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<address class="h3"><a class="moz-txt-link-freetext"
href="https://www.britannica.com/science/calendar/Calendar-reform-since-the-mid-18th-century">https://www.britannica.com/science/calendar/Calendar-reform-since-the-mid-18th-century</a><br>
</address>
<h1 class="h3"><a
href="https://www.britannica.com/topic/Aztec-calendar"
class="md-crosslink">The Mexican (Aztec) calendar</a></h1>
<p class="topic-paragraph">The calendar of the <a
href="https://www.britannica.com/topic/Aztec"
class="md-crosslink">Aztecs</a> was derived from earlier
calendars in the Valley of Mexico and was basically similar to
that of the Maya. The ritual <a
href="https://www.britannica.com/science/day"
class="md-crosslink autoxref">day</a> cycle was called <em><span
id="ref313521"></span><a
href="https://www.britannica.com/topic/tonalpohualli"
class="md-crosslink">tonalpohualli</a></em> and was formed, as
was the Mayan Tzolkin, by the <a
href="https://www.merriam-webster.com/dictionary/concurrence"
class="md-dictionary-link md-dictionary-tt-off"
data-term="concurrence">concurrence</a> of a cycle of numerals 1
through 13 with a cycle of 20 day names, many of them similar to
the day names of the Maya. The <em>tonalpohualli</em> could be
divided into four or five equal parts, each of four assigned to a
world quarter and a colour and including the centre of the world
if the parts were five. To the Aztecs, the 13-day period defined
by the day numerals was of prime importance, and each of 20 such
periods was under the patronage of a specific deity. A similar
list of 20 deities was associated with individual day names, and,
in addition, there was a list of 13 deities designated as Lords of
the Day, each accompanied by a flying creature, and a list of nine
deities known as Lords of the Night. The lists of deities vary
somewhat in different sources. They were probably used to
determine the fate of the days by the <span id="ref313522"></span>Tonalpouhque,
who were priests trained in calendrical divination. These priests
were consulted as to lucky days whenever an important enterprise
was undertaken or when a child was born. Children were often named
after the day of their birth; and tribal gods, who were legendary
heroes of the past, also bore calendar names.</p>
<span class="marker p1"></span><span class="marker AM1 am-inline"></span><span
class="marker MOD1 mod-inline"></span>
<p class="topic-paragraph">The Aztec <a
href="https://www.britannica.com/science/year"
class="md-crosslink autoxref">year</a> of 365 days was also
similar to the year of the Maya, though probably not synchronous
with it. It had 18 named months of 20 days each and an additional
five days, called <em>nemontemi</em>, which were considered to be
very unlucky. Though some colonial historians mention the use of
intercalary days, in Aztec annals there is no indication of a
correction in the length of the year. The years were named after
days that <a
href="https://www.britannica.com/science/autumn-season"
class="md-crosslink autoxref">fall</a> at intervals of 365 days,
and most scholars believe that these days held a fixed position in
the year, though there appears to be some disagreement as to
whether this position was the first day, the last day of the first
<a href="https://www.britannica.com/science/month"
class="md-crosslink autoxref">month</a>, or the last day of the
last month. Since 20 and 365 are both divisible by five, only four
day names—Acatl (Reed), Tecpatl (Flint), <a
href="https://www.merriam-webster.com/dictionary/Calli"
class="md-dictionary-link md-dictionary-tt-off"
data-term="Calli">Calli</a> (House), and Tochtli (Rabbit)—figure
in the names of the 52 years that form a cycle with the <em>tonalpohualli</em>.
