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2. The Christian calendar

The ``Christian calendar'' is the term I use to designate the calendar commonly in use, although its connection with Christianity is highly debatable.

The Christian calendar has years of 365 or 366 days. It is divided into 12 months that have no relationship to the motion of the moon. In parallel with this system, the concept of weeks groups the days in sets of 7.

Two main versions of the Christian calendar have existed in recent times: The Julian calendar and the Gregorian calendar. The difference between them lies in the way they approximate the length of the tropical year and their rules for calculating Easter.

 

2.1 What is the Julian calendar?

The Julian calendar was introduced by Julius Caesar in 45 BC. It was in common use until the 1500s, when countries started changing to the Gregorian calendar (section 2.2). However, some countries (for example, Greece and Russia) used it into this century, and the Orthodox church in Russia still uses it, as do some other Orthodox churches.

In the Julian calendar, the tropical year is approximated as 365 1/4 days = 365.25 days. This gives an error of 1 day in approximately 128 years.

The approximation 365 1/4 is achieved by having 1 leap year every 4 years.

 

2.1.1 What years are leap years?

The Julian calendar has 1 leap year every 4 years:

Every year divisible by 4 is a leap year.
However, this rule was not followed in the first years after the introduction of the Julian calendar in 45 BC. Due to a counting error, every 3rd year was a leap year in the first years of this calendar's existence. The leap years were:
45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, AD 8, AD 12, and every 4th year from then on.

Authories disagree about whether 45 BC was a leap year or not.

There were no leap years between 9 BC and AD 8 (or, according to some authorities, between 12 BC and AD 4). This period without leap years was decreed by emperor Augustus in order to make up for the surplus of leap years introduced previously, and it earned him a place in the calendar as the 8th month was named after him.

It is a curious fact that although the method of reckoning years after the (official) birthyear of Christ was not introduced until the 6th century, by some stroke of luck the Julian leap years coincide with years of our Lord that are divisible by 4.

 

2.1.2 What consequences did the use of the Julian calendar have?

The Julian calendar introduces an error of 1 day every 128 years. So every 128 years the tropical year shifts one day backwards with respect to the calendar. Furthermore, the method for calculating the dates for Easter was inaccurate and needed to be refined.

In order to remedy this, two steps were necessary: 1) The Julian calendar had to be replaced by something more adequate. 2) The extra days that the Julian calendar had inserted had to be dropped.

The solution to problem 1) was the Gregorian calendar described in section 2.2.

The solution to problem 2) depended on the fact that it was felt that 21 March was the proper day for vernal equinox (because 21 March was the date for vernal equinox during the Council of Nicaea in AD 325). The Gregorian calendar was therefore calibrated to make that day vernal equinox.

By 1582 vernal equinox had moved (1582-325)/128 days = approximately 10 days backwards. So 10 days had to be dropped.

 

   
2.2 What is the Gregorian calendar?

The Gregorian calendar is the one commonly used today. It was proposed by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII in accordance with instructions from the Council of Trent (1545-1563) to correct for errors in the older Julian Calendar. It was decreed by Pope Gregory XIII in a papal bull in February 1582.

In the Gregorian calendar, the tropical year is approximated as 365 97/400 days = 365.2425 days. Thus it takes approximately 3300 years for the tropical year to shift one day with respect to the Gregorian calendar.

The approximation 365 97/400 is achieved by having 97 leap years every 400 years.

 

2.2.1 What years are leap years?

The Gregorian calendar has 97 leap years every 400 years:

Every year divisible by 4 is a leap year.
However, every year divisible by 100 is not a leap year.
However, every year divisible by 400 is a leap year after all.
So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600, 2000, and 2400 are leap years.

(Destruction of a myth: There are no double leap years, i.e. no years with 367 days. See, however, the note on Sweden in section 2.2.4.)

 

   
2.2.2 Isn't there a 4000-year rule?

It has been suggested (by the astronomer John Herschel (1792-1871) among others) that a better approximation to the length of the tropical year would be 365 969/4000 days = 365.24225 days. This would dictate 969 leap years every 4000 years, rather than the 970 leap years mandated by the Gregorian calendar. This could be achieved by dropping one leap year from the Gregorian calendar every 4000 years, which would make years divisible by 4000 non-leap years.

This rule has, however, not been officially adopted.

 

2.2.3 Don't the Greek do it differently?

When the Orthodox church in Greece finally decided to switch to the Gregorian calendar in the 1920s, they tried to improve on the Gregorian leap year rules, replacing the ``divisible by 400'' rule by the following:

 

Every year which when divided by 900 leaves a remainder of 200 or 600 is a leap year.

This makes 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2800 non-leap years, whereas 2000, 2400, and 2900 are leap years. This will not create a conflict with the rest of the world until the year 2800.

This rule gives 218 leap years every 900 years, which gives us an average year of 365 218/900 days = 365.24222 days, which is certainly more accurate than the official Gregorian number of 365.2425 days.

However, this rule is not official in Greece.

[I have received an e-mail claiming that this system is official in Russia today. Information is very welcome.]

 

   
2.2.4 When did country X change from the Julian to the Gregorian calendar?

The papal bull of February 1582 decreed that 10 days should be dropped from October 1582 so that 15 October should follow immediately after 4 October, and from then on the reformed calendar should be used.

