February 24, 2005
History
of Our Calendar
Dr.
Frank J. Collazo
Before
today's Gregorian calendar was adopted, the older Julian calendar
was used. It was admirably close to
the actual length of the year, as it turns out, but the Julian calendar was
not so perfect that it didn't slowly shift off track over the following
centuries. But, hundreds of years
later, monks were the only ones with any free time for scholarly pursuits --
and they were discouraged from thinking about the matter of "secular
time" for any reason beyond figuring out when to observe Easter. In the Middle Ages, the study of the
measure of time was first viewed as prying too deeply into God's own affairs
-- and later thought of as a lowly, mechanical study, unworthy of serious
contemplation. As
a result, it wasn't until 1582, by which time Caesar's calendar had drifted a
full 10 days off course that Pope Gregory finally reformed the Julian
calendar. Ironically, by the time the
Catholic church buckled under the weight of the scientific reasoning that
pointed out the error, it had lost much of its power to implement the
fix. Protestant tract writers
responded to Gregory's calendar by calling him the "Roman Antichrist"
and claiming that its real purpose was to keep true Christians from
worshiping on the correct days. The
"new" calendar, as we know it today, was not adopted uniformly
across Europe until well into the 18th century. Here are a few historical
questions about our calendar: Has
the year always started on 1 January? Then what about leap years? Has the year always
started on 1 January?
For
the man on the street, yes. When
Julius Caesar introduced his calendar in 45 B.C.E., he made 1 January
the start of the year, and it was always the date on which the Solar Number
and the Golden Number were incremented.
However, the church didn't like the wild parties that took place at
the start of the new year, and in C.E. 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: |
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1 Mar X to 28/29 Feb X+1 1 Jan X to 31 Dec X 1 Jan X-1 to 31 Dec X-1 25 Mar X-1 to 24 Mar X 25 Mar X to 24 Mar X+1 Saturday before Easter X to Friday before Easter X+1 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 September, but they did not count years since the birth of Christ, instead they counted years since the creation of the world which they dated to 1 September 5509 B.C.E. 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: The historical year, which started on 1 January. The liturgical year, which started on the first Sunday in advent. 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, and from 1752 started on 1 January. (See the British Calendar Act of 1751.) What About Leap Years? If the year started on, for example, 1 March (two months later than our present year) when was the leap day inserted? When it comes to determining if a year is a leap year, since AD 8 the Julian calendar has always had 48 months between two leap days. So, in a country using a year starting on 1 March, 1439 would have been a leap year, because their February 1439 would correspond to February 1440 in the January-based reckoning. 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.
How Did Dionysius Date Christ's Birth? There are quite a few theories about this. Many of the theories are presented as if they were indisputable historical facts. The following are two theories that tend to be more accepted: 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 C.E. 14. If you combine these numbers you reach a birth year for Jesus that is strikingly close to the beginning of our year reckoning. This may have been the basis for Dionysius' calculations. Dionysius' original task was to calculate an Easter table. In the Julian calendar, the dates for Easter repeat every 532 years. The first year in Dionysius' Easter tables is C.E. 532. Is it a coincidence that the number 532 appears twice here? Or did Dionysius perhaps fix Jesus' birth year so that his own Easter tables would start exactly at the beginning of the second Easter cycle after Jesus' birth? Was Jesus born in the year 0? No. There are two reasons for this: There is no year 0, and Jesus was born before 4 B.C.E. 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. Dionysius let the year C.E.1 start one week after what he believed to be Jesus' birthday. Therefore, C.E.1 follows immediately after 1 B.C.E. with no intervening year zero. So a person who was born in 10 B.C.E. and died in C.E. 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, who died in 4 BCE. It is likely that Jesus was actually born around 7 B.C.E. The date of his birth is unknown; it may or may not be 25 December. Why do the 9th thru 12th months have names that mean 7th, 8th, 9th and 10th? September thru December were the seventh thru tenth months of a calendar used by the first Romans. Ancient historian and Greek biographer Plutarch, wrote in C.E. 75, about how they became displaced to two positions higher than their names would indicate. > Read excerpt of Plutarch's essay. > Read more about the early Roman calendar. Why does February have only 28 days? January and February both date from about the time of Rome's founding. They were added to a calendar that had been divided into ten month-like periods whose lengths varied from 20 to 35 or more days. A winter season was not included, so those period lengths are believed to have been intended to reflect growth stages of crops and cattle. When introduced, January was given 29 days and put at the beginning of the calendar year. February was given 23 days and put at the end. Then, for an undetermined period shortly after Rome's founding, months were said to have begun when a new moon was first sighted. At some later time, month lengths were separated from lunations and again became fixed. At that time, February's original length was extended by five days, which gave it a total of 28. |
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Our Year:
Calendars are normally based
on astronomical events, and the two most
Important astronomical objects are the sun and the
moon. Their cycles are very important
in the construction and understanding of calendars. Our concept of a year is based on the earth's motion around
the sun. The time from one fixed point, such as a solstice or equinox, to the
next is called a tropical year.