The cycle begins with a year 2 Reed and ends with a year 1 Rabbit,
which was regarded as a dangerous year of bad omen. At the end of
such a cycle, all household utensils and idols were discarded and
replaced by new ones, temples were renovated, and <a
href="https://www.britannica.com/topic/human-sacrifice"
class="md-crosslink autoxref">human sacrifice</a> was offered to
the <a href="https://www.britannica.com/place/Sun"
class="md-crosslink autoxref">Sun</a> at midnight on a
mountaintop as people awaited a new dawn.</p>
<span class="marker p2"></span><span class="marker AM2 am-inline"></span><span
class="marker MOD2 mod-inline"></span>
<p class="topic-paragraph">The year served to fix the time of
festivals, which took place at the end of each month. The new year
was celebrated by the making of a new fire, and a more elaborate
ceremony was held every four years, when the cycle had run through
the four day names. Every eight years was celebrated the
coincidence of the year with the 584-day period of the planet <a
href="https://www.britannica.com/place/Venus-planet"
class="md-crosslink autoxref">Venus</a>, and two 52-year cycles
formed “One Old Age,” when the day cycle, the year, and the period
of Venus all came together. All these periods were noted also by
the Maya.</p>
<span class="marker p3"></span><span class="marker AM3 am-inline"></span><span
class="marker MOD3 mod-inline"></span>
<p class="topic-paragraph">Where the Aztecs differed most
significantly from the Maya was in their more primitive number
system and in their less precise way of recording dates. Normally,
they noted only the day on which an event occurred and the name of
the current year. This is <a
href="https://www.merriam-webster.com/dictionary/ambiguous"
class="md-dictionary-link md-dictionary-tt-off"
data-term="ambiguous">ambiguous</a>, since the same day, as
designated in the way mentioned above, can occur twice in a year.
Moreover, years of the same name recur at 52-year intervals, and
Spanish colonial annals often disagree as to the length of time
between two events. Other discrepancies in the records are only
partially explained by the fact that different towns started their
year with different months. The most widely accepted correlation
of the calendar of Tenochtitlán with the Christian <a
href="https://www.britannica.com/science/Julian-calendar"
class="md-crosslink">Julian calendar</a> is based on the
entrance of Spanish conquistador <a
href="https://www.britannica.com/biography/Hernan-Cortes"
class="md-crosslink">Hernán Cortés</a> into that city on
November 8, 1519, and on the surrender of <a
href="https://www.britannica.com/biography/Cuauhtemoc"
class="md-crosslink">Cuauhtémoc</a> on <a
href="https://www.merriam-webster.com/dictionary/August"
class="md-dictionary-link md-dictionary-tt-off"
data-term="August">August</a> 13, 1521. According to this
correlation, the first date was a day 8 Wind, the ninth day of the
month Quecholli, in a year 1 Reed, the 13th year of a cycle.</p>
<span class="marker p4"></span><span class="marker AM4 am-inline"></span><span
class="marker MOD4 mod-inline"></span>
<p class="topic-paragraph">The Mexicans, as all other Mesoamericans,
believed in the periodic destruction and re-creation of the world.
The “<span id="ref313523"></span><a
href="https://www.britannica.com/topic/calendar-stone"
class="md-crosslink">Calendar Stone</a>” in the <a
href="https://www.britannica.com/topic/National-Museum-of-Anthropology"
class="md-crosslink autoxref">Museo Nacional de Antropología</a>
(National Museum of Anthropology) in <a
href="https://www.britannica.com/place/Mexico-City"
class="md-crosslink">Mexico City</a> depicts in its central
panel the date 4 Ollin (movement), on which they anticipated that
their current world would be destroyed by earthquake, and within
it the dates of previous holocausts: 4 Tiger, 4 Wind, 4 Rain, and
4 Water.</p>
<span class="marker p5"></span>
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="813">
<div class="md-assembly-wrapper" data-type="image"><a
style="--aspect-ratio: 16/9"
href="https://cdn.britannica.com/43/7043-050-DCF36CFF/Aztec-calendar-stone-National-Museum-of-Anthropology-1790.jpg"
class="position-relative d-flex align-items-center
justify-content-center media-overlay-link card-media"
data-href="/media/1/89368/813"><img
src="https://cdn.britannica.com/s:690x388,c:crop/43/7043-050-DCF36CFF/Aztec-calendar-stone-National-Museum-of-Anthropology-1790.jpg"
alt="Aztec calendar stone; in the National Museum of
Anthropology, Mexico City. The calendar, discovered in
1790, is a basaltic monolith. It weighs approximately 25
tons and is about 12 feet (3.7 metres) in diameter."