This was observed in Italy, Poland, Portugal, and Spain. Other Catholic countries followed shortly after, but Protestant countries were reluctant to change, and the Greek orthodox countries didn't change until the start of this century.

 

 


Changes in the 1500s required 10 days to be dropped.
Changes in the 1600s required 10 days to be dropped.
Changes in the 1700s required 11 days to be dropped.
Changes in the 1800s required 12 days to be dropped.
Changes in the 1900s required 13 days to be dropped.

 

 


(Exercise for the reader: Why is the error in the 1600s the same as in the 1500s.)

The following list contains the dates for changes in a number of countries. It is very strange that in many cases there seems to be some doubt among authorities about what the correct days are. Different sources give very different dates in some cases. Look at Bulgaria, for example. The list below does not include all the different opinions about when the change took place.

 

Alaska:
October 1867 when Alaska became part of the USA

 

Albania:
December 1912

 

Austria:
Different regions on different dates
Brixen, Salzburg and Tyrol:
         5 Oct 1583 was followed by 16 Oct 1583
Carinthia and Styria:
         14 Dec 1583 was followed by 25 Dec 1583
See also Czechoslovakia and Hungary

 

Belgium:
Then part of the Netherlands

 

Bulgaria:
Different authorities say
Sometime in 1915
18 Mar 1916 was followed by 1 Apr 1916
31 Mar 1916 was followed by 14 Apr 1916
3 Sep 1920 was followed by 17 Sep 1920

 

Canada:
Different regions followed the changes in Great Britain or France. [The details are hard to discover - can anybody help?]

 

China:
Different authorities say
18 Dec 1911 was followed by 1 Jan 1912
18 Dec 1928 was followed by 1 Jan 1929

 

Czechoslovakia (i.e. Bohemia and Moravia):

6 Jan 1584 was followed by 17 Jan 1584

 

Denmark (including Norway):

18 Feb 1700 was followed by 1 Mar 1700

 

Egypt:
1875

 

Estonia:
1918

 

Finland:
Then part of Sweden. (Note, however, that Finland later became part of Russia, which then still used the Julian calendar. The Gregorian calendar remained official in Finland, but some use of the Julian calendar was made.)

 

France:
9 Dec 1582 was followed by 20 Dec 1582
Alsace: 4 Feb 1682 was followed by 16 Feb 1682
Lorraine: 16 Feb 1760 was followed by 28 Feb 1760
Strasbourg: February 1682

 

Germany:
Different states on different dates:
Catholic states on various dates in 1583-1585
Prussia: 22 Aug 1610 was followed by 2 Sep 1610
Protestant states: 18 Feb 1700 was followed by 1 Mar 1700
(Many local variations)

 

Great Britain and Dominions (including what is now the USA):

2 Sep 1752 was followed by 14 Sep 1752

 

Greece:
9 Mar 1924 was followed by 23 Mar 1924
(Some sources say 1916 and 1920)

 

Hungary:
21 Oct 1587 was followed by 1 Nov 1587

 

Ireland:
See Great Britain

 

Italy:
4 Oct 1582 was followed by 15 Oct 1582

 

Japan:
Different authorities say:
19 Dec 1872 was followed by 1 Jan 1873
19 Dec 1892 was followed by 1 Jan 1893
18 Dec 1918 was followed by 1 Jan 1919

 

Latvia:
During German occupation 1915 to 1918

 

Lithuania:
1915

 

Luxemburg:
14 Dec 1582 was followed by 25 Dec 1582

 

Netherlands (including Belgium):

Holland, Zeeland, Brabant, Limburg and the southern provinces (currently Belgium):
         21 Dec 1582 was followed by 1 Jan 1583
Groningen:
         28 Feb 1583 was followed by 11 Mar 1583
         Gone back to Julian in the summer of 1584
         13 Dec 1700 was followed by 12 Jan 1701
Gelderland:
         30 Jun 1700 was followed by 12 Jul 1700
Utrecht and Overijssel:
         30 Nov 1700 was followed by 12 Dec 1700
Friesland and Drenthe:
         31 Dec 1700 was followed by 12 Jan 1701

 

Norway:
Then part of Denmark.

 

Poland:
4 Oct 1582 was followed by 15 Oct 1582

 

Portugal:
4 Oct 1582 was followed by 15 Oct 1582

 

Romania:
31 Mar 1919 was followed by 14 Apr 1919
(The Greek Orthodox parts of the country may have changed later)

 

Russia:
31 Jan 1918 was followed by 14 Feb 1918
(In the eastern parts of the country the change may not have occured until 1920)

 

Scotland:
See Great Britain

 

Spain:
4 Oct 1582 was followed by 15 Oct 1582

 

Sweden (including Finland):

17 Feb 1753 was followed by 1 Mar 1753 (see note below)

 

Switzerland:

Catholic cantons: 1583, 1584 or 1597
Protestant cantons:
         31 Dec 1700 was followed by 12 Jan 1701
(Many local variations)

 

Turkey:
Gregorian calendar introduced 1 Jan 1927

 

USA:
See Great Britain, of which it was then a colony. Also note the entry for Alaska.

