Eastern Roman, Byzantine, Latin, and Ottoman Empires. Throughout most of the Middle Ages, Constantinople was Europe's largest and wealthiest city.-Names:...
that only those centennial years (those ending in '00) that leave a remainder of 200 or 600 upon division by 900 would be leap years, decreasing the average year length to 365.242222 days. These remainders were chosen to delay the first year (after 1923) that this calendar would disagree with the Gregorian calendar as much as possible, until 2800. It was adopted by some Eastern Orthodox Churches under the names Revised Julian calendar
Revised Julian calendar
The Revised Julian calendar, also known as the Rectified Julian calendar, or, less formally, New calendar, is a calendar, originated in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the...
or New calendar, but was rejected by others.
Proposals
The Gregorian calendarGregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
is currently used by most of the world. There is also an international standard describing the calendar, ISO 8601
ISO 8601
ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988...
, with some differences to traditional conceptions in many cultures.
Since the last papal reform, several proposals have been offered to make the Western calendar more useful or regular. Very few reforms have gained official acceptance. The rather different decimal
Decimal calendar
A decimal calendar contains either ten days per week, a multiple of ten days in a month, or ten months per year. Examples that have been adopted are the calendar of Romulus, the Egyptian calendar, the Alexandrian calendar, the Coptic calendar, the Ethiopian calendar, and the French Republican...
French Republican Calendar
French Republican Calendar
The French Republican Calendar or French Revolutionary Calendar was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871...
was one such official reform, but was repealed shortly after its introduction. After World War II the newly-formed United Nations continued efforts of its predecessor, the League of Nations
League of Nations
The League of Nations was an intergovernmental organization founded as a result of the Paris Peace Conference that ended the First World War. It was the first permanent international organization whose principal mission was to maintain world peace...
, to establish a new calendar
World calendar
The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930.-Features:The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial, or perpetual, because it remains the same every year.Each quarter begins...
but postponed the issue after a veto from the USA, which was mainly based upon concerns of religious groups about the proposed days that would be out of the seven day week cycle and thus disrupt having a sabbath occur every seven days. Independently the World Council of Churches
World Council of Churches
The World Council of Churches is a worldwide fellowship of 349 global, regional and sub-regional, national and local churches seeking unity, a common witness and Christian service. It is a Christian ecumenical organization that is based in the Ecumenical Centre in Geneva, Switzerland...
still tries to find a common rule for the date of Easter, which might be eased by a new common calendar.
| 360 ÷ 7 = | 360 ÷ 12 = 30 |
| 364 ÷ 7 = 52 = 4 × 13 | 364 ÷ 12 = |
| 365 ÷ 7 = | 365 ÷ 12 = |
| 366 ÷ 7 = | 366 ÷ 12 = |
Reformers cite several problems with the Gregorian calendar:
- It is not perpetual. Each year starts on a different day of the week and calendars expire every year.
- It is difficult to determine the weekday of any given day of the year or month.
- Months are not equal in length nor regularly distributed across the year, requiring mnemonicMnemonicA mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...
s (e.g. “Thirty days hath SeptemberThirty days hath SeptemberThirty days hath September is a traditional English mnemonic rhyme, of which many variants are commonly used in English-speaking countries to remember the lengths of the months in the Julian and Gregorian calendars....
…”) or knuckle counting (ridges are 31, valleys are 30 except first valley [February] is 28 or 29 in a leap year) to remember which month is 28, 29, 30 or 31 days long. - The year’s four quarters (of three full months each) are not equal (being of 90/91, 91, 92 and 92 days respectively). Business quarters that are equal would make accounting easier.
- Its epoch (origin) is religious. The same applies to monthMonthA month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...
and weekdayWeekdayWeekday may either refer to only a day of the week which is part of the workweek thus not part of the weekend or to any of the days of the week.-Weekday as a day of the workweek:In most countries the days of the workweek are:# Monday# Tuesday# Wednesday...
names in many languages. - Each month has no connection with the lunar phaseLunar phaseA lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...
s.
It is impossible to solve all these issues in just one calendar.