data-width="1080" data-height="1085"></a></div>
</figure>
</div>
</div>
<section id="ref60227" data-level="3" data-has-spy="true">
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="813"><figcaption class="card-body">
<div class="md-assembly-caption text-muted font-14
font-serif">Aztec calendar stone; in the National Museum
of Anthropology, Mexico City. The calendar, discovered
in 1790, is a basaltic monolith. It weighs approximately
25 tons and is about 12 feet (3.7 metres) in diameter.</div>
<cite class="credit d-block mt-5">Courtesy of the Museo
Nacional de Antropología, Mexico City; photograph,
Mexican Ministry of Tourism</cite></figcaption></figure>
</div>
</div>
<span class="marker AM5 am-inline"></span><span class="marker MOD5
mod-inline"></span></section>
<span id="ref59341" data-level="2"></span>
<section id="ref60228" data-level="3" data-has-spy="true">
<h2 class="h3">Peru: the <span id="ref313524"></span><a
href="https://www.britannica.com/topic/Inca-calendar"
class="md-crosslink">Inca calendar</a></h2>
<p class="topic-paragraph">So little is known about the calendar
used by the <span id="ref313525"></span><a
href="https://www.britannica.com/topic/Inca"
class="md-crosslink">Incas</a> that one can hardly make a
statement about it for which a contrary opinion cannot be found.
Some workers in the field even assert that there was no formal
calendar but only a simple count of lunations. Since no written
language was used by the Incas, it is impossible to <a
href="https://www.britannica.com/topic/check-finance"
class="md-crosslink autoxref">check</a> contradictory
statements made by early colonial chroniclers. It was widely
believed that at least some of the <a
href="https://www.britannica.com/technology/quipu"
class="md-crosslink">quipu</a> (<em>khipu</em>) of the Incas
contained calendrical notations.</p>
<span class="marker p6"></span>
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="3173">
<div class="md-assembly-wrapper" data-type="image"><a
style="--aspect-ratio: 16/9"
href="https://cdn.britannica.com/04/3604-050-CFDB193D/Bookkeeper-rendering-accounts-Inca-ruler-Topa-Yupanqui.jpg"
class="position-relative d-flex align-items-center
justify-content-center media-overlay-link card-media"
data-href="/media/1/89368/3173"><img
src="https://cdn.britannica.com/s:690x388,c:crop/04/3604-050-CFDB193D/Bookkeeper-rendering-accounts-Inca-ruler-Topa-Yupanqui.jpg"
alt="Felipe Guamán Poma de Ayala: El primer nueva
corónica y buen gobierno, depiction of an Inca
bookkeeper using a quipu" data-width="859"
data-height="1287"></a></div>
</figure>
</div>
</div>
</section>
<section id="ref60228" data-level="3" data-has-spy="true">
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="3173"><figcaption class="card-body"><a
class="md-assembly-title font-weight-bold mb-5
d-inline-block font-16 font-sans-serif
media-overlay-link"
href="https://cdn.britannica.com/04/3604-050-CFDB193D/Bookkeeper-rendering-accounts-Inca-ruler-Topa-Yupanqui.jpg"
data-href="/media/1/89368/3173">Felipe Guamán Poma de
Ayala: <em>El primer nueva corónica y buen gobierno</em>,
depiction of an Inca bookkeeper using a quipu</a>
<div class="md-assembly-caption text-muted font-14
font-serif">Bookkeeper (right) rendering accounts to the
Inca ruler Topa Inca Yupanqui. The contents of the
storehouses (foreground and background) are recorded on
the bookkeeper's quipu of knotted strings. Drawing by
Felipe Guamán Poma de Ayala from <em>El primer nueva
corónica y buen gobierno</em>.</div>
<cite class="credit d-block mt-5">Courtesy, Library
Services Department, American Museum of Natural History,
New York City (Neg. No. 321546)</cite></figcaption></figure>
</div>
</div>
<span class="marker AM6 am-inline"></span><span class="marker MOD6
mod-inline"></span>
<p class="topic-paragraph">Most historians agree that the Incas
had a calendar based on the observation of both the Sun and the
<a href="https://www.britannica.com/place/Moon"
class="md-crosslink autoxref">Moon</a>, and their relationship
to the stars. Names of 12 lunar months are recorded, as well as
their association with festivities of the agricultural cycle;
