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Source: http://astro.nmsu.edu/~lhuber/leaphist.html



This information is reprinted from the Explanatory
Supplement to the Astronomical Almanac, P. Kenneth
Seidelmann, editor, with permission from University
Science Books, Sausalito, CA 94965.

Another place on the WWW to look for calendar information is Calendar Zone.



                                  Calendars
                                    by L. E. Doggett




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Table of Contents
    1. Introduction...................................................................................................... 3
          1.1 Astronomical Bases of Calendars .......................................................... 4
          1.2 Nonastronomical Bases of Calendars: the Week ................................... 6
          1.3 Calendar Reform and Accuracy ............................................................. 6
          1.4 Historical Eras and Chronology .............................................................. 7
    2. The Gregorian Calendar .................................................................................. 9
          2.1 Rules for Civil Use .................................................................................. 9
          2.2 Ecclesiastical Rules.............................................................................. 10
          2.3 History of the Gregorian Calendar ........................................................ 12
    3. The Hebrew Calendar ................................................................................... 14
          3.1 Rules .................................................................................................... 14
          3.2 History of the Hebrew Calendar ........................................................... 19
    4. The Islamic Calendar..................................................................................... 20
          4.1 Rules .................................................................................................... 20
          4.2 History of the Islamic Calendar............................................................. 21
    5. The Indian Calendar ...................................................................................... 23
          5.1 Rules for Civil Use ................................................................................ 23
          5.2 Principles of the Religious Calendar ..................................................... 24
          5.3 History of the Indian Calendar .............................................................. 26
    6. The Chinese Calendar................................................................................... 28
          6.1 Rules .................................................................................................... 29
          6.2 History of the Chinese Calendar........................................................... 32
    7. Julian Day Numbers and Julian Date ............................................................ 35
    8. The Julian Calendar ...................................................................................... 35
          8.1 Rules .................................................................................................... 35
          8.2 History of the Julian Calendar .............................................................. 35
    9. Calendar Conversion Algorithms ................................................................... 37
    10. References .................................................................................................. 38




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1. Introduction

A calendar is a system of organizing units of time for the purpose of reckoning time
over extended periods. By convention, the day is the smallest calendrical unit of time;
the measurement of fractions of a day is classified as timekeeping. The generality of
this definition is due to the diversity of methods that have been used in creating
calendars. Although some calendars replicate astronomical cycles according to fixed
rules, others are based on abstract, perpetually repeating cycles of no astronomical
significance. Some calendars are regulated by astronomical observations, some
carefully and redundantly enumerate every unit, and some contain ambiguities and
discontinuities. Some calendars are codified in written laws; others are transmitted by
oral tradition.


The common theme of calendar making is the desire to organize units of time to
satisfy the needs and preoccupations of society. In addition to serving practical
purposes, the process of organization provides a sense, however illusory, of
understanding and controlling time itself. Thus calendars serve as a link between
mankind and the cosmos. It is little wonder that calendars have held a sacred status
and have served as a source of social order and cultural identity. Calendars have
provided the basis for planning agricultural, hunting, and migration cycles, for
divination and prognostication, and for maintaining cycles of religious and civil events.
Whatever their scientific sophistication, calendars must ultimately be judged as social
contracts, not as scientific treatises.


According to a recent estimate (Fraser, 1987), there are about forty calendars used
in the world today. This chapter is limited to the half-dozen principal calendars in
current use. Furthermore, the emphasis of the chapter is on function and calculation
rather than on culture. The fundamental bases of the calendars are given, along with
brief historical summaries. Although algorithms are given for correlating these
systems, close examination reveals that even the standard calendars are subject to
local variation. With the exception of the Julian calendar, this chapter does not deal


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with extinct systems. Inclusion of the Julian calendar is justified by its everyday use in
historical studies.


Despite a vast literature on calendars, truly authoritative references, particularly in
English, are difficult to find. Aveni (1989) surveys a broad variety of calendrical
systems, stressing their cultural contexts rather than their operational details. Parise
(1982) provides useful, though not infallible, tables for date conversion.
Fotheringham (1935) and the Encyclopedia of Religion and Ethics (1910), in its
section on "Calendars," offer basic information on historical calendars. The sections
on "Calendars" and "Chronology" in all editions of the Encyclopedia Britannica
provide useful historical surveys. Ginzel (1906) remains an authoritative, if dated,
standard of calendrical scholarship. References on individual calendars are given in
the relevant sections.


1.1 Astronomical Bases of Calendars

The principal astronomical cycles are the day (based on the rotation of the Earth on
its axis), the year (based on the revolution of the Earth around the Sun), and the
month (based on the revolution of the Moon around the Earth). The complexity of
calendars arises because these cycles of revolution do not comprise an integral
number of days, and because astronomical cycles are neither constant nor perfectly
commensurable with each other,


The tropical year is defined as the mean interval between vernal equinoxes; it
corresponds to the cycle of the seasons. The following expression, based on the
orbital elements of Laskar (1986), is used for calculating the length of the tropical
year:


365.2421896698 - 0.00000615359 T - 7.29×10-10 T2 + 2.64×10-10 T3 [days]
            (JD - 2451545.0)
where T =                      and JD is the Julian day number.
                 36525




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However, the interval from a particular vernal equinox to the next may vary from this
mean by several minutes.


The synodic month, the mean interval between conjunctions of the Moon and Sun,
corresponds to the cycle of lunar phases. The following expression for the synodic
month is based on the lunar theory of Chapront-Touze' and Chapront (1988):


29.5305888531 + 0.00000021621 T - 3.64E×10-10 T2 [days].
            (JD - 2451545.0)
Again T =                      and JD is the Julian day number.
                 36525


Any particular phase cycle may vary from the mean by up to seven hours.
In the preceding formulas, T is measured in Julian centuries of Terrestrial Dynamical
Time (TDT), which is independent of the variable rotation of the Earth. Thus, the
lengths of the tropical year and synodic month are here defined in days of 86400
seconds of International Atomic Time (TAI).


From these formulas we see that the cycles change slowly with time. Furthermore,
the formulas should not be considered to be absolute facts; they are the best
approximations possible today. Therefore, a calendar year of an integral number of
days cannot be perfectly synchronized to the tropical year. Approximate
synchronization of calendar months with the lunar phases requires a complex
sequence of months of 29 and 30 days. For convenience it is common to speak of a
lunar year of twelve synodic months, or 354.36707 days.


Three distinct types of calendars have resulted from this situation. A solar calendar,
of which the Gregorian calendar in its civil usage is an example, is designed to
maintain synchrony with the tropical year. To do so, days are intercalated (forming
leap years) to increase the average length of the calendar year. A lunar calendar,
such as the Islamic calendar, follows the lunar phase cycle without regard for the
tropical year. Thus the months of the Islamic calendar systematically shift with
respect to the months of the Gregorian calendar. The third type of calendar, the
lunisolar calendar, has a sequence of months based on the lunar phase cycle; but


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every few years a whole month is intercalated to bring the calendar back in phase
with the tropical year. The Hebrew and Chinese calendars are examples of this type
of calendar.