 

Wales:
See Great Britain

 

Yugoslavia:
1919

Sweden has a curious history. Sweden decided to make a gradual change from the Julian to the Gregorian calendar. By dropping every leap year from 1700 through 1740 the eleven superfluous days would be omitted and from 1 Mar 1740 they would be in sync with the Gregorian calendar. (But in the meantime they would be in sync with nobody!)

So 1700 (which should have been a leap year in the Julian calendar) was not a leap year in Sweden. However, by mistake 1704 and 1708 became leap years. This left Sweden out of synchronisation with both the Julian and the Gregorian world, so they decided to go back to the Julian calendar. In order to do this, they inserted an extra day in 1712, making that year a double leap year! So in 1712, February had 30 days in Sweden.

Later, in 1753, Sweden changed to the Gregorian calendar by dropping 11 days like everyone else.

 

   
2.3 What day is the leap day?

It is 24 February!

Weird? Yes! The explanation is related to the Roman calendar and is found in section 2.6.1.

From a numerical point of view, of course 29 February is the extra day. But from the point of view of celebration of feast days, the following correspondence between days in leap years and non-leap years has traditionally been used:

 


Non-leap year Leap year
22 February 22 February
23 February 23 February
  24 February (extra day)
24 February 25 February
25 February 26 February
26 February 27 February
27 February 28 February
28 February 29 February


For example, the feast of St. Leander has been celebrated on 27 February in non-leap years and on 28 February in leap years.

The EU (European Union) in their infinite wisdom have decided that starting in the year 2000, 29 February is to be the leap day. This will affect countries such as Sweden and Austria that celebrate ``name days'' (i.e. each day is associated with a name).

It appears that the Roman Catholic Church already uses 29 February as the leap day.

 

   
2.4 What is the Solar Cycle?

In the Julian calendar the relationship between the days of the week and the dates of the year is repeated in cycles of 28 years. In the Gregorian calendar this is still true for periods that do not cross years that are divisible by 100 but not by 400.

A period of 28 years is called a Solar Cycle. The Solar Number of a year is found as:

 

\begin{displaymath}Solar Number = (year + 8) \bmod 28 + 1
\end{displaymath}

 

In the Julian calendar there is a one-to-one relationship between the Solar Number and the day on which a particular date falls.

(The leap year cycle of the Gregorian calendar is 400 years, which is 146,097 days, which curiously enough is a multiple of 7. So in the Gregorian calendar the equivalent of the ``Solar Cycle'' would be 400 years, not 7400=2800 years as one might be tempted to believe.)

 

2.5 What day of the week was 2 August 1953?

To calculate the day on which a particular date falls, the following algorithm may be used (the divisions are integer divisions, in which remainders are discarded):

 


\begin{tabular}{\vert ll\vert}
\hline
& \rule{0cm}{0.5cm}$a = \frac{14 - month}{...
...s 0 for a Sunday, 1 for a Monday, 2 for a Tuesday, etc.}\\
\hline
\end{tabular}

 

 


Example: On what day of the week was the author born?

My birthday is 2 August 1953 (Gregorian, of course).


 

\begin{displaymath}\begin{array}{rl}
a & = \frac{14 - 8}{12} = 0 \\
y & = 1953 ...
...+ 4 + 15 ) \bmod 7 \\
& = 2443 \bmod 7 \\
& = 0
\end{array}\end{displaymath}

 

I was born on a Sunday.

 

2.6 What is the Roman calendar?

Before Julius Caesar introduced the Julian calendar in 45 BC, the Roman calendar was a mess, and much of our so-called ``knowledge'' about it seems to be little more than guesswork.

Originally, the year started on 1 March and consisted of only 304 days or 10 months (Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December). These 304 days were followed by an unnamed and unnumbered winter period. The Roman king Numa Pompilius (c. 715-673 BC, although his historicity is disputed) allegedly introduced February and January (in that order) between December and March, increasing the length of the year to 354 or 355 days. In 450 BC, February was moved to its current position between January and March.

In order to make up for the lack of days in a year, an extra month, Intercalaris or Mercedonius, (allegedly with 22 or 23 days though some authorities dispute this) was introduced in some years. In an 8 year period the length of the years were:

1: 12 months or 355 days
2: 13 months or 377 days
3: 12 months or 355 days
4: 13 months or 378 days
5: 12 months or 355 days
6: 13 months or 377 days
7: 12 months or 355 days
8: 13 months or 378 days
A total of 2930 days corresponding to a year of 366 1/4 days. This year was discovered to be too long, and therefore 7 days were later dropped from the 8th year, yielding 365.375 days per year.

This is all theory. In practice it was the duty of the priesthood to keep track of the calendars, but they failed miserably, partly due to ignorance, partly because they were bribed to make certain years long and other years short. Furthermore, leap years were considered unlucky and were therefore avoided in time of crisis, such as the Second Punic War.

In order to clean up this mess, Julius Caesar made his famous calendar reform in 45 BC. We can make an educated guess about the length of the months in the years 47 and 46 BC:

 


  47 BC 46 BC
January 29 29
February 28 24
Intercalaris   27
March 31 31
April 29 29
May 31 31
June 29 29
Quintilis 31 31
Sextilis 29 29
September 29 29
October 31 31
November 29 29
Undecember   33
Duodecember   34
December 29 29
Total 355 445


The length of the months from 45 BC onward were the same as the ones we know today.