but there is no suggestion of the widespread use of a numerical
system for counting time, although a quinary decimal system,
with names of numbers at least up to 10,000, was used for other
purposes. The organization of work on the basis of six <span
id="ref664718"></span><a
href="https://www.britannica.com/science/week"
class="md-crosslink">weeks</a> of nine days suggests the
further possibility of a count by triads that could result in a
formal month of 30 days.</p>
<span class="marker p7"></span><span class="marker AM7 am-inline"></span><span
class="marker MOD7 mod-inline"></span>
<p class="topic-paragraph">A count of this sort was described by
German naturalist and explorer <a
href="https://www.britannica.com/biography/Alexander-von-Humboldt"
class="md-crosslink">Alexander von Humboldt</a> for a <span
id="ref313527"></span><a
href="https://www.britannica.com/topic/Chibcha"
class="md-crosslink">Chibcha</a> tribe living outside of the
Inca empire, in the mountainous region of <a
href="https://www.britannica.com/place/Colombia"
class="md-crosslink">Colombia</a>. The description is based on
an earlier manuscript by a village priest, and one authority has
dismissed it as “wholly imaginary,” but this is not necessarily
the case. The smallest unit of this calendar was a numerical
count of three days, which, interacting with a similar count of
10 days, formed a standard 30-day “month.” Every third year was
made up of 13 moons, the others having 12. This formed a cycle
of 37 moons, and 20 of these cycles made up a period of 60
years, which was subdivided into four parts and could be
multiplied by 100. A period of 20 months is also mentioned.
Although the account of the Chibcha system cannot be accepted at
face value, if there is any truth in it at all it is suggestive
of devices that may have been used also by the Incas.</p>
<span class="marker p8"></span><span class="marker AM8 am-inline"></span><span
class="marker MOD8 mod-inline"></span>
<p class="topic-paragraph">In one account, it is said that the
Inca <span id="ref313528"></span><a
href="https://www.britannica.com/topic/Viracocha"
class="md-crosslink">Viracocha</a> established a year of 12
months, each beginning with the <a
href="https://www.britannica.com/topic/New-Moon-Jewish-festival"
class="md-crosslink autoxref">New Moon</a>, and that his
successor, <span id="ref313529"></span><a
href="https://www.britannica.com/biography/Pachacuti-Inca-Yupanqui"
class="md-crosslink">Pachacuti</a>, finding confusion in
regard to the year, built the sun towers in order to keep a
check on the calendar. Since Pachacuti reigned less than a
century before the conquest, it may be that the contradictions
and the meagreness of information on the Inca calendar are due
to the fact that the system was still in the process of being
revised when the Spaniards first arrived.</p>
<span class="marker p9"></span><span class="marker AM9 am-inline"></span><span
class="marker MOD9 mod-inline"></span><span class="md-signature"><a
href="https://www.britannica.com/contributor/Tatiana-Proskouriakoff/2376">Tatiana
Proskouriakoff</a></span>
<p class="topic-paragraph">Despite the uncertainties, further
research has made it clear that at least at <span
id="ref313530"></span><a
href="https://www.britannica.com/place/Cuzco"
class="md-crosslink">Cuzco</a>, the capital city of the Incas,
there was an official calendar of the sidereal–lunar type, based
on the sidereal month of 27 <span class="md-fraction
md-fraction-oblique"><span><sup>1</sup>/<sub>3</sub></span></span>
days. It consisted of 328 nights (12 × 27 <span
class="md-fraction md-fraction-oblique"><span><sup>1</sup>/<sub>3</sub></span></span>)
and began on June 8/9, coinciding with the heliacal rising (the
rising just after sunset) of the Pleiades; it ended on the first
Full Moon after the June solstice (the <a
href="https://www.britannica.com/science/winter-solstice"
class="md-crosslink autoxref">winter solstice</a> for the
Southern Hemisphere). This sidereal–lunar calendar fell short of
the solar year by 37 days, which consequently were intercalated.