Its length is currently 365.242190 days, but it varies. Around 1900 its length was 365.242196 days,
and around 2100 it will be 365.242184 days. (This definition of the tropical
year is not quite accurate, see section astronomic issues
for more details.)
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Illuminations
of Dante's Divine Comedy |
Our
concept of a month is based on the moon's motion around the earth, although
this connection has been broken in the calendar commonly used now. The time from one new moon to the next is
called a synodic month, and its length is currently 29.5305889 days,
but it varies. Around 1900 its length
was 29.5305886 days, and around 2100 it will be 29.5305891 days.
Note
that these numbers are averages. The
actual length of a particular year may vary by several minutes due to the
influence of the gravitational force from other planets. Similarly, the time
between two new moons may vary by several hours due to a number of factors,
including changes in the gravitational force from the sun, and the moon's
orbital inclination.
It
is unfortunate that the length of the tropical year is not a multiple of the
length of the synodic month. This means
that with 12 months per year, the relationship between our month and the moon
cannot be maintained.
However,
19 tropical years is 234.997 synodic months, which is very close to an
integer. So every 19 years the phases
of the moon fall on the same dates (if it were not for the skewness introduced
by leap years). Nineteen years is
called a Metonic cycle (after Meton, an astronomer from Athens in the 5th
century B.C.E.). So, to summarize,
please note: A tropical year is 365.24219
days, and a synodic month is 29.53059 days.
Nineteen
tropical years is close to an integral number of synodic months. The Christian calendar
(Gregorian calendar) is based on the motion of the earth around the sun, while
the months have no connection with the motion of the moon. On the other hand, the Islamic calendar is
based on the motion of the moon, while the year has no connection with the
motion of the earth around the sun.
Finally,
the Jewish calendar
combines both, in that its years are linked to the motion of the earth around
the sun, and its months are linked to the motion of the moon. See also related information in another
exhibit, Daylight Saving Time.
Astronomical Basis
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.
What are different measures
of the year? What
are Different Measures of the Year? The tropical year is defined as
the mean interval between vernal equinoxes; it corresponds to the cycle of
the seasons. Our calendar year is
linked to the tropical year as measured between two March equinoxes, as
originally established by Caesar and Sosigenes. 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 - 7.29E-10 T2 + 2.64E-10 T3 [days]
where T = (JD - 2451545.0) / 36525 and JD is the Julian day number. However, the interval from a particular
vernal equinox to the next may vary from this mean by several minutes. Another kind of year is called the sidereal year,
which is the time it takes the earth to orbit the sun. In the year 2000, the length of the
Tropical Year = 365.24219 days. 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. Again
T = (JD - 2451545.0)/36525 and JD is the Julian day number). 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.
00000021621 T - 3.64E-10 T2 [days] of the Sidereal Year =
365.2564. |
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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
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.
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 if the
cycles are accurate. 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.