1.2 Nonastronomical Bases of Calendars: the Week

[omitted]


1.3 Calendar Reform and Accuracy

In most societies a calendar reform is an extraordinary event. Adoption of a calendar
depends on the forcefulness with which it is introduced and on the willingness of
society to accept it. For example, the acceptance of the Gregorian calendar as a
worldwide standard spanned more than three centuries.


The legal code of the United States does not specify an official national calendar.
Use of the Gregorian calendar in the United States stems from an Act of Parliament
of the United Kingdom in 1751, which specified use of the Gregorian calendar in
England and its colonies. However, its adoption in the United Kingdom and other
countries was fraught with confusion, controversy, and even violence (Bates, 1952;
Gingerich, 1983; Hoskin, 1983). It also had a deeper cultural impact through the
disruption of traditional festivals and calendrical practices (MacNeill, 1982).


Because calendars are created to serve societal needs, the question of a calendar's
accuracy is usually misleading or misguided. A calendar that is based on a fixed set
of rules is accurate if the rules are consistently applied. For calendars that attempt to
replicate astronomical cycles, one can ask how accurately the cycles are replicated.
However, astronomical cycles are not absolutely constant, and they are not known
exactly. In the long term, only a purely observational calendar maintains synchrony
with astronomical phenomena. However, an observational calendar exhibits short-
term uncertainty, because the natural phenomena are complex and the observations
are subject to error.




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1.4 Historical Eras and Chronology

The calendars treated in this chapter, except for the Chinese calendar, have counts
of years from initial epochs. In the case of the Chinese calendar and some calendars
not included here, years are counted in cycles, with no particular cycle specified as
the first cycle. Some cultures eschew year counts altogether but name each year
after an event that characterized the year. However, a count of years from an initial
epoch is the most successful way of maintaining a consistent chronology. Whether
this epoch is associated with an historical or legendary event, it must be tied to a
sequence of recorded historical events.


This is illustrated by the adoption of the birth of Christ as the initial epoch of the
Christian calendar. This epoch was established by the sixth-century scholar
Dionysius Exiguus, who was compiling a table of dates of Easter. An existing table
covered the nineteen-year period denoted 228-247, where years were counted from
the beginning of the reign of the Roman emperor Diocletian. Dionysius continued the
table for a nineteen-year period, which he designated Anni Domini Nostri Jesu Christi
532-550. Thus, Dionysius' Anno Domini 532 is equivalent to Anno Diocletian 248. In
this way a correspondence was established between the new Christian Era and an
existing system associated with historical records. What Dionysius did not do is
establish an accurate date for the birth of Christ. Although scholars generally believe
that Christ was born some years before A.D. 1, the historical evidence is too sketchy
to allow a definitive dating.


Given an initial epoch, one must consider how to record preceding dates. Bede, the
eighth-century English historian, began the practice of counting years backward from
A.D. 1 (see Colgrave and Mynors, 1969). In this system, the year A.D. 1 is preceded
by the year 1 B.C., without an intervening year 0. Because of the numerical
discontinuity, this "historical" system is cumbersome for comparing ancient and
modern dates. Today, astronomers use +1 to designate A.D. 1. Then +1 is naturally
preceded by year 0, which is preceded by year -1. Since the use of negative
numbers developed slowly in Europe, this "astronomical" system of dating was




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delayed until the eighteenth century, when it was introduced by the astronomer
Jacques Cassini (Cassini, 1740).


Even as use of Dionysius' Christian Era became common in ecclesiastical writings of
the Middle Ages, traditional dating from regnal years continued in civil use. In the
sixteenth century, Joseph Justus Scaliger tried to resolve the patchwork of historical
eras by placing everything on a single system (Scaliger, 1583). Instead of introducing
negative year counts, he sought an initial epoch in advance of any historical record.
His numerological approach utilized three calendrical cycles: the 28-year solar cycle,
the nineteen-year cycle of Golden Numbers, and the fifteen-year indiction cycle. The
solar cycle is the period after which weekdays and calendar dates repeat in the
Julian calendar. The cycle of Golden Numbers is the period after which moon phases
repeat (approximately) on the same calendar dates. The indiction cycle was a
Roman tax cycle. Scaliger could therefore characterize a year by the combination of
numbers (S,G,I), where S runs from 1 through 28, G from 1 through 19, and I from 1
through 15. Scaliger noted that a given combination would recur after 7980 (= 28×19
×15) years. He called this a Julian Period, because it was based on the Julian
calendar year. For his initial epoch Scaliger chose the year in which S, G, and I were
all equal to 1. He knew that the year 1 B.C. was characterized by the number 9 of the
solar cycle, by the Golden Number 1, and by the number 3 of the indiction cycle, i.e.,
(9,1,3). He found that the combination (1,1,1) occurred in 4713 B.C. or, as
astronomers now say, -4712. This serves as year 1 of Scaliger's Julian Period. It was
later adopted as the initial epoch for the Julian day numbers.




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2. The Gregorian Calendar

The Gregorian calendar today serves as an international standard for civil use. In
addition, it regulates the ceremonial cycle of the Roman Catholic and Protestant
churches. In fact, its original purpose was ecclesiastical. Although a variety of other
calendars are in use today, they are restricted to particular religions or cultures.


2.1 Rules for Civil Use

Years are counted from the initial epoch defined by Dionysius Exiguus, and are
divided into two classes: common years and leap years. A common year is 365 days
in length; a leap year is 366 days, with an intercalary day, designated February 29,
preceding March 1. Leap years are determined according to the following rule:


   Every year that is exactly divisible by 4 is a leap year, except for years that are
                                exactly divisible by 100;
     these centurial years are leap years only if they are exactly divisible by 400.


As a result the year 2000 is a leap year, whereas 1900 and 2100 are not leap years.
These rules can be applied to times prior to the Gregorian reform to create a
proleptic Gregorian calendar. In this case, year 0 (1 B.C.) is considered to be exactly
divisible by 4, 100, and 400; hence it is a leap year.


The Gregorian calendar is thus based on a cycle of 400 years, which comprises
146097 days. Since 146097 is evenly divisible by 7, the Gregorian civil calendar
exactly repeats after 400 years. Dividing 146097 by 400 yields an average length of
365.2425 days per calendar year, which is a close approximation to the length of the
tropical year. Comparison with Equation 1.1-1 reveals that the Gregorian calendar
accumulates an error of one day in about 2500 years. Although various adjustments
to the leap-year system have been proposed, none has been instituted.




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Within each year, dates are specified according to the count of days from the
beginning of the month. The order of months and number of days per month were
adopted from the Julian calendar.
                                          Table 2.1.1
                           Months of the Gregorian Calendar
                           1. January 31 7. July            31
                           2. February 28* 8. August        31
                           3. March        31 9. September 30

                           4. April        30 10. October   31
                           5. May          31 11. November 30
                           6. June         30 12. December 31


                        * In a leap year, February has 29 days.