Most plans evolve around the solar year of little more than 365 days. This number does not divide well by seven or twelve, which are the traditional numbers of days per week and months per year respectively. The nearby numbers 360, 364 and 366 are divisible in better ways. There are also lunar centric proposals.
| Comparison of proposed solar calendar reforms (Gregorian/ISO Date ISO 8601 ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988... equivalents) |
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|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Author | New Year's Day | Jan 1 | Feb 2 | Mar 3 | Apr 4 | May 5 | Jun 6 | Jul 7 | Aug 8 | Sep 9 | Oct 10 | Nov 11 | Dec 12 | 13 | Extra calendrical days | Intercalary days | Starts at |
| Comte International Fixed Calendar The International Fixed calendar is a solar calendar proposal for calendar reform designed by Moses B... |
Monday, Jan 1 (01.01) |
28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | Leap D. (07.00), Year Day (13.29) | Leap D. (07.00) (W27.0) | |
| Colligan Pax Calendar The Pax calendar was invented by James A. Colligan in 1930 as a reform of the Gregorian calendar.Unlike other proposals such as the International Fixed Calendar and the World Calendar, it preserves the 7-day week by intercalating a week to a perpetual year of 52 weeks = 364 days.The year is divided... |
Sunday, Jan 1 |
28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | 28 | Leap week 13.1-7 moving Dec from 13 to 14 | ||
| Achelis World calendar The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930.-Features:The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial, or perpetual, because it remains the same every year.Each quarter begins... |
Sunday, Jan 1 |
31 | 30 | 30 | 31 | 30 | 30 | 31 | 30 | 30 | 31 | 30 | 30 | World Leap D. (Jun 31/ 07.00), World D. (Dec 31/ 12.31) | World Leap D. (Jun 31/ 07.00) (W27.0) | ||
| Asimov | Sunday, N. Winter Solstice, 01.01 |
91 | 91 | 91 | 91 | D-92 (04.92), B-92 (02.92) | B-92 (02.92) (W27.0) | ||||||||||
| Bromberg Symmetry454 The Symmetry454 Calendar is a proposal for Gregorian calendar reform. It is a perpetual solar calendar that conserves the traditional 7-day week, has symmetrical equal quarters, and starts every month on Monday.-References:... |
Monday, Jan 1 |
28 | 35 | 28 | 28 | 35 | 28 | 28 | 35 | 28 | 28 | 35 | 28 | Dec 29–35 | |||
| McClennon Common-Civil-Calendar-and-Time The Common-Civil-Calendar-and-Time Calendar is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days.... |
Monday, Jan 1 |
30 | 30 | 31 | 30 | 30 | 31 | 30 | 30 | 31 | 30 | 30 | 31 | * | *Newton 07.1–7, moves following months forward 1 | ||
| Cesare Emiliani Holocene calendar The Human Era, also known as the Holocene calendar or Holocene era , is a year numbering system that adds exactly 10,000 years to the currently world-dominant Anno Domini and Common Era system, placing its first year near the beginning of the Holocene epoch and the Neolithic revolution... |
Jan 1 | 31 | 28 | 31 | 30 | 31 | 30 | 31 | 31 | 30 | 31 | 30 | 31 | Same as Gregorian | January 1, 10000 BC | ||
Perpetual calendars
Many calendar reforms have offered solutions to make the calendar perpetualPerpetual calendar
A perpetual calendar is a calendar which is good for a span of many years, such as the Runic calendar.- General information :...
. These reforms would make it easy to work out the day of week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...
of a particular date
Calendar date
A date in a calendar is a reference to a particular day represented within a calendar system. The calendar date allows the specific day to be identified. The number of days between two dates may be calculated. For example, "24 " is ten days after "14 " in the Gregorian calendar. The date of a...
, and would make changing calendars each year unnecessary.
There are, roughly speaking, two options to achieve this goal: leap week calendar
Leap week calendar
A leap week calendar is a calendar system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, but some - such as the ISO week number calendar - are simply conveniences for specific...
s and intercalary day
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...
s. Both make it easier to work out the day of week by having exactly 52 weeks in each year. The former add a whole 53rd week every five or six years, the latter have an extra day not belonging to any week and two such in leap years. Proposals mainly differ in their selection of a leap rule, placing of the leap item (usually middle or end of the year), in the start day of the week and year, in the number (12 or 13) and size of months and in connected naming; some are compatible to the week date of ISO 8601.