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. |
What are Equinoxes
and Solstices? Equinoxes and solstices are frequently used as anchor points
for calendars. For people in the
northern hemisphere, Winter solstice is the time in December when the sun
reaches its southernmost latitude. At
this time we have the shortest day.
The date is near 21 December.
Summer solstice is the time in June when the sun reaches its northernmost latitude. At this time we have the longest day. The date is near 21 June. Vernal equinox is the time in March when the sun passes the equator moving from the southern to the northern hemisphere. Day and night have approximately the same length. The date is near 20 March. Autumnal equinox is the time in September when the sun passes the equator moving from the northern to the southern hemisphere. Day and night have approximately the same length. The date is near 22 September. For
people in the southern hemisphere these events are shifted half a year. The astronomical "tropical year"
is frequently defined as the time between, say, two vernal equinoxes, but
this is not actually true. Currently
the time between two vernal equinoxes is slightly greater than the tropical
year. The reason is that the earth's
position in its orbit at the time of solstices and equinoxes shifts slightly
each year (taking approximately 21,000 years to move all the way around the
orbit). This, combined with the fact
that the earth's orbit is not completely circular, causes the equinoxes and
solstices to shift with respect to each other. The astronomer's mean tropical year is really a somewhat
artificial average of the period between the time when the sun is in any
given position in the sky with respect to the equinoxes and the next time the
sun is in the same position. |
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Did
the church study astronomy? Yes, they
did. Although the Roman Catholic
Church once waged a long and bitter war on science and astronomy
(particularly condemning Galileo), in general, they were quite involved in
astronomy. The church gave more financial
and social support to the study of astronomy for over six centuries, from the
recovery of ancient learning during the late Middle Ages into the
Enlightenment, than any other, and probably, all other, institutions. The church was not necessarily seeking
knowledge for knowledge's sake, a traditional aim of pure science. Rather, like many patrons, it wanted
something practical in return for its investments: mainly the improvement of
the calendar so church officials could more accurately establish the date of
Easter.
When
to celebrate the feast of Christ's resurrection had become a bureaucratic
crisis in the church. Traditionally,
Easter fell on the Sunday after the first full moon of spring. But by the
12th century, the usual ways to predict that date had gone awry. To set a date for Easter Sunday years in
advance, and thus reinforce the church's power and unity, popes and
ecclesiastical officials had for centuries relied on astronomers who pondered
over old manuscripts and devised instruments that set them at the forefront
of the scientific revolution. In
its scientific zeal, the church adapted cathedrals across Europe, and a tower
at the Vatican itself, so their darkened vaults could serve as solar
observatories. Beams of sunlight that fell past religious art and marble
columns not only inspired the faithful but provided astronomers with
information about the Sun, the Earth and their celestial relationship. Among other things, solar images projected
on cathedral floors disclosed the passage of dark spots across the Sun's
face, a blemish in the heavens, which theologians once thought to be without
flaw. Over the centuries,
observatories were built in cathedrals and churches throughout Europe,
including those in Rome, Paris, Milan, Florence, Bologna, Palermo, Brussels
and Antwerp. Didn't the church
condemn Galileo? Yes. The traditional view of the church's hostility toward
science grew out of its famous feud with Galileo, condemned to house arrest
in 1632 for astronomical heresy. Since
antiquity, astronomers had put Earth at the center of planetary motions, a
view the church had embraced. But
Galileo, using the new telescope, became convinced that the planets in fact
moved around the Sun, a view Nicholas Copernicus, a Polish astronomer, had
championed.
Ø
Read Nicholas
Copernicus, On the
Revolutions. Ø
Read Galileo
Galilei, Dialogue Concerning the Two Chief World Systems. The
censure of Galileo, at age 70, hurt the image of the church for
centuries. Pope John Paul II finally
acknowledged in 1992, 359 years later, that the church had erred in
condemning the scientific giant.