Occasionally one reads the following story:

``Julius Caesar made all odd numbered months 31 days long, and all even numbered months 30 days long (with February having 29 days in non-leap years). In 44 BC Quintilis was renamed `Julius' (July) in honour of Julius Caesar, and in 8 BC Sextilis became `Augustus' in honour of emperor Augustus. When Augustus had a month named after him, he wanted his month to be a full 31 days long, so he removed a day from February and shifted the length of the other months so that August would have 31 days.''

This story, however, has no basis in actual fact. It is a fabrication possibly dating back to the 14th century.

 

   
2.6.1 How did the Romans number days?

The Romans didn't number the days sequentially from 1. Instead they had three fixed points in each month:

 

``Kalendae''
(or ``Calendae''), which was the first day of the month.
``Idus'',
which was the 13th day of January, February, April, June, August, September, November, and December, or the 15th day of March, May, July, or October.
``Nonae'',
which was the 9th day before Idus (counting Idus itself as the 1st day).

The days between Kalendae and Nonae were called ``the 4th day before Nonae'', ``the 3rd day before Nonae'', and ``the 2nd day before Nonae''. (The 1st day before Nonae would be Nonae itself.)

Similarly, the days between Nonae and Idus were called ``the Xth day before Idus'', and the days after Idus were called ``the Xth day before Kalendae (of the next month)''.

Julius Caesar decreed that in leap years the ``6th day before Kalendae of March'' should be doubled. So in contrast to our present system, in which we introduce an extra date (29 February), the Romans had the same date twice in leap years. The doubling of the 6th day before Kalendae of March is the origin of the word bissextile. If we create a list of equivalences between the Roman days and our current days of February in a leap year, we get the following:

 


7th day before Kalendae of March 23 February
6th day before Kalendae of March 24 February
6th day before Kalendae of March 25 February
5th day before Kalendae of March 26 February
4th day before Kalendae of March 27 February
3rd day before Kalendae of March 28 February
2nd day before Kalendae of March 29 February
Kalendae of March 1 March


You can see that the extra 6th day (going backwards) falls on what is today 24 February. For this reason 24 February is still today considered the ``extra day'' in leap years (see section 2.3). However, at certain times in history the second 6th day (25 Feb) has been considered the leap day.

Why did Caesar choose to double the 6th day before Kalendae of March? It appears that the leap month Intercalaris/Mercedonius of the pre-reform calendar was not placed after February, but inside it, namely between the 7th and 6th day before Kalendae of March. It was therefore natural to have the leap day in the same position.

 

2.7 Has the year always started on 1 January?

For the man in the street, yes. When Julius Caesar introduced his calendar in 45 BC, he made 1 January the start of the year, and it was always the date on which the Solar Number and the Golden Number (see section 2.9.3) were incremented.

However, the church didn't like the wild parties that took place at the start of the new year, and in AD 567 the council of Tours declared that having the year start on 1 January was an ancient mistake that should be abolished.

Through the middle ages various New Year dates were used. If an ancient document refers to year X, it may mean any of 7 different periods in our present system:

 

$\bullet$
1 Mar X to 28/29 Feb X+1
$\bullet$
1 Jan X to 31 Dec X
$\bullet$
1 Jan X-1 to 31 Dec X-1
$\bullet$
25 Mar X-1 to 24 Mar X
$\bullet$
25 Mar X to 24 Mar X+1
$\bullet$
Saturday before Easter X to Friday before Easter X+1
$\bullet$
25 Dec X-1 to 24 Dec X

Choosing the right interpretation of a year number is difficult, so much more as one country might use different systems for religious and civil needs.

The Byzantine Empire used a year starting on 1 Sep, but they didn't count years since the birth of Christ, instead they counted years since the creation of the world which they dated to 1 September 5509 BC.

Since about 1600 most countries have used 1 January as the first day of the year. Italy and England, however, did not make 1 January official until around 1750.

In England (but not Scotland) three different years were used:

 

$\bullet$
The historical year, which started on 1 January.
$\bullet$
The liturgical year, which started on the first Sunday in advent.
$\bullet$
The civil year, which
         from the 7th to the 12th century started on 25 December,
         from the 12th century until 1751 started on 25 March,
         from 1752 started on 1 January.

 

2.8 What is the origin of the names of the months?

A lot of languages, including English, use month names based on Latin. Their meaning is listed below. However, some languages (Czech and Polish, for example) use quite different names.

 

January
Latin: Januarius. Named after the god Janus.
February
Latin: Februarius. Named after Februa, the purification festival.
March
Latin: Martius. Named after the god Mars.
April
Latin: Aprilis. Named either after the goddess Aphrodite or the Latin word aperire, to open.
May
Latin: Maius. Probably named after the goddess Maia.
June
Latin: Junius. Probably named after the goddess Juno.
July
Latin: Julius. Named after Julius Caesar in 44 BC. Prior to that time its name was Quintilis from the word quintus, fifth, because it was the 5th month in the old Roman calendar.
August
Latin: Augustus. Named after emperor Augustus in 8 BC. Prior to that time the name was Sextilis from the word sextus, sixth, because it was the 6th month in the old Roman calendar.
September
Latin: September. From the word septem, seven, because it was the 7th month in the old Roman calendar.
October
Latin: October. From the word octo, eight, because it was the 8th month in the old Roman calendar.
November
Latin: November. From the word novem, nine, because it was the 9th month in the old Roman calendar.
December
Latin: December. From the word decem, ten, because it was the 10th month in the old Roman calendar.