This <a href="https://www.britannica.com/science/intercalation"
class="md-crosslink autoxref">intercalation</a>, and thus the
place of the sidereal–lunar within the solar year, was fixed by
following the cycle of the Sun as it “strengthened” to <a
href="https://www.britannica.com/science/summer-season"
class="md-crosslink autoxref">summer</a> (December) <span
id="ref313531"></span><a
href="https://www.britannica.com/science/solstice"
class="md-crosslink">solstice</a> and “weakened” afterward,
and by noting a similar cycle in the visibility of the Pleiades.</p>
<span class="marker p10"></span><span class="marker AM10
am-inline"></span><span class="marker MOD10 mod-inline"></span><span
class="md-signature"><a
href="https://www.britannica.com/contributor/Tatiana-Proskouriakoff/2376">Tatiana
Proskouriakoff</a></span><span class="md-signature"><a
href="https://www.britannica.com/contributor/Colin-Alistair-Ronan/2514">Colin
Alistair Ronan</a></span></section>
<span id="ref59341" data-level="2"></span>
<section id="ref60229" data-level="3" data-has-spy="true">
<h2 class="h3">North <span id="ref313532"></span><a
href="https://www.britannica.com/topic/American-Indian"
class="md-crosslink">American Indian</a> time counts</h2>
<p class="topic-paragraph">No North American Indian tribe had a
true calendar—a single <a
href="https://www.merriam-webster.com/dictionary/integrated"
class="md-dictionary-link md-dictionary-tt-off"
data-term="integrated">integrated</a> system of denoting days
and longer periods of time. Usually, intervals of time were
counted independently of one another. The day was a basic unit
recognized by all tribes, but there is no record of aboriginal
names for days. A common device for keeping track of days was a
bundle of sticks of known number, from which one was extracted
for every day that passed, until the bundle was exhausted.
Longer periods of time were usually counted by moons, which
began with the New Moon, or conjunction of the Sun and Moon.
Years were divided into four seasons, occasionally five, and
when counted were usually designated by one of the seasons;
e.g., a North American Indian might say that a certain event had
happened 10 winters ago. Among sedentary agricultural tribes,
the cycle of the seasons was of great ritual importance, but the
time of the beginning of the year varied. Some observed it about
the time of the <a
href="https://www.britannica.com/science/vernal-equinox"
class="md-crosslink autoxref">vernal equinox</a>, others in
the fall. The <a href="https://www.britannica.com/topic/Hopi"
class="md-crosslink">Hopi</a> tribe of northern Arizona held a
new-fire ceremony in November. The <a
href="https://www.britannica.com/topic/Creek-people"
class="md-crosslink">Creek</a> ceremony, known as the Busk,
was held late in July or in August, but it is said that each
Creek town or settlement set its own date for the celebration.</p>
<span class="marker p11"></span>
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="96044">
<div class="md-assembly-wrapper" data-type="image"><a
style="--aspect-ratio: 16/9"
href="https://cdn.britannica.com/36/99036-050-71FCAC1D/painting-calendar-Kiowa-buffalo-James-Mooney-photograph-1895.jpg"
class="position-relative d-flex align-items-center
justify-content-center media-overlay-link card-media"
data-href="/media/1/89368/96044"><img
src="https://cdn.britannica.com/s:690x388,c:crop/36/99036-050-71FCAC1D/painting-calendar-Kiowa-buffalo-James-Mooney-photograph-1895.jpg"
alt="Kiowa calendar painting of the years 1833–92 on
buffalo hide, photograph by James Mooney, 1895."