Although some scholars claim that Rome's handling of Galileo made
Copernican astronomy a forbidden topic among faithful Catholics for two
centuries, in fact, Rome's support of astronomy was considerable. The church tended to regard all the
systems of the mathematical astronomy as fictions. That interpretation gave Catholic writers scope to develop
mathematical and observational astronomy almost as they pleased, despite the
tough wording of the condemnation of Galileo. How did the
observatories work?
Typically,
the building, dark inside, needed only a small hole in the roof to allow a
beam of sunlight to strike the floor below, producing a clear image of the
solar disk. In effect, the church had
been turned into a pinhole camera, in which light passes through a small hole
into a darkened interior, forming an image on the opposite side. On each sunny day, the solar image would
sweep across the church floor and, exactly at noon, cross a long metal rod
that was the observatory's most important and precise part. The
noon crossings over the course of a year would reach the line's extremities
-- which usually marked the summer and winter solstices, when the Sun is
farthest north and south of the Equator.
The circuit, among other things, could be used to measure the year's
duration with great precision. The
path on the floor was known as a meridian line, like the north-south
meridians of geographers. The rod, in
keeping with its setting and duties, was often surrounded by rich tile inlays
and zodiacal motifs. The instruments
lost much of their astronomical value around the middle of the 18th century
as telescopes began to exceed them in power.
But the observatories still played a significant role because the
solar timepieces were often used to correct errors in mechanical clocks and
even to set time for railroads. One
of the observatories also impressed Charles Dickens, who in his book
"Pictures from Italy" wrote that he found little to like in Bologna
except "the Church of San Petronio, where the sunbeams mark the time
among the kneeling people." Today,
the surviving cathedral solar instruments are lovely anachronisms that baffle
most visitors, who are usually unaware of their original use or historical
importance. In the book, The Sun
in the Church, author Dr. Heilbron, describes his astonishment with
seeing the old instruments in Bologna, Italy, at the Basilica of San
Petronio. "The church itself was
beautiful, somber," Dr. Heilbron recalled. "When the sun crawled across that floor, there was nothing
else. That's what you had to look
at. It was intense." In
the great Basilica of San Petronio, a solar observatory was erected in 1576
by Egnatio Danti, a mathematician and Dominican friar who worked for Cosimo I
dei Medici, the Grand Duke of Tuscany, who also advised Pope Gregory on
calendar reform. The church
observatory produced data long before the telescope existed. By 1582, the Gregorian calendar had been
established, creating the modern year of 365 days and an occasional leap year
of 366 days. Danti was rewarded with
a commission to build a solar observatory in the Vatican itself within the
Torre dei Venti, or Tower of the Winds. The golden age of the cathedral
observatories came later, between 1650 and 1750, and helped to disprove the
astronomical dogma that the church had defended with such militancy in the
case of Galileo. How did Cassini
prove Kepler was right?
Among the best known of the rebel observers was Giovanni Cassini, an Italian astronomer who gained fame for discovering moons of Saturn and the gaps in its rings that still bear his name. Around 1655, Cassini persuaded the builders of the Basilica of San Petronio that they should include a major upgrade of Danti's old meridian line, making it larger and far more accurate, its entry hole for daylight moved up to be some 90 feet high, atop a lofty vault. "Most illustrious nobles of Bologna," Cassini boasted in a flier drawn up for the new observatory, "the kingdom of astronomy is now yours." The exaggeration turned out to have some merit as Cassini used the observatory to investigate the "orbit" of the Sun, quietly suggesting that it actually stood still while the Earth moved. Cassini decided to use his observations to try to confirm
the theories of Johannes Kepler, the German astronomer who had proposed in
1609 that the planets moved in elliptical orbits not the circles that
Copernicus had envisioned.
If
true, that meant the Earth over the course of a year would pull slightly
closer and farther away from the Sun.