 

2.9 What is Easter?

In the Christian world, Easter (and the days immediately preceding it) is the celebration of the death and resurrection of Jesus in (approximately) AD 30.

 

2.9.1 When is Easter? (Short answer)

Easter Sunday is the first Sunday after the first full moon after vernal equinox.

 

2.9.2 When is Easter? (Long answer)

The calculation of Easter is complicated because it is linked to (an inaccurate version of) the Hebrew calendar.

Jesus was crucified immediately before the Jewish Passover, which is a celebration of the Exodus from Egypt under Moses. Celebration of Passover started on the 14th or 15th day of the (spring) month of Nisan. Jewish months start when the moon is new, therefore the 14th or 15th day of the month must be immediately after a full moon.

It was therefore decided to make Easter Sunday the first Sunday after the first full moon after vernal equinox. Or more precisely: Easter Sunday is the first Sunday after the ``official'' full moon on or after the ``official'' vernal equinox.

The official vernal equinox is always 21 March.

The official full moon may differ from the real full moon by one or two days.

(Note, however, that historically, some countries have used the real (astronomical) full moon instead of the official one when calculating Easter. This was the case, for example, of the German Protestant states, which used the astronomical full moon in the years 1700-1776. A similar practice was used Sweden in the years 1740-1844 and in Denmark in the 1700s.)

The full moon that precedes Easter is called the Paschal full moon. Two concepts play an important role when calculating the Paschal full moon: The Golden Number and the Epact. They are described in the following sections.

The following sections give details about how to calculate the date for Easter. Note, however, that while the Julian calendar was in use, it was customary to use tables rather than calculations to determine Easter. The following sections do mention how to calcuate Easter under the Julian calendar, but the reader should be aware that this is an attempt to express in formulas what was originally expressed in tables. The formulas can be taken as a good indication of when Easter was celebrated in the Western Church from approximately the 6th century.

 

   
2.9.3 What is the Golden Number?

Each year is associated with a Golden Number.

Considering that the relationship between the moon's phases and the days of the year repeats itself every 19 years (as described in chapter 1), it is natural to associate a number between 1 and 19 with each year. This number is the so-called Golden Number. It is calculated thus:

 

\begin{displaymath}GoldenNumber = (year \bmod 19)+1
\end{displaymath}

 

New moon will fall on (approximately) the same date in two years with the same Golden Number.

 

2.9.4 What is the Epact?

Each year is associated with an Epact.

The Epact is a measure of the age of the moon (i.e. the number of days that have passed since an ``official'' new moon) on a particular date.

In the Julian calendar, 8 + the Epact is the age of the moon at the start of the year. In the Gregorian calendar, the Epact is the age of the moon at the start of the year.

The Epact is linked to the Golden Number in the following manner:

Under the Julian calendar, 19 years were assumed to be exactly an integral number of synodic months, and the following relationship exists between the Golden Number and the Epact:

 

\begin{displaymath}Epact = (11 \times (GoldenNumber-1)) \bmod 30
\end{displaymath}

 

If this formula yields zero, the Epact is by convention frequently designated by the symbol * and its value is said to be 30. Weird? Maybe, but people didn't like the number zero in the old days.

Since there are only 19 possible golden numbers, the Epact can have only 19 different values: 1, 3, 4, 6, 7, 9, 11, 12, 14, 15, 17, 18, 20, 22, 23, 25, 26, 28, and 30.

 

 


The Julian system for calculating full moons was inaccurate, and under the Gregorian calendar, some modifications are made to the simple relationship between the Golden Number and the Epact.

In the Gregorian calendar the Epact should be calculated thus (the divisions are integer divisions, in which remainders are discarded):

1.
Use the Julian formula:

 

\begin{displaymath}Epact = (11 \times (GoldenNumber-1)) \bmod 30
\end{displaymath}

 

2.
Adjust the Epact, taking into account the fact that 3 out of 4 centuries have one leap year less than a Julian century:

 

\begin{displaymath}Epact = Epact - (3\times century)/4
\end{displaymath}

 

(For the purpose of this calculation century=20 is used for the years 1900 through 1999, and similarly for other centuries, although this contradicts the rules in section 2.10.3.)

 

3.
Adjust the Epact, taking into account the fact that 19 years is not exactly an integral number of synodic months:

 

\begin{displaymath}Epact = Epact + (8\times century + 5)/25
\end{displaymath}

 

(This adds one to the Epact 8 times every 2500 years.)

 

4.
Add 8 to the Epact to make it the age of the moon on 1 January:

 

Epact = Epact + 8

 

 

5.
Add or subtract 30 until the Epact lies between 1 and 30.

In the Gregorian calendar, the Epact can have any value from 1 to 30.

 

 


Example: What was the Epact for 1992?


 

\begin{displaymath}\begin{array}{l@{\hspace{5mm}}l}
& GoldenNumber = 1992 \bmod ...
...5 = 17\\
4.& Epact = 17 + 8 = 25\\
5.& Epact = 25
\end{array}\end{displaymath}

 

The Epact for 1992 was 25.