data-width="975" data-height="800"></a></div>
</figure>
</div>
</div>
</section>
<div class="grid">
<div class="topic-content col-sm pr-lg-60">
<section id="ref" data-level="1">
<section id="ref" data-level="2">
<section id="ref60229" data-level="3" data-has-spy="true">
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="96044"><figcaption
class="card-body">
<div class="md-assembly-caption text-muted font-14
font-serif">Kiowa calendar painting of the years
1833–92 on buffalo hide, photograph by James
Mooney, 1895.</div>
<cite class="credit d-block mt-5">"Seventeenth
Annual Report of the Bureau of American
Ethnology to the Smithsonian Institution,
1895-96," by James Mooney.</cite></figcaption></figure>
</div>
</div>
<span class="marker AM11 am-inline"></span><span
class="marker MOD11 mod-inline"></span>
<p class="topic-paragraph">As years were determined by
seasons and not by a fixed number of days, the
correlation of moons and years was also approximate and
not a function of a daily count. Most tribes reckoned 12
moons to a year. Some northern tribes, notably those of
<a href="https://www.britannica.com/place/New-England"
class="md-crosslink autoxref">New England</a>, and the
<a href="https://www.britannica.com/topic/Cree"
class="md-crosslink">Cree</a> tribes, counted 13. The
Indians of the northwest coast divided their years into
two parts, counting six moons to each part, and the <a
href="https://www.britannica.com/topic/Kiowa"
class="md-crosslink">Kiowa</a> split one of their 12
moons between two unequal seasons, beginning their year
with a Full Moon.</p>
<span class="marker p12"></span><span class="marker AM12
am-inline"></span><span class="marker MOD12 mod-inline"></span>
<p class="topic-paragraph">The naming of moons is perhaps
the first step in transforming them into months. The <span
id="ref313533"></span><a
href="https://www.britannica.com/topic/Zuni"
class="md-crosslink">Zuni</a> Indians of <a
href="https://www.britannica.com/place/New-Mexico"
class="md-crosslink autoxref">New Mexico</a> named the
first six moons of the year, referring to the remainder
by colour <a
href="https://www.merriam-webster.com/dictionary/designations"
class="md-dictionary-link md-dictionary-tt-off"
data-term="designations">designations</a> associated
with the four cardinal (horizontal) directions, and the
zenith and the nadir. Only a few Indian tribes attempted
a more precise correlation of moons and years. The <span
id="ref313534"></span><a
href="https://www.britannica.com/topic/Creek-people"
class="md-crosslink">Creeks</a> are said to have added
a moon between each pair of years, and the <span
id="ref313535"></span><a
href="https://www.britannica.com/topic/Haida"
class="md-crosslink">Haida</a> from time to time
inserted a “between moon” in the division of their year
into two parts. It is said that an unspecified tribe of
the <a href="https://www.britannica.com/topic/Sioux"
class="md-crosslink">Sioux</a> or the <a
href="https://www.britannica.com/topic/Ojibwa"
class="md-crosslink">Ojibwa</a> (Chippewa) made a
practice of adding a “lost moon” when 30 moons had
waned.</p>
<span class="marker p13"></span><span class="marker AM13
am-inline"></span><span class="marker MOD13 mod-inline"></span>
<p class="topic-paragraph">A tally of years following an
important event was sometimes kept on a notched stick.
The best-known record <a
href="https://www.merriam-webster.com/dictionary/commemorates"
class="md-dictionary-link md-dictionary-tt-off"
data-term="commemorates">commemorates</a> the
spectacular <a
href="https://www.britannica.com/science/meteor-shower"
class="md-crosslink">meteor shower</a> (the Leonids)
of 1833. Some northern tribes recorded series of events
by pictographs, and one such record, said to have been
originally painted on a buffalo robe and known as the
“Lone-Dog Winter Count,” covers a period of 71 years
beginning with 1800.</p>
<span class="marker p14"></span><span class="marker AM14
am-inline"></span><span class="marker MOD14 mod-inline"></span>
<p class="topic-paragraph">Early explorers had little
opportunity to learn about the calendrical devices of
the Indians, which were probably held sacred and secret.