At least in theory, Cassini's observatory could test Kepler's idea,
since the Sun's projected disk on the cathedral floor would shrink slightly
as the distance grew and would expand as the gap lessened. Such an experiment could also address
whether there was any merit to the ancient system of Ptolemy, some
interpretations of which had the Earth moving around the Sun in an eccentric
circular orbit. Ptolemy's
Sun at its closest approach moved closer to the Earth than Kepler's Sun did,
in theory making the expected solar image larger and the correctness of the
rival theories easy to distinguish. For
the experiment to succeed, Cassini could tolerate measurement errors no
greater than 0.3 inches in the Sun's projected face, which ranged from 5 to
33 inches wide, depending on the time of year. No telescope of the day could achieve that precision. The experiment was run around 1655, and
after much trial and error, succeeded.
Cassini and his Jesuit allies confirmed Kepler's version of the
Copernican theory. Between
1655 and 1736, astronomers used the solar observatory at San Petronio to make
4,500 observations, aiding substantially the tide of scientific advance. |
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What years are leap
years?
Leap
years were introduced to keep New Year's Day on autumnal equinox. But this Turned
out to be difficult to handle, because equinox is not easy to predict. In fact, the first decree implementing the
calendar (5 Oct 1793) contained two contradictory rules, as it
stated that the first day of each year would be that of the autmunal
equinox. Every fourth year would be a
le leap year. In
practice, the first calendars were based on the equinoxial condition. To remove the confusion, a rule similar to
the one used in the Gregorian Calendar (including a 4000 year rule) was
proposed by the calendar's author, Charles Rommes, but his proposal ran into
political problems. In short, during
the time when the French Revolutionary Calendar was in use, the following
years were leap years: 3, 7, and 11. Is 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. Do the Greeks 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. What day is the leap
day?
It
is 24 February! Weird? Yes!
The explanation is related to the Roman calendar. 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:
For
example, the feast of St. Leander has been celebrated on 27 February in
non-leap years and on 28 February in leap years. Many countries are gradually changing the
leap day from the 24th to the 29th.
This affects countries such as Sweden and Austria that celebrate
"name days" (i.e. each day is associated with a name). 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: Solar Number = (year + 8) mod 28 + 1 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 7 x 400 = 2800 years as one might be tempted to believe.) When can I reuse my
1992 calendar?
Let
us first assume that you are only interested in which dates fall on which
days of the week; you are not interested in the dates for Easter and other
irregular holidays. Let us further
confine ourselves to the years 1901-2099. With
these restrictions, the answer is as follows: If y If year X is a leap year, you can reuse its calendar in year X+28. Ø If year X is the first year after a leap year, you can reuse its calendar in years X+6, X+17, and X+28. Ø If year X is the second year after a leap year, you can reuse its calendar in years X+11, X+17, and X+28. Ø If year X is the third year after a leap year, you can reuse its calendar in years X+11, X+22, and X+28. Note
that the expression X+28 occurs in all four items above. So you can always reuse your calendar
every 28 years. But
if you also want your calendar's indication of Easter and other Christian
holidays to be correct, the rules are far too complex to be put to a simple
formula. Sometimes calendars can be
reused after just six years. For
example, the calendars for the years 1981 and 1987 are identical, even when
it comes to the date for Easter. But
sometimes a very long time can pass before a calendar can be reused; if you
happen to have a calendar from 1940, you won't be able to reuse it until the
year 5280! 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. In
the U.S.A. a month-day-year format is common: 12/25/1998
12-25-1998 Most
other countries use a day-month-year format, such as: 25.12.1998
25/12/1998
25/12-1998
25.XII.1998 International
standard ISO-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 March 2004; to an American it is
3 February 2004; and to a person using the international standard
it would be 4 March 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. How does one count
years?