2.2 Ecclesiastical Rules

The ecclesiastical calendars of Christian churches are based on cycles of movable
and immovable feasts. Christmas is the principal immovable feast, with its date set at
December 25. Easter is the principal movable feast, and dates of most other movable
feasts are determined with respect to Easter. However, the movable feasts of the
Advent and Epiphany seasons are Sundays reckoned from Christmas and the Feast
of the Epiphany, respectively.


In the Gregorian calendar, the date of Easter is defined to occur on the Sunday
following the ecclesiastical Full Moon that falls on or next after March 21. This should
not be confused with the popular notion that Easter is the first Sunday after the first
Full Moon following the vernal equinox. In the first place, the vernal equinox does not
necessarily occur on March 21. In addition, the ecclesiastical Full Moon is not the
astronomical Full Moon -- it is based on tables that do not take into account the full
complexity of lunar motion. As a result, the date of an ecclesiastical Full Moon may
differ from that of the true Full Moon. However, the Gregorian system of leap years




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and lunar tables does prevent progressive departure of the tabulated data from the
astronomical phenomena.


The ecclesiastical Full Moon is defined as the fourteenth day of a tabular lunation,
where day 1 corresponds to the ecclesiastical New Moon. The tables are based on
the Metonic cycle, in which 235 mean synodic months occur in 6939.688 days. Since
nineteen Gregorian years is 6939.6075 days, the dates of Moon phases in a given
year will recur on nearly the same dates nineteen years laters. To prevent the 0.08
day difference between the cycles from accumulating, the tables incorporate
adjustments to synchronize the system over longer periods of time. Additional
complications arise because the tabular lunations are of 29 or 30 integral days. The
entire system comprises a period of 5700000 years of 2081882250 days, which is
equated to 70499183 lunations. After this period, the dates of Easter repeat
themselves.


The following algorithm for computing the date of Easter is based on the algorithm of
Oudin (1940). It is valid for any Gregorian year, Y. All variables are integers and the
remainders of all divisions are dropped. The final date is given by M, the month, and
D, the day of the month.


                                         C = Y/100,
                                     N = Y - 19*(Y/19),
                                      K = (C - 17)/25,
                           I = C - C/4 - (C - K)/3 + 19*N + 15,
                                      I = I - 30*(I/30),
                    I = I - (I/28)*(1 - (I/28)*(29/(I + 1))*((21 - N)/11)),
                              J = Y + Y/4 + I + 2 - C + C/4,
                                       J = J - 7*(J/7),
                                          L = I - J,
                                   M = 3 + (L + 40)/44,
                                  D = L + 28 - 31*(M/4).




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2.3 History of the Gregorian Calendar

The Gregorian calendar resulted from a perceived need to reform the method of
calculating dates of Easter. Under the Julian calendar the dating of Easter had
become standardized, using March 21 as the date of the equinox and the Metonic
cycle as the basis for calculating lunar phases. By the thirteenth century it was
realized that the true equinox had regressed from March 21 (its supposed date at the
time of the Council of Nicea, +325) to a date earlier in the month. As a result, Easter
was drifting away from its springtime position and was losing its relation with the
Jewish Passover. Over the next four centuries, scholars debated the "correct" time
for celebrating Easter and the means of regulating this time calendrically. The Church
made intermittent attempts to solve the Easter question, without reaching a
consensus.


By the sixteenth century the equinox had shifted by ten days, and astronomical New
Moons were occurring four days before ecclesiastical New Moons. At the behest of
the Council of Trent, Pope Pius V introduced a new Breviary in 1568 and Missal in
1570, both of which included adjustments to the lunar tables and the leap-year
system. Pope Gregory XIII, who succeeded Pope Pius in 1572, soon convened a
commission to consider reform of the calendar, since he considered his
predecessor's measures inadequate.


The recommendations of Pope Gregory's calendar commission were instituted by the
papal bull "Inter Gravissimus," signed on 1582 February 24. Ten days were deleted
from the calendar, so that 1582 October 4 was followed by 1582 October 15, thereby
causing the vernal equinox of 1583 and subsequent years to occur about March 21.
And a new table of New Moons and Full Moons was introduced for determining the
date of Easter.


Subject to the logistical problems of communication and governance in the sixteenth
century, the new calendar was promulgated through the Roman-Catholic world.
Protestant states initially rejected the calendar, but gradually accepted it over the
coming centuries. The Eastern Orthodox churches rejected the new calendar and


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continued to use the Julian calendar with traditional lunar tables for calculating Easter.
Because the purpose of the Gregorian calendar was to regulate the cycle of Christian
holidays, its acceptance in the non-Christian world was initially not at issue. But as
international communications developed, the civil rules of the Gregorian calendar
were gradually adopted around the world.


Anyone seriously interested in the Gregorian calendar should study the collection of
papers resulting from a conference sponsored by the Vatican to commemorate the
four-hundredth anniversary of the Gregorian Reform (Coyne et al., 1983).




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3. The Hebrew Calendar

As it exists today, the Hebrew calendar is a lunisolar calendar that is based on
calculation rather than observation. This calendar is the official calendar of Israel and
is the liturgical calendar of the Jewish faith.


In principle the beginning of each month is determined by a tabular New Moon
(molad) that is based on an adopted mean value of the lunation cycle. To ensure that
religious festivals occur in appropriate seasons, months are intercalated according to
the Metonic cycle, in which 235 lunations occur in nineteen years.


By tradition, days of the week are designated by number, with only the seventh day,
Sabbath, having a specific name. Days are reckoned from sunset to sunset, so that
day 1 begins at sunset on Saturday and ends at sunset on Sunday. The Sabbath
begins at sunset on Friday and ends at sunset on Saturday.


3.1 Rules

Years are counted from the Era of Creation, or Era Mundi, which corresponds to -
3760 October 7 on the Julian proleptic calendar. Each year consists of twelve or
thirteen months, with months consisting of 29 or 30 days. An intercalary month is
introduced in years 3, 6, 8, 11, 14, 17, and 19 in a nineteen-year cycle of 235
lunations. The initial year of the calendar, A.M. (Anno Mundi) 1, is year 1 of the
nineteen-year cycle.


The calendar for a given year is established by determining the day of the week of
Tishri 1 (first day of Rosh Hashanah or New Year's Day) and the number of days in
the year. Years are classified according to the number of days in the year (see Table
3.1.1).




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                                        Table 3.1.1
                    Classification of Years in the Hebrew Calendar

                                       Deficient Regular Complete
                    Ordinary year      353          354   355
                    Leap year          383          384   385


                                        Table 3.1.2
                            Months of the Hebrew Calendar
                           1. Tishri     30    7. Nisan     30

                           2. Heshvan 29*      8. Iyar      29
                           3. Kislev     30** 9. Sivan      30
                           4. Tevet      29    10. Tammuz 29
                           5. Shevat     30    11. Av       30
                           6. Adar       29*** 12. Elul     29
                      * In a complete year, Heshvan has 30 days.
                       ** In a deficient year, Kislev has 29 days.
     *** In a leap year Adar I has 30 days; it is followed by Adar II with 29 days.


                                        Table 3.1.3
                         Terminology of the Hebrew Calendar
Deficient (haser) month: a month comprising 29 days.