Contact with Europeans and their <a
href="https://www.britannica.com/topic/church-year"
class="md-crosslink autoxref">Christian calendar</a>
doubtless altered many aboriginal practices. Thus,
present knowledge of the systems used in the past may
not reflect their true complexity.</p>
<span class="marker p15"></span><span class="marker AM15
am-inline"></span><span class="marker MOD15 mod-inline"></span><span
class="md-signature"><a
href="https://www.britannica.com/contributor/Tatiana-Proskouriakoff/2376">Tatiana
Proskouriakoff</a></span></section>
</section>
</section>
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before-article"></span><span class="marker h4"></span>
<section id="ref59346" data-level="1" data-has-spy="true">
<h1 class="h1">The Western calendar and calendar reforms</h1>
<p class="topic-paragraph">The calendar now in general
worldwide use had its origin in the desire for a <a
href="https://www.britannica.com/science/solar-calendar"
class="md-crosslink">solar calendar</a> that kept in
step with the <a
href="https://www.britannica.com/science/season"
class="md-crosslink">seasons</a> and possessed fixed
rules of <a
href="https://www.britannica.com/science/intercalation"
class="md-crosslink">intercalation</a>. Because it
developed in Western Christendom, it had also to provide a
method for <a
href="https://www.britannica.com/science/dating-geochronology"
class="md-crosslink autoxref">dating</a> movable
religious feasts, the timing of which had been based on a
lunar reckoning. To <a
href="https://www.merriam-webster.com/dictionary/reconcile"
class="md-dictionary-link md-dictionary-tt-off"
data-term="reconcile">reconcile</a> the lunar and solar
schemes, features of the <a
href="https://www.britannica.com/science/Roman-republican-calendar"
class="md-crosslink">Roman republican calendar</a> and
the <a
href="https://www.britannica.com/science/Egyptian-calendar"
class="md-crosslink">Egyptian calendar</a> were
combined.</p>
<span class="marker p1"></span><span class="marker AM1
am-inline"></span><span class="marker MOD1 mod-inline"></span>
<p class="topic-paragraph">The Roman republican calendar was
basically a lunar reckoning and became increasingly out of
phase with the <span id="ref664721"></span><a
href="https://www.britannica.com/science/season"
class="md-crosslink">seasons</a> as time passed. By
about 50 <span class="text-smallcaps">bce</span> the
vernal <a
href="https://www.britannica.com/science/equinox-astronomy"
class="md-crosslink">equinox</a> that should have fallen
late in <a
href="https://www.britannica.com/topic/March-month"
class="md-crosslink">March</a> fell on the Ides of May,
some eight weeks later, and it was plain that this error
would continue to increase. Moreover, the behaviour of the
Pontifices (<em>see above</em> <a
href="https://www.britannica.com/science/calendar/The-early-Roman-calendar#ref60215"
class="md-crosslink">The early Roman calendar</a>) made
it necessary to seek a fixed rule of intercalation in
order to put an end to arbitrariness in inserting months.</p>
<span class="marker p2"></span><span class="marker AM2
am-inline"></span><span class="marker MOD2 mod-inline"></span>
<p class="topic-paragraph">In addition to the problem of
intercalation, it was clear that the average Roman
republican year of 366.25 days would always show a
continually increasing disparity with the seasons,
amounting to one <a
href="https://www.britannica.com/science/month"
class="md-crosslink autoxref">month</a> every 30 years,
or three months a century. But the great difficulty facing
any reformer was that there seemed to be no way of
effecting a change that would still allow the months to
remain in step with the phases of the <a
href="https://www.britannica.com/place/Moon"
class="md-crosslink autoxref">Moon</a> and the year with
the seasons. It was necessary to make a fundamental break
with traditional reckoning to devise an efficient seasonal
calendar.</p>
<span class="marker p3"></span><span class="marker AM3
am-inline"></span><span class="marker MOD3 mod-inline"></span>
<section id="ref59347" data-level="2" data-has-spy="true">
<h2 class="h2">The <span id="ref313538"></span><a
href="https://www.britannica.com/science/Julian-calendar"
class="md-crosslink">Julian calendar</a></h2>
<p class="topic-paragraph">In the mid-1st century <span
class="text-smallcaps">bce</span><span id="ref313537"></span><a
href="https://www.britannica.com/topic/Julius-Caesar-by-Shakespeare"
class="md-crosslink">Julius Caesar</a> invited
astronomer <span id="ref313539"></span><a
href="https://www.britannica.com/biography/Sosigenes-of-Alexandria"
class="md-crosslink">Sosigenes of Alexandria</a> to
advise him about the reform of the calendar, and
Sosigenes decided that the only practical step was to
abandon the <a
href="https://www.britannica.com/science/lunar-calendar"
class="md-crosslink">lunar calendar</a> altogether.