In
about C.E. 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
C.E. 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 B.C.E. immediately precede C.E. 1
with no intervening year zero. Note,
however, that astronomers frequently use another way of numbering the years
B.C.E. Instead of 1 B.C.E. they
use 0, instead of 2 B.C.E. they use -1, instead of 3 B.C.E. they
use -2, etc. It
is frequently claimed that it was the venerable Bede (673-735) who introduced
B.C. dating. Although Bede seems to have used the term on at least one occasion,
it is generally believed that B.C. dates were not used until the middle of
the 17th century. In
this section I have used C.E. 1 = 754 AUC. This is the most likely equivalence
between the two systems. However,
some authorities state that C.E. 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. |
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When did the 3rd
millennium start? The first millennium started in AD 1, so the millennia are
counted in this manner:
1st
millennium: 1-1000 Thus,
the 3rd millennium and, similarly, the 21st century started on 1 Jan
2001. This is the cause of some
heated debate, especially since some dictionaries and encyclopedias say that
a century starts in years that end in 00.
Furthermore, the change 1999/2000 is obviously much more spectacular
than the change 2000/2001. Let
us propose a few compromises: Any
100-year period is a century.
Therefore the period from 23 June 2004 to 22 June 2104 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, the 21st century started in 2001, but the 2000s started in
2000. What do A.D., B.C.,
C.E., and B.C.E. stand for?
Years
before the birth of Christ are in English traditionally identified using the
abbreviation B.C. ("Before Christ"). Years after the birth of Christ are traditionally identified
using the Latin abbreviation AD ("Anno Domini", that is, "In
the Year of the Lord"). Some
people, who want to avoid the reference to Christ that is implied in these
terms, prefer the abbreviations BCE ("Before the Common Era" or
"Before the Christian Era") and CE ("Common Era" or
"Christian Era"). Historical Eras and
Chronology: The calendars described in this exhibit, 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.E., 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 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.E.
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.E. 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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Countries' Calendar
Reform: 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). When did countries
change from Julian to Gregorian calendars?
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 the
1900s. 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. For example, when Soviet Russia undertook
its calendar reform in February 1918, they moved from the Julian calendar to
the Gregorian. This move resulted in
a loss of 13 days, so that February 1, 1918, became February 14. 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. The
list below does not include all the different opinions about when the change
took place. (See
the British Calendar Act of
1751.) |
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The
Jewish calendar
The
current definition of the Jewish calendar is generally said to have been set
down by
the
Sanhedrin president Hillel II in approximately C.E. 359. The original details of his calendar are,
however, uncertain. The Jewish calendar
is used for religious purposes by Jews all over the world, and it is the
official calendar of Israel. The Jewish
calendar is a combined solar/lunar calendar, in that it strives to have its
years coincide with the tropical year and its months coincide with the synodic
months. This is a complicated goal, and
the rules for the Jewish calendar are correspondingly fascinating.
Lunisolar calendars use months to approximate the
tropical year. Examples are the Jewish
and Chinese calendars. Since 12 months
are about 11 days shorter than the tropical year, a leap month (also called
intercalary month) is inserted about every third year to keep the calendar in
tune with the seasons. The big question
is how to do this. A simple method is
to just base it on nature. In ancient
Israel, the religious leaders would determine the date for Passover each spring
by seeing if the roads were dry enough for the pilgrims and if the lambs were
ready for slaughter. If not, they would
add one more month. An aboriginal tribe
in Taiwan would go out to sea with lanterns near the new moon at the beginning
of spring. If the migrating flying fish
appeared, there would be fish for New Year's reunion dinner. If not, they would try their luck next
month.
What does a Jewish
year look like?
An
ordinary (non-leap) year has 353, 354, or 355 days. A leap year has 383, 384, or 385 days. The three lengths of the years are termed, "deficient,"
"regular," and "complete," respectively. An ordinary year has 12 months, a leap year
has 13 months. Every month starts
(approximately) on the day of a new moon.
The months and their lengths are:
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Name |
Length in a |
Length in a |
Length in a |
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Tishri |
30 |
30 |
30 |
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Heshvan |
29 |
29 |
30 |
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Kislev |
29 |
30 |
30 |
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Tevet |
29 |
29 |
29 |
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Shevat |
30 |
30 |
30 |
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Adar I |
30 |
30 |
30) |
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Adar II |
29 |
29 |
29 |
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Nisan |
30 |
30 |
30 |
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Iyar |
29 |
29 |
29 |
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Sivan |
30 |
30 |
30 |
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Tammuz |
29 |
29 |
29 |
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Av |
30 |
30 |
30 |
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Elul |
29 |
29 |
29 |
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Total: |
353 or 383 |
354 or 384 |
355 or 385 |
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The
month Adar I is only present in leap years. In non-leap years Adar II is simply called "Adar."