The World Calendar
World calendar
The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930.-Features:The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial, or perpetual, because it remains the same every year.Each quarter begins...
, favored by the UN in the 1950s, and the International Fixed Calendar
International Fixed Calendar
The International Fixed calendar is a solar calendar proposal for calendar reform designed by Moses B...
, quite popular among economists between the World Wars, are proposals that start each year on a Sunday. The remaining 364 days then form 52 weeks of 7 days. The World Calendar has every quarter beginning on the same day of week.
In the World Calendar, the 365th and 366th day are considered holidays and named Worlds Day and Leap Year Day. These "off-calendar" days stand outside the seven-day week and caused some religious groups to strongly oppose adoption of The World Calendar. Such concerns helped prevent the World Calendar from being adopted. Supporters of the World Calendar, however, argue that the religious groups' opposition overlooked every individual's right to celebrate these holidays as extra days of worship, or Sabbaths. This option, they reason, maintained the seven-day worship cycle for those who share that concern, while allowing benefits of a perpetual calendar to be shared by all.
Leap week calendar
Leap week calendar
A leap week calendar is a calendar system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, but some - such as the ISO week number calendar - are simply conveniences for specific...
s add a leap week of seven days to the calendar every five or six years to keep the calendar roughly in step with the tropical year
Tropical year
A tropical year , for general purposes, is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice...
. They have years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week.
Some calendars have quarters of regularly patterned uneven months e.g. a 35 day (five week) month and a pair of 28 day (four week) months, with a leap week appended to the final month when needed. The Common Civil Calendar and Time calendar has months of 30 and 31 days, but inserts a leap week in the middle of the year, when needed.
The 53-week calendar, used in government and in business for fiscal years, is a variant of this concept. Each year of this calendar can be up to 371 days long.
Still other proposals abandon attempts to make the calendar perpetual, instead opting for eleven 30-day months and one long month at the end of 35 (or 36) days.
13-month calendars
Some calendar reformers seek to equalize the length of each month in the year. This is often accomplished by creating a calendar that has 13 months of 4 weeks (28 days) each, making 364 days.An early 13-month proposal was the 1849 Positivist calendar
Positivist calendar
The positivist calendar was a calendar reform proposal by Auguste Comte in 1849. After revising the earlier work of Marco Mastrofini, Comte's proposed calendar was a solar calendar which had 13 months of 28 days, and an additional festival day commemorating the dead, totalling 365 days.This extra...
, created by Auguste Comte
Auguste Comte
Isidore Auguste Marie François Xavier Comte , better known as Auguste Comte , was a French philosopher, a founder of the discipline of sociology and of the doctrine of positivism...
. It was based on a 364-day year which included one or two “blank” days. Each of the 13 months had 28 days and exactly four weeks, and each started on a Monday. The International Fixed Calendar
International Fixed Calendar
The International Fixed calendar is a solar calendar proposal for calendar reform designed by Moses B...
is a more modern descendant of this calendar.
Some proposals add one or two days to the calendar each year to account for the annual solar cycle, while others keep these days off the calendar entirely, to make the calendar perpetual.
Around 1930 Colligan invented the Pax Calendar
Pax Calendar
The Pax calendar was invented by James A. Colligan in 1930 as a reform of the Gregorian calendar.Unlike other proposals such as the International Fixed Calendar and the World Calendar, it preserves the 7-day week by intercalating a week to a perpetual year of 52 weeks = 364 days.The year is divided...
, which avoids off-calendar days by adding a 7-day leap week to the perpetual 364-day year for 71 out of 400 years.
In 1987, Jose Arguelles
Jose Arguelles
Joseph Anthony Arguelles , better known as José Argüelles, was a world-renowned author, artist, visionary and educator. He was the founder of Planet Art Network and the Foundation for the Law of Time. He held a Ph.D...
proposed the Dreamspell
Dreamspell
The Dreamspell is an esoteric calendar based on a modern interpretation of the Maya calendar by New Age spiritualist, Mayanist philosopher, and author José Argüelles, initiated in 1987 and released as a board game in 1993.-General overview of the calendar:...
calendar.