Full (male) month: a month comprising 30 days.
Ordinary year: a year comprising 12 months, with a total of 353, 354, or 355 days.
Leap year: a year comprising 13 months, with a total of 383, 384, or 385 days.
Complete year (shelemah): a year in which the months of Heshvan and Kislev both
contain 30 days.
Deficient year (haser): a year in which the months of Heshvan and Kislev both
contain 29 days.
Regular year (kesidrah): a year in which Heshvan has 29 days and Kislev has 30


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days.
Halakim(singular, helek): "parts" of an hour; there are 1080 halakim per hour.
Molad(plural, moladot): "birth" of the Moon, taken to mean the time of conjunction for
modern calendric purposes.
Dehiyyah(plural, dehiyyot): "postponement"; a rule delaying 1 Tishri until after the
molad.


The months of Heshvan and Kislev vary in length to satisfy requirements for the
length of the year (see Table 3.1.1). In leap years, the 29-day month Adar is
designated Adar II, and is preceded by the 30-day intercalary month Adar I.


For calendrical calculations, the day begins at 6 P.M., which is designated 0 hours.
Hours are divided into 1080 halakim; thus one helek is 3 1/3 seconds. (Terminology
is explained in Table 3.1.3.) Calendrical calculations are referred to the meridian of
Jerusalem -- 2 hours 21 minutes east of Greenwich.


Rules for constructing the Hebrew calendar are given in the sections that follow.
Cohen (1981), Resnikoff (1943), and Spier (1952) provide reliable guides to the rules
of calculation.

3.1.1 Determining Tishri 1

The calendar year begins with the first day of Rosh Hashanah (Tishri 1). This is
determined by the day of the Tishri molad and the four rules of postponements
(dehiyyot). The dehiyyot can postpone Tishri 1 until one or two days following the
molad. Tabular new moons (maladot) are reckoned from the Tishri molad of the year
A.M. 1, which occurred on day 2 at 5 hours, 204 halakim (i.e., 11:11:20 P.M. on
Sunday, -3760 October 6, Julian proleptic calendar). The adopted value of the mean
lunation is 29 days, 12 hours, 793 halakim (29.530594 days). To avoid rounding and
truncation errors, calculation should be done in halakim rather than decimals of a day,
since the adopted lunation constant is expressed exactly in halakim.


                                      Table 3.1.1.1


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                      Lunation Constants for Determining Tishri 1
                      Lunations       Weeks-Days-Hours-Halakim
                      1           =           4-1-12-0793

                      12          =           50-4-08-0876
                      13          =           54-5-21-0589
                      235         =          991-2-16-0595
Lunation constants required in calculations are shown in Table 3.1.1.1. By
subtracting off the weeks, these constants give the shift in weekdays that occurs after
each cycle.
The dehiyyot are as follows:
(a) If the Tishri molad falls on day 1, 4, or 6, then Tishri 1 is postponed one day.
(b) If the Tishri molad occurs at or after 18 hours (i.e., noon), then Tishri 1 is
postponed one day. If this causes Tishri 1 to fall on day 1, 4, or 6, then Tishri 1 is
postponed an additional day to satisfy dehiyyah (a).
(c) If the Tishri molad of an ordinary year (i.e., of twelve months) falls on day 3 at or
after 9 hours, 204 halakim, then Tishri 1 is postponed two days to day 5, thereby
satisfying dehiyyah (a).
(d) If the first molad following a leap year falls on day 2 at or after 15 hours, 589
halakim, then Tishri 1 is postponed one day to day 3.

3.1.2 Reasons for the Dehiyyot

Dehiyyah (a) prevents Hoshana Rabba (Tishri 21) from occurring on the Sabbath and
prevents Yom Kippur (Tishri 10) from occurring on the day before or after the
Sabbath.
Dehiyyah (b) is an artifact of the ancient practice of beginning each month with the
sighting of the lunar crescent. It is assumed that if the molad (i.e., the mean
conjunction) occurs after noon, the lunar crescent cannot be sighted until after 6 P.M.,
which will then be on the following day.
Dehiyyah (c) prevents an ordinary year from exceeding 355 days. If the Tishri molad
of an ordinary year occurs on Tuesday at or after 3:11:20 A.M., the next Tishri molad
will occur at or after noon on Saturday. According to dehiyyah (b), Tishri 1 of the next
year must be postponed to Sunday, which by dehiyyah (a) occasions a further


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postponement to Monday. This results in an ordinary year of 356 days. Postponing
Tishri 1 from Tuesday to Thursday produces a year of 354 days.
Dehiyyah (d) prevents a leap year from falling short of 383 days. If the Tishri molad
following a leap year is on Monday, at or after 9:32:43 1/3 A.M., the previous Tishri
molad (thirteen months earlier) occurred on Tuesday at or after noon. Therefore, by
dehiyyot (b) and (a), Tishri 1 beginning the leap year was postponed to Thursday. To
prevent a leap year of 382 days, dehiyyah (d) postpones by one day the beginning of
the ordinary year.
A thorough discussion of both the functional and religious aspects of the dehiyyot is
provided by Cohen (1981).

3.1.3 Determining the Length of the Year

An ordinary year consists of 50 weeks plus 3, 4, or 5 days. The number of excess
days identifies the year as being deficient, regular, or complete, respectively. A leap
year consists of 54 weeks plus 5, 6, or 7 days, which again are designated deficient,
regular, or complete, respectively. The length of a year can therefore be determined
by comparing the weekday of Tishri 1 with that of the next Tishri 1.


First consider an ordinary year. The weekday shift after twelve lunations is 04-08-876.
For example if a Tishri molad of an ordinary year occurs on day 2 at 0 hours 0
halakim (6 P.M. on Monday), the next Tishri molad will occur on day 6 at 8 hours 876
halakim. The first Tishri molad does not require application of the dehiyyot, so Tishri
1 occurs on day 2. Because of dehiyyah (a), the following Tishri 1 is delayed by one
day to day 7, five weekdays after the previous Tishri 1. Since this characterizes a
complete year, the months of Heshvan and Kislev both contain 30 days.


The weekday shift after thirteen lunations is 05-21-589. If the Tishri molad of a leap
year occurred on day 4 at 20 hours 500 halakim, the next Tishri molad will occur on
day 3 at 18 hours 9 halakim. Becuase of dehiyyot (b), Tishri 1 of the leap year is
postponed two days to day 6. Because of dehiyyot (c), Tishri 1 of the following year is
postponed two days to day 5. This six-day difference characterizes a regular year, so
that Heshvan has 29 days and Kislev has 30 days.



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3.2 History of the Hebrew Calendar

The codified Hebrew calendar as we know it today is generally considered to date
from A.M. 4119 (+359), though the exact date is uncertain. At that time the patriarch
Hillel II, breaking with tradition, disseminated rules for calculating the calendar. Prior
to that time the calendar was regarded as a secret science of the religious authorities.
The exact details of Hillel's calendar have not come down to us, but it is generally
considered to include rules for intercalation over nineteen-year cycles. Up to the
tenth century A.D., however, there was disagreement about the proper years for
intercalation and the initial epoch for reckoning years.