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Note
that in a regular year the numbers 30 and 29 alternate. A complete year is created by adding a day
to Heshvan, whereas a deficient year is created by removing a day from
Kislev. The alteration of 30 and 29
ensures that when the year starts with a new moon, so does each month. What years are leap
years?
A
year is a leap year if the number year mod 19 is one of the
following: 0, 3, 6, 8, 11, 14, or 17.
The value for year in this formula is the 'Anno Mundi' described below. |
What years are
deficient, regular, and complete?
That is the wrong question to ask. The correct question to ask is: When does a Jewish year begin? Once you have answered that question (see below) the length of the year is the number of days between 1 Tishri in one year and 1 Tishri in the following year.
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When is New Year's
Day?
That
depends. Jews have 4 different days to choose from:
Only
the first two dates are celebrated today. When does a Jewish
day begin?
A
Jewish-calendar day does not begin at midnight, but at either sunset or when
three medium-sized stars should be visible, depending on the religious
circumstance. Sunset
marks the start of the 12 night hours, whereas sunrise marks the start of the
12-day hours. This means that night
hours may be longer or shorter than day hours, depending on the season. When does a Jewish
year begin?
The
first day of the calendar year, Rosh HaShanah, on 1 Tishri is determined
as follows: The new year starts on the day of the new moon that occurs about 354 days (or 384 days if the previous year was a leap year) after 1 Tishri of the previous year. Ø If this would cause the new year to start on a Sunday, Wednesday, or Friday, delay it by one day. (because we want to avoid that Yom Kippur (10 Tishri) falls on a Friday or Sunday, and that Hoshanah Rabba (21 Tishri) falls on a Sabbath (Saturday)). Ø If two consecutive years start 356 days apart (an illegal year length), delay the start of the first year by two days. Ø If two consecutive years start 382 days apart (an illegal year length), delay the start of the second year by one day.
Note:
Rule 4 can only come into play if the first year was supposed to start on a
Tuesday. Therefore a two-day delay is used rather that a one-day delay, as
the year must not start on a Wednesday as stated in rule 3. When is the new
moon?
A
calculated new moon is used. In order
to understand the calculations, one must know that an hour is subdivided into
1080 'parts'. The calculations are as
follows: The new moon that started
the year AM 1, occurred 5 hours and 204 parts after sunset (i.e. just
before midnight on Julian date 6 October 3761 B.C.E.). The new moon of any particular year is
calculated by extrapolating from this time, using a synodic month of 29 days
12 hours and 793 parts. Note
that 18:00 Jerusalem time (15:39 UTC) is used instead of sunset in
all these cases. How does one count
years?
Years
are counted since the creation of the world, which is assumed to have taken
place in 3761 B.C.E. In that
year, AM 1 started (AM = Anno Mundi = year of the world). In the year C.E. 1998 we have
witnessed the start of Jewish year AM 5759. |
The Islamic Calendar: The
Islamic calendar (or Hijri calendar) is a purely lunar calendar. It contains
12 months that are based on the motion of the moon, and because 12 synodic
months is only 12 x 29.53=354.36 days, the Islamic calendar is consistently
shorter than a tropical year, and therefore it shifts with respect to the
Christian calendar.
The
calendar is based on the Qur'an (Sura IX, 36-37) and its proper
observance is a sacred duty for Muslims.
The Islamic calendar is the official calendar in countries around the
Gulf, especially Saudi Arabia. But
other Muslim countries use the Gregorian calendar for civil purposes and only
turn to the Islamic calendar for religious purposes. So you can't print an
Islamic calendar in advance? How does one count years?
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