Dividing the years into 52 weeks creates 13 months of 28 days (4 weeks), and 4 quarters of 91 days (13 weeks). This creates months and quarters of fixed, even duration, but not quarters containing a whole number of months.
Lunisolar calendars
- See also: Lunisolar calendarLunisolar calendarA lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the calendar will...
Lunisolar calendars usually have 12 or 13 months of 29 or 30 days.
Some propose to improve leap rules of existing calendars, such as the Hebrew calendar
Hebrew calendar
The Hebrew calendar , or Jewish calendar, is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits , and daily Psalm reading, among many ceremonial uses...
. The Rectified Hebrew calendar uses a more accurate leap cycle of 4366 months per 353-year cycle, with 130 leap years per cycle, and a progressively shorter molad
Molad
Molad is a Hebrew word meaning "birth" that also generically refers to the time at which the New Moon is "born". The word is ambiguous, however, because depending on the context it could refer to the actual or mean astronomical lunar conjunction , or the molad of the traditional Hebrew...
interval, intended to replace the 19-year leap cycle and the constant molad interval of the traditional fixed arithmetic Hebrew calendar
Hebrew calendar
The Hebrew calendar , or Jewish calendar, is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits , and daily Psalm reading, among many ceremonial uses...
, respectively.
Naming
Calendar proposals that introduce a thirteenth month or change the Julian-Gregorian system of months often also propose new names for these months. New names have also been proposed for days out of the week cycle (e.g. 365th and leap) and weeks out of the month cycle.Proposals to change the traditional month and weekday names are less frequent. The Gregorian calendar obtains its names mostly from gods of historical religions (e.g. Thursday from Nordic Thor
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...
or March from Roman Mars) or leaders of vanished empires (July and August from the first Cæsars), or ordinals that got out of synchronization (September through December, originally seventh through tenth, now ninth through twelfth).
Calendar reformers, therefore, seek to correct what they see as deficiencies by focusing on more homogeneous sets of individuals, who usually share common traits.
Examples
Comte’s Positivist calendarPositivist calendar
The positivist calendar was a calendar reform proposal by Auguste Comte in 1849. After revising the earlier work of Marco Mastrofini, Comte's proposed calendar was a solar calendar which had 13 months of 28 days, and an additional festival day commemorating the dead, totalling 365 days.This extra...
, for example, proposed naming the 13 months in his calendar after figures from religion, literature, philosophy and science.
Similarly, the Hermetic Lunar Week Calendar uses 12 or 13 lunar months named after 13 contributors to research on psychoactive plants and chemicals.
The Simple Lunisolar Calendar names its months after the letters of the Greek alphabet
Greek alphabet
The Greek alphabet is the script that has been used to write the Greek language since at least 730 BC . The alphabet in its classical and modern form consists of 24 letters ordered in sequence from alpha to omega...
.
Such as Karl Palmen have suggested, to reuse an existing 13 × 4 naming system.
The one found is playing cards. Thus either months are numbered Ace, Two through Ten, Jack, Queen and King with four weeks each, named after the four suit
Suit (cards)
In playing cards, a suit is one of several categories into which the cards of a deck are divided. Most often, each card bears one of several symbols showing to which suit it belongs; the suit may alternatively or in addition be indicated by the color printed on the card...
s (♠♣♥♦); or the roles are reversed if the calendar has four quarters with thirteen weeks each. Leap days or weeks are assigned the Joker. This system has internationalisation problems, though, because even where the 52-card deck is known, the order of suits may vary.
Also the contemporary roman alphabet has 26 letters, which could be used, together with a further binary indicator, as keys for 52 weeks.
See also
- List of calendars
- Calculating the day of the weekCalculating the day of the weekThis article details various mathematical algorithms to calculate the day of the week for any particular date in the past or future.A typical application is to calculate the day of the week on which someone was born or some other special event occurred....
- Common-Civil-Calendar-and-TimeCommon-Civil-Calendar-and-TimeThe Common-Civil-Calendar-and-Time Calendar is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days....