 

   
2.9.5 How does one calculate Easter then?

To find Easter the following algorithm is used:

 

1.
Calculate the Epact as described in the previous section.

 

2.
For the Julian calendar: Add 8 to the Epact. (For the Gregorian calendar, this has already been done in step 4 of the calculation of the Epact). Subtract 30 if the sum exceeds 30.

 

3.
Look up the Epact (as possibly modified in step 2) in this table to find the date for the Paschal full moon:

 

\begin{tabular}{\vert cr@{ }l\vert cr@{ }l\vert cr@{ }l\vert}
\hline
Epact & \m...
...pril\\
10 & 3& April & 20 & 24& March & 30 & 13& April\\
\hline
\end{tabular}
4.
Easter Sunday is the first Sunday following the above full moon date. If the full moon falls on a Sunday, Easter Sunday is the following Sunday.

An Epact of 25 requires special treatment, as it has two dates in the above table. There are two equivalent methods for choosing the correct full moon date:

 

A)
Choose 18 April, unless the current century contains years with an epact of 24, in which case 17 April should be used.

 

B)
If the Golden Number is >11 choose 17 April, otherwise choose 18 April.

The proof that these two statements are equivalent is left as an exercise to the reader. (The frustrated ones may contact me for the proof.)

 

 


Example: When was Easter in 1992?

In the previous section we found that the Golden Number for 1992 was 17 and the Epact was 25. Looking in the table, we find that the Paschal full moon was either 17 or 18 April. By rule B above, we choose 17 April because the Golden Number >11.

17 April 1992 was a Friday. Easter Sunday must therefore have been 19 April.

 

2.9.6 Isn't there a simpler way to calculate Easter?

This is an attempt to boil down the information given in the previous sections (the divisions are integer divisions, in which remainders are discarded):

 


 

\begin{tabular}{\vert rcl\vert}
\hline
\rule{0cm}{0.5cm}$G$\space &$=$ & $year \...
...y$\space &$=$ & $L + 28 - 31\times\frac{EasterMonth}{4}$\\
\hline
\end{tabular}


This algorithm is based in part on the algorithm of Oudin (1940) as quoted in ``Explanatory Supplement to the Astronomical Almanac'', P. Kenneth Seidelmann, editor.

People who want to dig into the workings of this algorithm, may be interested to know that

G
is the Golden Number-1
H
is 23-Epact (modulo 30)
I
is the number of days from 21 March to the Paschal full moon
J
is the weekday for the Paschal full moon (0=Sunday, 1=Monday, etc.)
L
is the number of days from 21 March to the Sunday on or before the Paschal full moon (a number between -6 and 28)

 

2.9.7 Is there a simple relationship between two consecutive Easters?

Suppose you know the Easter date of the current year, can you easily find the Easter date in the next year? No, but you can make a qualified guess.

If Easter Sunday in the current year falls on day X and the next year is not a leap year, Easter Sunday of next year will fall on one of the following days: X-15, X-8, X+13 (rare), or X+20.

If Easter Sunday in the current year falls on day X and the next year is a leap year, Easter Sunday of next year will fall on one of the following days: X-16, X-9, X+12 (extremely rare), or X+19. (The jump X+12 occurs only once in the period 1800-2099, namely when going from 2075 to 2076.)

If you combine this knowledge with the fact that Easter Sunday never falls before 22 March and never falls after 25 April, you can narrow the possibilities down to two or three dates.

 

   
2.9.8 How frequently are the dates for Easter repeated?

The sequence of Easter dates repeats itself every 532 years in the Julian calendar. The number 532 is the product of the following numbers:

19 (the Metonic cycle or the cycle of the Golden Number)
28 (the Solar cycle, see section 2.4)

The sequence of Easter dates repeats itself every 5,700,000 years in the Gregorian calendar. The number 5,700,000 is the product of the following numbers:

19 (the Metonic cycle or the cycle of the Golden Number)
400 (the Gregorian equivalent of the Solar cycle, see section 2.4)
25 (the cycle used in step 3 when calculating the Epact)
30 (the number of different Epact values)

 

2.9.9 What about Greek Easter?

The Greek Orthodox Church does not always celebrate Easter on the same day as the Catholic and Protestant countries. The reason is that the Orthodox Church uses the Julian calendar when calculating Easter. This is case even in the churches that otherwise use the Gregorian calendar.

When the Greek Orthodox Church in 1923 decided to change to the Gregorian calendar (or rather: a Revised Julian Calendar), they chose to use the astronomical full moon as the basis for calculating Easter, rather than the ``official'' full moon described in the previous sections. And they chose the meridian of Jerusalem to serve as definition of when a Sunday starts. However, except for some sporadic use the 1920s, this system was never adopted in practice.

 

2.9.10 What will happen after 2001?

At at meeting in Aleppo, Syria (5-10 March 1997), organised by the World Council of Churches and the Middle East Council of Churches, representatives of several churches and Christian world communions suggested that the discrepancies between Easter calculations in the Western and the Eastern churches could be resolved by adopting astronomically accurate calculations of the vernal equinox and the full moon, instead of using the algorithm presented in section 2.9.5. The meridian of Jerusalem should be used for the astronomical calculations.