Information on calendrical practices prior to Hillel is fragmentary and often
contradictory. The earliest evidence indicates a calendar based on observations of
Moon phases. Since the Bible mentions seasonal festivals, there must have been
intercalation. There was likely an evolution of conflicting calendrical practices.


The Babylonian exile, in the first half of the sixth century B.C., greatly influenced the
Hebrew calendar. This is visible today in the names of the months. The Babylonian
influence may also have led to the practice of intercalating leap months.


During the period of the Sanhedrin, a committee of the Sanhedrin met to evaluate
reports of sightings of the lunar crescent. If sightings were not possible, the new
month was begun 30 days after the beginning of the previous month. Decisions on
intercalation were influenced, if not determined entirely, by the state of vegetation
and animal life. Although eight-year, nineteen-year, and longer- period intercalation
cycles may have been instituted at various times prior to Hillel II, there is little
evidence that they were employed consistently over long time spans.




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4. The Islamic Calendar

The Islamic calendar is a purely lunar calendar in which months correspond to the
lunar phase cycle. As a result, the cycle of twelve lunar months regresses through
the seasons over a period of about 33 years. For religious purposes, Muslims begin
the months with the first visibility of the lunar crescent after conjunction. For civil
purposes a tabulated calendar that approximates the lunar phase cycle is often used.


The seven-day week is observed with each day beginning at sunset. Weekdays are
specified by number, with day 1 beginning at sunset on Saturday and ending at
sunset on Sunday. Day 5, which is called Jum'a, is the day for congregational
prayers. Unlike the Sabbath days of the Christians and Jews, however, Jum'a is not a
day of rest. Jum'a begins at sunset on Thursday and ends at sunset on Friday.


4.1 Rules

Years of twelve lunar months are reckoned from the Era of the Hijra, commemorating
the migration of the Prophet and his followers from Mecca to Medina. This epoch, 1
A.H. (Anno Higerae) Muharram 1, is generally taken by astronomers (Neugebauer,
1975) to be Thursday, +622 July 15 (Julian calendar). This is called the astronomical
Hijra epoch. Chronological tables (e.g., Mayr and Spuler, 1961; Freeman-Grenville,
1963) generally use Friday, July 16, which is designated the civil epoch. In both
cases the Islamic day begins at sunset of the previous day.


For religious purposes, each month begins in principle with the first sighting of the
lunar crescent after the New Moon. This is particularly important for establishing the
beginning and end of Ramadan. Because of uncertainties due to weather, however,
a new month may be declared thirty days after the beginning of the preceding month.
Although various predictive procedures have been used for determining first visibility,
they have always had an equivocal status. In practice, there is disagreement among
countries, religious leaders, and scientists about whether to rely on observations,



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which are subject to error, or to use calculations, which may be based on poor
models.


Chronologists employ a thirty-year cyclic calendar in studying Islamic history. In this
tabular calendar, there are eleven leap years in the thirty-year cycle. Odd-numbered
months have thirty days and even-numbered months have twenty-nine days, with a
thirtieth day added to the twelfth month, Dhu al-Hijjah (see Table 4.1.1). Years 2, 5, 7,
10, 13, 16, 18, 21, 24, 26, and 29 of the cycle are designated leap years. This type of
calendar is also used as a civil calendar in some Muslim countries, though other
years are sometimes used as leap years. The mean length of the month of the thirty-
year tabular calendar is about 2.9 seconds less than the synodic period of the Moon.
                                          Table 4.1.1
                          Months of Tabular Islamic Calendar

                       1. Muharram** 30 7. Rajab**                30
                       2. Safar           29 8. Sha'ban           29
                       3. Rabi'a I        30 9. Ramadan***        30
                       4. Rabi'a II       29 10. Shawwal          29

                       5. Jumada I        30 11. Dhu al-Q'adah** 30
                       6. Jumada II       29 12. Dhu al-Hijjah** 29*
                      * In a leap year, Dhu al-Hijjah has 30 days.
                                        ** Holy months.
                                      *** Month of fasting.

4.1.1 Visibility of the Crescent Moon

[omitted]


4.2 History of the Islamic Calendar

The form of the Islamic calendar, as a lunar calendar without intercalation, was laid
down by the Prophet in the Qur'an (Sura IX, verse 36-37) and in his sermon at the
Farewell Pilgrimage. This was a departure from the lunisolar calendar commonly




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used in the Arab world, in which months were based on first sightings of the lunar
crescent, but an intercalary month was added as deemed necessary.
Caliph 'Umar I is credited with establishing the Hijra Era in A.H. 17. It is not known
how the initial date was determined. However, calculations show that the
astronomical New Moon (i.e., conjunction) occurred on +622 July 14 at 0444 UT
(assuming delta-T = 1.0 hour), so that sighting of the crescent most likely occurred on
the evening of July 16.




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5. The Indian Calendar

As a result of a calendar reform in A.D. 1957, the National Calendar of India is a
formalized lunisolar calendar in which leap years coincide with those of the Gregorian
calendar (Calendar Reform Committee, 1957). However, the initial epoch is the Saka
Era, a traditional epoch of Indian chronology. Months are named after the traditional
Indian months and are offset from the beginning of Gregorian months (see Table
5.1.1).
In addition to establishing a civil calendar, the Calendar Reform Committee set
guidelines for religious calendars, which require calculations of the motions of the
Sun and Moon. Tabulations of the religious holidays are prepared by the India
Meteorological Department and published annually in The Indian Astronomical
Ephemeris.
Despite the attempt to establish a unified calendar for all of India, many local
variations exist. The Gregorian calendar continues in use for administrative purposes,
and holidays are still determined according to regional, religious, and ethnic traditions
(Chatterjee, 1987).


5.1 Rules for Civil Use

Years are counted from the Saka Era; 1 Saka is considered to begin with the vernal
equinox of A.D. 79. The reformed Indian calendar began with Saka Era 1879, Caitra
1, which corresponds to A.D. 1957 March 22. Normal years have 365 days; leap
years have 366. In a leap year, an intercalary day is added to the end of Caitra. To
determine leap years, first add 78 to the Saka year. If this sum is evenly divisible by 4,
the year is a leap year, unless the sum is a multiple of 100. In the latter case, the
year is not a leap year unless the sum is also a multiple of 400. Table 5.1.1 gives the
sequence of months and their correlation with the months of the Gregorian calendar.




                                       Table 5.1.1



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                          Months of the Indian Civil Calendar
                          Days Correlation of Indian/Gregorian
         1. Caitra        30*   Caitra 1                          March 22*

         2. Vaisakha      31    Vaisakha 1                        April 21
         3. Jyaistha      31    Jyaistha 1                        May 22
         4. Asadha        31    Asadha 1                          June 22
         5. Sravana       31    Sravana 1                         July 23
         6. Bhadra        31    Bhadra 1                          August 23

         7. Asvina        30    Asvina 1                          September 23
         8. Kartika       30    Kartika 1                         October 23
         9. Agrahayana 30       Agrahayana 1                      November 22
         10. Pausa        30    Pausa 1                           December 22
         11. Magha        30    Magha 1                           January 21

         12. Phalguna 30        Phalguna 1                        February 20
      * In a leap year, Caitra has 31 days and Caitra 1 coincides with March 21.