- Metric timeMetric timeMetric time is the measure of time interval using the metric system, which defines the second as the base unit of time, and multiple and submultiple units formed with metric prefixes, such as kiloseconds and milliseconds. It does not define the time of day, as this is defined by various time...
Specific proposals
There have been many specific calendar proposals to replace the Gregorian Calendar:The following count one or more days outside the standard seven-day week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...
:
- Positivist calendarPositivist calendarThe positivist calendar was a calendar reform proposal by Auguste Comte in 1849. After revising the earlier work of Marco Mastrofini, Comte's proposed calendar was a solar calendar which had 13 months of 28 days, and an additional festival day commemorating the dead, totalling 365 days.This extra...
- The World CalendarWorld calendarThe World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930.-Features:The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial, or perpetual, because it remains the same every year.Each quarter begins...
- International Fixed CalendarInternational Fixed CalendarThe International Fixed calendar is a solar calendar proposal for calendar reform designed by Moses B...
- Invariable CalendarInvariable CalendarIn April 1900, Professor L. A. Grosclaude of Geneva proposed the Invariable Calendar, New Era Calendar, or Normal Calendar with 12 months and four 91-day quarters of exactly 13 weeks. An additional day, termed New Year's Day, that was not any day of the week and not part of any month, would occur...
- World Season Calendar
- Tranquility CalendarTranquility CalendarThe Tranquility calendar is a solar calendar proposal for calendar reform designed by Jeff Siggins providing for a year of 13 months of 28 days each, with one day at the end of each year belonging to no month or week, and a leap day approximately every 4 years....
The following are leap week calendar
Leap week calendar
A leap week calendar is a calendar system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, but some - such as the ISO week number calendar - are simply conveniences for specific...
s:
- Pax CalendarPax CalendarThe Pax calendar was invented by James A. Colligan in 1930 as a reform of the Gregorian calendar.Unlike other proposals such as the International Fixed Calendar and the World Calendar, it preserves the 7-day week by intercalating a week to a perpetual year of 52 weeks = 364 days.The year is divided...
- Common Civil Calendar and Time: C&TCommon-Civil-Calendar-and-TimeThe Common-Civil-Calendar-and-Time Calendar is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days....
- Symmetry454Symmetry454The Symmetry454 Calendar is a proposal for Gregorian calendar reform. It is a perpetual solar calendar that conserves the traditional 7-day week, has symmetrical equal quarters, and starts every month on Monday.-References:...
The following track the moon as well as the sun:
- Hermetic Lunar Week Calendar
- Rectified Hebrew calendar
There have also been proposals to revise the way years are numbered:
- Holocene calendarHolocene calendarThe Human Era, also known as the Holocene calendar or Holocene era , is a year numbering system that adds exactly 10,000 years to the currently world-dominant Anno Domini and Common Era system, placing its first year near the beginning of the Holocene epoch and the Neolithic revolution...
- Jomon EraJapanese era nameThe Japanese era calendar scheme is a common calendar scheme used in Japan, which identifies a year by the combination of the and the year number within the era...
External links
- Specific proposals
- Calendar Reform by Rick McCarty
- C&T calendar home page
- The 13-Month Sol Calendar
- The Ecliptic Calendar
- The New Earth Calendar (13-month calendar)
- The Abysmal Calendar (13-month solilunar calendar)
- The PETIN-METON Calendar
- Johns Hopkins press release on C&T
- Bob McClennon's Refomed Weekly Calendar (Leap week rule has a drafting error)
- The Symmetry454 Calendar home page
- The Rectified Hebrew Calendar home page
- The 30x11 Calendar home page
- Tranquility Calendar (13-month calendar)
- Aristean Calendar
- The Kluznickian Calendar (13 months)
- Calendar 13 (13 month calendar)
- Collections
- Calendar Zone Reform Calendars
- A New Calendar - options for calendar reform
- Leap week calendars in which each year has either 364 or 371 days
- The 13-Moon Change movement
- Calculations
- The Lengths of the Seasons (numerical integration analysis)
- The Length of the Lunar Cycle (numerical integration analysis)
- Solar Calendar Leap Rules (astronomical evaluations)
- Catholic Encyclopedia "Reform of the Calendar" Historical information