The new method for calculating Easter should take effect from the year 2001. In that year the Julian and Gregorian Easter dates coincide (on 15 April Gregorian/2 April Julian), and it is therefore a reasonable starting point for the new system.

Whether this new system will actually be adopted, remains to be seen. So the answer to the question heading this section is: I don't know.

If the new system is introduced, churches using the Gregorian calendar will hardly notice the change. Only once during the period 2001-2025 will these churches note a difference: In 2019 the Gregorian method gives an Easter date of 21 April, but the proposed new method gives 24 March.

Note that the new method makes an Easter date of 21 March possible. This date was not possible under the Julian or Gregorian algorithms. (Under the new method, Easter will fall on 21 March in the year 2877. You're all invited to my house on that date!)

 

   
2.10 How does one count years?

In about AD 523, the papal chancellor, Bonifatius, asked a monk by the name of Dionysius Exiguus to devise a way to implement the rules from the Nicean council (the so-called ``Alexandrine Rules'') for general use.

Dionysius Exiguus (in English known as Denis the Little) was a monk from Scythia, he was a canon in the Roman curia, and his assignment was to prepare calculations of the dates of Easter. At that time it was customary to count years since the reign of emperor Diocletian; but in his calculations Dionysius chose to number the years since the birth of Christ, rather than honour the persecutor Diocletian.

Dionysius (wrongly) fixed Jesus' birth with respect to Diocletian's reign in such a manner that it falls on 25 December 753 AUC (ab urbe condita, i.e. since the founding of Rome), thus making the current era start with AD 1 on 1 January 754 AUC.

How Dionysius established the year of Christ's birth is not known (see section 2.10.1 for a couple of theories). Jesus was born under the reign of king Herod the Great, who died in 750 AUC, which means that Jesus could have been born no later than that year. Dionysius' calculations were disputed at a very early stage.

When people started dating years before 754 AUC using the term ``Before Christ'', they let the year 1 BC immediately precede AD 1 with no intervening year zero.

Note, however, that astronomers frequently use another way of numbering the years BC. Instead of 1 BC they use 0, instead of 2 BC they use -1, instead of 3 BC they use -2, etc.

See also section 2.10.2.

It is frequently claimed that it was the venerable Bede (673-735) who introduced BC dating. This is probably not true.

In this section I have used AD 1 = 754 AUC. This is the most likely equivalence between the two systems. However, some authorities state that AD 1 = 753 AUC or 755 AUC. This confusion is not a modern one, it appears that even the Romans were in some doubt about how to count the years since the founding of Rome.

 

   
2.10.1 How did Dionysius date Christ's birth?

There are quite a few theories about this. And many of the theories are presented as if they were indisputable historical fact.

Here are two theories that I personally consider likely:

 

1.
According to the Gospel of Luke (3:1 & 3:23) Jesus was ``about thirty years old'' shortly after ``the fifteenth year of the reign of Tiberius Caesar''. Tiberius became emperor in AD 14. If you combine these numbers you reach a birthyear for Jesus that is strikingly close to the beginning of our year reckoning. This may have been the basis for Dionysius' calculations.

 

2.
Dionysius' original task was to calculate an Easter table. In the Julian calendar, the dates for Easter repeat every 532 years (see section 2.9.8). The first year in Dionysius' Easter tables is AD 532. Is it a coincidence that the number 532 appears twice here? Or did Dionysius perhaps fix Jesus' birthyear so that his own Easter tables would start exactly at the beginning of the second Easter cycle after Jesus' birth?

 

   
2.10.2 Was Jesus born in the year 0?

No.

There are two reasons for this:

  • There is no year 0.
  • Jesus was born before 4 BC.

The concept of a year ``zero'' is a modern myth (but a very popular one). Roman numerals do not have a figure designating zero, and treating zero as a number on an equal footing with other numbers was not common in the 6th century when our present year reckoning was established by Dionysius Exiguus (see section 2.10). Dionysius let the year AD 1 start one week after what he believed to be Jesus' birthday.

Therefore, AD 1 follows immediately after 1 BC with no intervening year zero. So a person who was born in 10 BC and died in AD 10, would have died at the age of 19, not 20.

Furthermore, Dionysius' calculations were wrong. The Gospel of Matthew tells us that Jesus was born under the reign of king Herod the Great, and he died in 4 BC. It is likely that Jesus was actually born around 7 BC. The date of his birth is unknown; it may or may not be 25 December.

 

   
2.10.3 When does the 21st century start?

The first century started in AD 1. The second century must therefore have started a hundred years later, in AD 101, and the 21st century must start 2000 years after the first century, i.e. in the year 2001.

This is the cause of some heated debate, especially since some dictionaries and encyclopaedias say that a century starts in years that end in 00.

Let me propose a few compromises:

Any 100-year period is a century. Therefore the period from 23 June 1998 to 22 June 2098 is a century. So please feel free to celebrate the start of a century any day you like!

Although the 20th century started in 1901, the 1900s started in 1900. Similarly, we can celebrate the start of the 2000s in 2000 and the start of the 21st century in 2001.

Finally, let's take a lesson from history:

When 1899 became 1900 people celebrated the start of a new century.
When 1900 became 1901 people celebrated the start of a new century.
Two parties! Let's do the same thing again!