5.2 Principles of the Religious Calendar

Religious holidays are determined by a lunisolar calendar that is based on
calculations of the actual postions of the Sun and Moon. Most holidays occur on
specified lunar dates (tithis), as is explained later; a few occur on specified solar
dates. The calendrical methods presented here are those recommended by the
Calendar Reform Committee (1957). They serve as the basis for the calendar
published in The Indian Astronomical Ephemeris. However, many local calendar
makers continue to use traditional astronomical concepts and formulas, some of
which date back 1500 years.
The Calendar Reform Committee attempted to reconcile traditional calendrical
practices with modern astronomical concepts. According to their proposals,
precession is accounted for and calculations of solar and lunar position are based on
accurate modern methods. All astronomical calculations are performed with respect




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to a Central Station at longitude 82o30' East, latitude 23o11' North. For religious
purposes solar days are reckoned from sunrise to sunrise.
A solar month is defined as the interval required for the Sun's apparent longitude to
increase by 30o, corresponding to the passage of the Sun through a zodiacal sign
(rasi). The initial month of the year, Vaisakha, begins when the true longitude of the
Sun is 23o 15' (see Table 5.2.1). Because the Earth's orbit is elliptical, the lengths of
the months vary from 29.2 to 31.2 days. The short months all occur in the second
half of the year around the time of the Earth's perihelion passage.
                                       Table 5.2.1
                     Solar Months of the Indian Religious Calendar
                         Sun's Longitude Approx. Duration Approx. Greg. Date
                         deg min          d
        1. Vaisakha      23 15            30.9               Apr. 13

        2. Jyestha       53 15            31.3               May 14
        3. Asadha        83 15            31.5               June 14
        4. Sravana       113 15           31.4               July 16
        5. Bhadrapada 143 15              31.0               Aug. 16

        6. Asvina        173 15           30.5               Sept. 16
        7. Kartika       203 15           30.0               Oct. 17
        8. Margasirsa 233 15              29.6               Nov. 16
        9. Pausa         263 15           29.4               Dec. 15
        10. Magha        293 15           29.5               Jan. 14

        11. Phalgura     323 15           29.9               Feb. 12
        12. Caitra       353 15           30.3               Mar. 14
Lunar months are measured from one New Moon to the next (although some groups
reckon from the Full Moon). Each lunar month is given the name of the solar month
in which the lunar month begins. Because most lunations are shorter than a solar
month, there is occasionally a solar month in which two New Moons occur. In this
case, both lunar months bear the same name, but the first month is described with
the prefix adhika, or intercalary. Such a year has thirteen lunar months. Adhika



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months occur every two or three years following patterns described by the Metonic
cycle or more complex lunar phase cycles.
More rarely, a year will occur in which a short solar month will pass without having a
New Moon. In that case, the name of the solar month does not occur in the calendar
for that year. Such a decayed (ksaya) month can occur only in the months near the
Earth's perihelion passage. In compensation, a month in the first half of the year will
have had two New Moons, so the year will still have twelve lunar months. Ksaya
months are separated by as few as nineteen years and as many as 141 years.
Lunations are divided into 30 tithis, or lunar days. Each tithi is defined by the time
required for the longitude of the Moon to increase by 12o over the longitude of the
Sun. Thus the length of a tithi may vary from about 20 hours to nearly 27 hours.
During the waxing phases, tithis are counted from 1 to 15 with the designation Sukla.
Tithis for the waning phases are designated Krsna and are again counted from 1 to
15. Each day is assigned the number of the tithi in effect at sunrise. Occasionally a
short tithi will begin after sunrise and be completed before the next sunrise. Similarly
a long tithi may span two sunrises. In the former case, a number is omitted from the
day count. In the latter, a day number is carried over to a second day.


5.3 History of the Indian Calendar

The history of calendars in India is a remarkably complex subject owing to the
continuity of Indian civilization and to the diversity of cultural influences. In the mid-
1950s, when the Calendar Reform Committee made its survey, there were about 30
calendars in use for setting religious festivals for Hindus, Buddhists, and Jainists.
Some of these were also used for civil dating. These calendars were based on
common principles, though they had local characteristics determined by long-
established customs and the astronomical practices of local calendar makers. In
addition, Muslims in India used the Islamic calendar, and the Indian government used
the Gregorian calendar for administrative purposes.
Early allusions to a lunisolar calendar with intercalated months are found in the
hymns from the Rig Veda, dating from the second millennium B.C. Literature from
1300 B.C. to A.D. 300, provides information of a more specific nature. A five-year
lunisolar calendar coordinated solar years with synodic and sidereal lunar months.



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Indian astronomy underwent a general reform in the first few centuries A.D., as
advances in Babylonian and Greek astronomy became known. New astronomical
constants and models for the motion of the Moon and Sun were adapted to traditional
calendric practices. This was conveyed in astronomical treatises of this period known
as Siddhantas, many of which have not survived. The Surya Siddhanta, which
originated in the fourth century but was updated over the following centuries,
influenced Indian calendrics up to and even after the calendar reform of A.D. 1957.
Pingree (1978) provides a survey of the development of mathematical astronomy in
India. Although he does not deal explicitly with calendrics, this material is necessary
for a full understanding of the history of India's calendars.




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6. The Chinese Calendar

The Chinese calendar is a lunisolar calendar based on calculations of the positions of
the Sun and Moon. Months of 29 or 30 days begin on days of astronomical New
Moons, with an intercalary month being added every two or three years. Since the
calendar is based on the true positions of the Sun and Moon, the accuracy of the
calendar depends on the accuracy of the astronomical theories and calculations.


Although the Gregorian calendar is used in the Peoples' Republic of China for
administrative purposes, the traditional Chinese calendar is used for setting
traditional festivals and for timing agricultural activities in the countryside. The
Chinese calendar is also used by Chinese communities around the world.


                                        Table 6.1.1
                     Chinese Sexagenary Cycle of Days and Years

                    Celestial Stems (天干) Earthly Branches (地支)
                    1. jia 甲                 1. zi (rat) 子 (鼠)
                    2. yi 乙                  2. chou (ox) 丑 (牛)
                    3. bing 丙                3. yin (tiger) 寅 (虎)
                    4. ding 丁                4. mao (hare) 卯 (免)

                    5. wu 戊                  5. chen (dragon) 辰 (龍)
                    6. ji 己                  6. si (snake) 巳 (蛇)
                    7. geng 庚                7. wu (horse) 午 (馬)
                    8. xin 辛                 8. wei (sheep) 未 (羊)

                    9. ren 壬                 9. shen (monkey) 申 (猴)
                    10. gui 癸                10. you (fowl) 酉 (雞)
                                             11. xu (dog) 戌 (犬)
                                             12. hai (pig) 亥 (豬)




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                                       Year Names
     1. jia-zi 甲子        16. ji-mao 己卯       31. jia-wu 甲午        46. ji-you 己酉