 

   
2.10.4 What do AD, BC, CE, and BCE stand for?

Years before the birth of Christ are in English traditionally identified using the abbreviation BC (``Before Christ'').

Years after the birth of Christ are traditionally identified using the abbreviation AD (``Anno Domini'', that is, ``In the Year of the Lord'').

Some people, who dislike the reference to Christianity that is implied in these terms, prefer the abbreviations BCE (``Before the Common Era'') and CE (``Common Era'').

 

   
2.11 What is the Indiction?

The Indiction was used in the middle ages to specify the position of a year in a 15 year taxation cycle. It was introduced by emperor Constantine the Great on 1 September 312 and abolished [whatever that means] in 1806.

The Indiction may be calculated thus:

 

\begin{displaymath}Indiction = (year + 2) \bmod 15 + 1
\end{displaymath}

 

The Indiction has no astronomical significance.

The Indiction did not always follow the calendar year. Three different Indictions may be identified:

 

1.
The Pontifical or Roman Indiction, which started on New Year's Day (being either 25 December, 1 January, or 25 March).
2.
The Greek or Constantinopolitan Indiction, which started on 1 September.
3.
The Imperial Indiction or Indiction of Constantine, which started on 24 September.

 

   
2.12 What is the Julian Period?

The Julian period (and the Julian day number) must not be confused with the Julian calendar.

The French scholar Joseph Justus Scaliger (1540-1609) was interested in assigning a positive number to every year without having to worry about BC/AD. He invented what is today known as the Julian Period.

The Julian Period probably takes its name from the Julian calendar, although it has been claimed that it is named after Scaliger's father, the Italian scholar Julius Caesar Scaliger (1484-1558).

Scaliger's Julian period starts on 1 January 4713 BC (Julian calendar) and lasts for 7980 years. AD 1998 is thus year 6711 in the Julian period. After 7980 years the number starts from 1 again.

Why 4713 BC and why 7980 years? Well, in 4713 BC the Indiction (see section 2.11), the Golden Number (see section 2.9.3) and the Solar Number (see section 2.4) were all 1. The next times this happens is 151928=7980 years later, in AD 3268.

Astronomers have used the Julian period to assign a unique number to every day since 1 January 4713 BC. This is the so-called Julian Day (JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC to noon UTC on 2 January 4713 BC.

This means that at noon UTC on 1 January AD 2000, JD 2,451,545 will start.

This can be calculated thus:

From 4713 BC to AD 2000 there are 6712 years.
In the Julian calendar, years have 365.25 days, so 6712 years correspond to 6712365.25=2,451,558 days. Subtract from this the 13 days that the Gregorian calendar is ahead of the Julian calendar, and you get 2,451,545.

Often fractions of Julian day numbers are used, so that 1 January AD 2000 at 15:00 UTC is referred to as JD 2,451,545.125.

Note that some people use the term ``Julian day number'' to refer to any numbering of days. NASA, for example, use the term to denote the number of days since 1 January of the current year.

 

2.12.1 Is there a formula for calculating the Julian day number?

Try this one (the divisions are integer divisions, in which remainders are discarded):

 


\begin{tabular}{\vert rcl\vert}
\hline
\rule{0cm}{0.5cm} $a$\space &$=$ & $\frac...
... & $day + \frac{153m+2}{5} + 365y + \frac{y}{4} - 32083$\\
\hline
\end{tabular}

JDN is the Julian day number that starts at noon UTC on the specified date.

The algorithm works fine for AD dates. If you want to use it for BC dates, you must first convert the BC year to a negative year (e.g., 10 BC = -9). The algorith works correctly for all dates after 4800 BC, i.e. at least for all positive Julian day numbers.

To convert the other way (i.e., to convert a Julian day number, JDN, to a day, month, and year) these formulas can be used (again, the divisions are integer divisions):

 


\begin{tabular}{\vert rcl\vert}
\hline
\multicolumn{3}{\vert l\vert}{For the Gre...
...m} $year $\space &$=$ & $100b + d - 4800 + \frac{m}{10}$\\
\hline
\end{tabular}

 

2.12.2 What is the modified Julian day number?

Sometimes a modified Julian day number (MJD) is used which is 2,400,000.5 less than the Julian day number. This brings the numbers into a more manageable numeric range and makes the day numbers change at midnight UTC rather than noon.

MJD 0 thus started on 17 Nov 1858 (Gregorian) at 00:00:00 UTC.

 

2.13 What is the correct way to write dates?

The answer to this question depends on what you mean by ``correct''. Different countries have different customs.

Most countries use a day-month-year format, such as:

25.12.1998          25/12/1998          25/12-1998          25.XII.1998

In the U.S.A. a month-day-year format is common:

12/25/1998          12-25-1998

International standard IS-8601 mandates a year-month-day format, namely either

1998-12-25 or 19981225.

In all of these systems, the first two digits of the year are frequently omitted:

25.12.98          12/25/98          98-12-25

This confusion leads to misunderstandings. What is 02-03-04? To most people it is 2 Mar 2004; to an American it is 3 Feb 2004; and to a person using the international standard it would be 4 Mar 2002.

If you want to be sure that people understand you, I recommend that you

  • write the month with letters instead of numbers, and
  • write the years as 4-digit numbers.