     2. yi-chou 乙丑       17. geng-chen 庚辰 32. yi-wei 乙未           47. geng-xu 庚戌
     3. bing-yin 丙寅      18. xin-si 辛巳       33. bing-shen 丙申 48. xin-hai 辛亥
     4. ding-mao 丁卯      19. ren-wu 壬午       34. ding-you 丁酉 49. ren-zi 壬子

     5. wu-chen 戊辰       20. gui-wei 癸未      35. wu-xu 戊戌         50. gui-chou 癸丑

     6. ji-si 己巳         21. jia-shen 甲申     36. ji-hai 己亥        51. jia-yin 甲寅

     7. geng-wu 庚午       22. yi-you 乙酉       37. geng-zi 庚子       52. yi-mao 乙卯
     8. xin-wei 辛未       23. bing-xu 丙戌      38. xin-chou 辛丑 53. bing-chen 丙辰

     9. ren-shen 壬申      24. ding-hai 丁亥     39. ren-yin 壬寅       54. ding-si 丁巳
     10. gui-you 癸酉      25. wu-zi 戊子        40. gui-mao 癸卯       55. wu-wu 戊午

     11. jia-xu 甲戌       26. ji-chou 己丑      41. jia-chen 甲辰      56. ji-wei 己未

     12. yi-hai 乙亥       27. geng-yin 庚寅     42. yi-si 乙巳         57. geng-shen 庚申

     13. bing-zi 丙子      28. xin-mao 辛卯      43. bing-wu 丙午       58. xin-you 辛酉

     14. ding-chou 丁丑 29. ren-chen 壬辰        44. ding-wei 丁未 59. ren-xu 壬戌
     15. wu-yin 戊寅       30. gui-si 癸巳       45. wu-shen 戊申 60. gui-hai 癸亥


6.1 Rules

There is no specific initial epoch for counting years. In historical records, dates were
specified by counts of days and years in sexagenary cycles and by counts of years
from a succession of eras established by reigning monarchs.


The sixty-year cycle consists of a set of year names that are created by pairing a
name from a list of ten Celestial Stems with a name from a list of twelve Terrestrial
Branches, following the order specified in Table 6.1.1. The Celestial Stems are
specified by Chinese characters that have no English translation; the Terrestrial
Branches are named after twelve animals. After six repetitions of the set of stems
and five repetitions of the branches, a complete cycle of pairs is completed and a
new cycle begins. The initial year (jia-zi 甲子) of the current cycle began on 1984
February 2.


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Days are measured from midnight to midnight. The first day of a calendar month is
the day on which the astronomical New Moon (i.e., conjunction) is calculated to occur.
Since the average interval between successive New Moons is approximately 29.53
days, months are 29 or 30 days long. Months are specified by number from 1 to 12.
When an intercalary month is added, it bears the number of the previous month, but
is designated as intercalary. An ordinary year of twelve months is 353, 354, or 355
days in length; a leap year of thirteen months is 383, 384, or 385 days long.


The conditions for adding an intercalary month are determined by the occurrence of
the New Moon with respect to divisions of the tropical year. The tropical year is
divided into 24 solar terms, in 15o segments of solar longitude. These divisions are
paired into twelve Sectional Terms (Jieqi 節氣) and twelve Principal Terms (Zhongqi),
as shown in Table 6.1.2. These terms are numbered and assigned names that are
seasonal or meteorological in nature. For convenience here, the Sectional and
Principal Terms are denoted by S and P, respectively, followed by the number.
Because of the ellipticity of the Earth's orbit, the interval between solar terms varies
with the seasons.


Reference works give a variety of rules for establishing New Year's Day and for
intercalation in the lunisolar calendar. Since the calendar was originally based on the
assumption that the Sun's motion was uniform through the seasons, the published
rules are frequently inadequate to handle special cases.


The following rules (Liu and Stephenson, in press) are currently used as the basis for
calendars prepared by the Purple Mountain Observatory (1984):
(1) The first day of the month is the day on which the New Moon occurs.
(2) An ordinary year has twelve lunar months; an intercalary year has thirteen lunar
months.
(3) The Winter Solstice (term P-11) always falls in month 11.
(4) In an intercalary year, a month in which there is no Principal Term is the
intercalary month. It is assigned the number of the preceding month, with the further
designation of intercalary. If two months of an intercalary year contain no Principal


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Term, only the first such month after the Winter Solstice is considered intercalary.
(5) Calculations are based on the meridian 120° East.
The number of the month usually corresponds to the number of the Principal Term
occurring during the month. In rare instances, however, there are months that have
two Principal Terms, with the result that a nonintercalary month will have no Principal
Term. As a result the numbers of the months will temporarily fail to correspond to the
numbers of the Principal Terms. These cases can be resolved by strictly applying
rules 2 and 3.
                                         Table 6.1.2
                                  Chinese Solar Terms
                                               Sun's              Approx. Greg.
Term*                    Name                                                     Duration
                                               Longitude          Date
                         Beginning of
S-1     Lichun 立春                              315                Feb. 4
                         Spring
P-1     Yushui 雨水        Rain Water            330                Feb. 19         29.8
S-2     Jingzhe 驚蟄       Waking of Insects 345                    Mar. 6
P-2     Chunfen 春分       Spring Equinox        0                  Mar. 21         30.2

S-3     Qingming 清明 Pure Brightness            15                 Apr. 5
P-3     Guyu 榖雨          Grain Rain            30                 Apr. 20         30.7
                         Beginning of
S-4     Lixia 立夏                               45                 May 6
                         Summer
P-4     Xiaoman 小滿       Grain Full            60                 May 21          31.2

        Mangzhong 芒
S-5                      Grain in Ear          75                 June 6
        種
P-5     Xiazhi 夏至        Summer Solstice       90                 June 22         31.4
S-6     Xiaoshu 小署       Slight Heat           105                July 7

P-6     Dashu 大署         Great Heat            120                July 23         31.4
                         Beginning of
S-7     Liqiu 立秋                               135                Aug. 8
                         Autumn
P-7     Chushu 處署        Limit of Heat         150                Aug. 23         31.1



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S-8    Bailu 白露          White Dew            165                 Sept. 8
P-8    Qiufen 秋分         Autumnal Equinox 180                     Sept. 23       30.7
S-9    Hanlu 寒露          Cold Dew             195                 Oct. 8
       Shuangjiang 霜
P-9                      Descent of Frost     210                 Oct. 24        30.1
       降
                         Beginning of
S-10 Lidong 立冬                                225                 Nov. 8
                         Winter
P-10 Xiaoxue 小雪          Slight Snow          240                 Nov. 22        29.7
S-11 Daxue 大雪            Great Snow           255                 Dec. 7

P-11 Dongzhi 冬至          Winter Solstice      270                 Dec. 22        29.5
S-12 Xiaohan 小寒          Slight Cold          285                 Jan. 6
P-12 Dahan 大寒            Great Cold           300                 Jan. 20        29.5
* Terms are classified as Sectional (Jieqi 節氣) or Principal (Zhongqi), followed by the
                                  number of the term.


In general, the first step in calculating the Chinese calendar is to check for the
existence of an intercalary year. This can be done by determining the dates of Winter
Solstice and month 11 before and after the period of interest, and then by counting
the intervening New Moons.


Published calendrical tables are often in disagreement about the Chinese calendar.
Some of the tables are based on mean, or at least simplified, motions of the Sun and
Moon. Some are calculated for other meridians than 120° East. Some incorporate a
rule that the eleventh, twelfth, and first months are never followed by an intercalary
month. This is sometimes not stated as a rule, but as a consequence of the rapid
change in the Sun's longitude when the Earth is near perihelion. However, this
statement is incorrect when the motions of the Sun and Moon are accurately
calculated.


6.2 History of the Chinese Calendar




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In China the calendar was a sacred document, spopnsored and promulgated by the
reigning monarch. For more than two millennia, a Bureau of Astronomy made
astronomical observations, calculated astronomical events such as eclipses,
prepared astrological predictions, and maintained the calendar (Needham, 1959).
After all, a successful calendar not only served practical needs, but also confirmed
the consonance between Heaven and the imperial court.


Analysis of surviving astronomical records inscribed on oracle bones reveals a
Chinese lunisolar calendar, with intercalation of lunar months, dating back to the
Shang dynasty of the fourteenth century B.C. Various intercalation schemes were
developed for the early calendars, including the nineteen-year and 76-year lunar
phase cycles that came to be known in the West as the Metonic cycle and Callipic
cycle.


From the earliest records, the beginning of the year occurred at a New Moon near
the winter solstice. The choice of month for beginning the civil year varied with time
and place, however. In the late second century B.C., a calendar reform established
the practice, which continues today, of requiring the winter solstice to occur in month
11. This reform also introduced the intercalation system in which dates of New
Moons are compared with the 24 solar terms. However, calculations were based on
the mean motions resulting from the cyclic relationships. Inequalities in the Moon's
motions were incorporated as early as the seventh century A.D. (Sivin, 1969), but the
Sun's mean longitude was used for calculating the solar terms until 1644 (Liu and
Stephenson, in press).


Years were counted from a succession of eras established by reigning emperors.
Although the accession of an emperor would mark a new era, an emperor might also
declare a new era at various times within his reign. The introduction of a new era was
an attempt to reestablish a broken connection between Heaven and Earth, as
personified by the emperor. The break might be revealed by the death of an emperor,
the occurrence of a natural disaster, or the failure of astronomers to predict a
celestial event such as an eclipse. In the latter case, a new era might mark the
introduction of new astronomical or calendrical models.


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Sexagenary cycles were used to count years, months, days, and fractions of a day
using the set of Celestial Stems and Terrestrial Branches described in Section 6.1.
Use of the sixty-day cycle is seen in the earliest astronomical records. By contrast
the sixty-year cycle was introduced in the first century A.D. or possibly a century
earlier (Tung, 1960; Needham, 1959). Although the day count has fallen into disuse
in everyday life, it is still tabulated in calendars. The initial year (jia-zi, 甲子) of the
current year cycle began on 1984 February 2, which is the third day (bing-yin, 丙寅)
of the day cycle.
Western (pre-Copernican) astronomical theories were introduced to China by Jesuit
missionaries in the seventeenth century. Gradually, more modern Western concepts
became known. Following the revolution of 1911, the traditional practice of counting
years from the accession of an emperor was abolished.




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7. Julian Day Numbers and Julian Date

[omitted]




8. The Julian Calendar

The Julian calendar, introduced by Juliius Caesar in -45, was a solar calendar with
months of fixed lengths. Every fourth year an intercalary day was added to maintain
synchrony between the calendar year and the tropical year. It served as a standard
for European civilization until the Gregorian Reform of +1582.


Today the principles of the Julian calendar continue to be used by chronologists. The
Julian proleptic calendar is formed by applying the rules of the Julian calendar to
times before Caesar's reform. This provides a simple chronological system for
correlating other calendars and serves as the basis for the Julian day numbers.


8.1 Rules

Years are classified as normal years of 365 days and leap years of 366 days. Leap
years occur in years that are evenly divisible by 4. For this purpose, year 0 (or 1 B.C.)
is considered evenly divisible by 4. The year is divided into twelve formalized months
that were eventually adopted for the Gregorian calendar.


8.2 History of the Julian Calendar

The year -45 has been called the "year of confusion," because in that year Julius
Caesar inserted 90 days to bring the months of the Roman calendar back to their
traditional place with respect to the seasons. This was Caesar's first step in replacing
a calendar that had gone badly awry. Although the pre-Julian calendar was lunisolar
in inspiration, its months no longer followed the lunar phases and its year had lost
step with the cycle of seasons (see Michels, 1967; Bickerman, 1974). Following the



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advice of Sosigenes, an Alexandrine astronomer, Caesar created a solar calendar
with twelve months of fixed lengths and a provision for an intercalary day to be added
every fourth year. As a result, the average length of the Julian calendar year was
365.25 days. This is consistent with the length of the tropical year as it was known at
the time.


Following Caesar's death, the Roman calendrical authorities misapplied the leap-year
rule, with the result that every third, rather than every fourth, year was intercalary.
Although detailed evidence is lacking, it is generally believed that Emperor Augustus
corrected the situation by omitting intercalation from the Julian years -8 through +4.
After this the Julian calendar finally began to function as planned.


Through the Middle Ages the use of the Julian calendar evolved and acquired local
peculiarities that continue to snare the unwary historian. There were variations in the
initial epoch for counting years, the date for beginning the year, and the method of
specifying the day of the month. Not only did these vary with time and place, but also
with purpose. Different conventions were sometimes used for dating ecclesiastical
records, fiscal transactions, and personal correspondence.


Caesar designated January 1 as the beginning of the year. However, other
conventions flourished at different times and places. The most popular alternatives
were March 1, March 25, and December 25. This continues to cause problems for
historians, since, for example, +998 February 28 as recorded in a city that began its
year on March 1, would be the same day as +999 February 28 of a city that began
the year on January 1.
Days within the month were originally counted from designated division points within
the month: Kalends, Nones, and Ides. The Kalends is the first day of the month. The
Ides is the thirteenth of the month, except in March, May, July, and October, when it
is the fifteenth day. The Nones is always eight days before the Ides (see Table 8.2.1).
Dates falling between these division points are designated by counting inclusively
backward from the upcoming division point. Intercalation was performed by repeating
the day VI Kalends March, i.e., inserting a day between VI Kalends March (February
24) and VII Kalends March (February 23).


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By the eleventh century, consecutive counting of days from the beginning of the
month came into use. Local variations continued, however, including counts of days
from dates that commemorated local saints. The inauguration and spread of the
Gregorian calendar resulted in the adoption of a uniform standard for recording dates.


Cappelli (1930), Grotefend and Grotefend (1941), and Cheney (1945) offer guidance
through the maze of medieval dating.




9. Calendar Conversion Algorithms

[omitted]




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10. References

[to be added later]



This information is reprinted from the Explanatory
Supplement to the Astronomical Almanac, P. Kenneth
Seidelmann, editor, with permission from University
Science Books, Sausalito, CA 94965.
Page author: Lyle Huber <lhuber@nmsu.edu>



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