Months must be arranged on a seasonal basis, and a
tropical (solar) year used, as in the Egyptian calendar,
but with its length taken as 365 <span
class="md-fraction md-fraction-oblique"><span><sup>1</sup>/<sub>4</sub></span></span>
days.</p>
<span class="marker p4"></span><span class="marker AM4
am-inline"></span><span class="marker MOD4 mod-inline"></span>
<p class="topic-paragraph">To remove the immense
discrepancy between calendar date and equinox, it was
decided that the year known in modern times as 46 <span
class="text-smallcaps">bce</span> should have two
intercalations. The first was the customary
intercalation of the Roman republican calendar due that
year, the insertion of 23 days following February 23.
The second intercalation, to bring the calendar in step
with the equinoxes, was achieved by inserting two
additional months between the end of <a
href="https://www.britannica.com/topic/November-month"
class="md-crosslink">November</a> and the beginning of
<a href="https://www.britannica.com/topic/December"
class="md-crosslink">December</a>. This insertion
amounted to an addition of 67 days, making a year of no
less than 445 days and causing the beginning of March 45
<span class="text-smallcaps">bce</span> in the Roman
republican calendar to <a
href="https://www.britannica.com/science/autumn-season"
class="md-crosslink autoxref">fall</a> on what is
still called January 1 of the Julian calendar.</p>
<span class="marker p5"></span><span class="marker AM5
am-inline"></span><span class="marker MOD5 mod-inline"></span>
<p class="topic-paragraph">Previous errors having been
corrected, the next step was to prevent their
recurrence. Here Sosigenes’ suggestion about a tropical
year was adopted and any pretense to a lunar calendar
was rejected. The figure of 365.25 days was accepted for
the tropical year, and, to achieve this by a simple
civil reckoning, Caesar directed that a calendar year of
365 days be adopted and that an extra <a
href="https://www.britannica.com/science/day"
class="md-crosslink autoxref">day</a> be intercalated
every fourth year. Since <a
href="https://www.britannica.com/topic/February"
class="md-crosslink">February</a> ordinarily had 28
days, February 24 was the sixth day (using <a
href="https://www.merriam-webster.com/dictionary/inclusive"
class="md-dictionary-link md-dictionary-tt-off"
data-term="inclusive">inclusive</a> numbering) before
the Kalendae, or beginning of March, and was known as
the <em>sexto-kalendae</em>; the intercalary day, when
it appeared, was in effect a “doubling” of the <em>sexto-kalendae</em>
and was called the <em>bis-sexto-kalendae</em>. This
practice led to the term <em>bissextile</em> being used
to refer to such a <a
href="https://www.britannica.com/science/leap-year-calendar"
class="md-crosslink">leap year</a>. The name leap year
is a later <a
href="https://www.merriam-webster.com/dictionary/connotation"
class="md-dictionary-link md-dictionary-tt-off"
data-term="connotation">connotation</a>, probably
derived from the <a
href="https://www.britannica.com/topic/Old-Norse-language"
class="md-crosslink">Old Norse</a> <em>hlaupa</em>
(“to leap”) and used because, in a bissextile year, any
fixed festival after February leaps forward, falling on
the second weekday from that on which it fell the
previous year, not on the next weekday as it would do in
an ordinary year.</p>
<span class="marker p6"></span><span class="marker AM6
am-inline"></span><span class="marker MOD6 mod-inline"></span>
<p class="topic-paragraph">Apparently, the <span
id="ref313540"></span><a
href="https://www.britannica.com/topic/pontifex"
class="md-crosslink">Pontifices</a> misinterpreted the
edict and inserted the intercalation too frequently. The
error arose because of the Roman practice of inclusive
numbering, so that an intercalation once every fourth
year meant to them intercalating every three years,
because a bissextile year was counted as the first year
of the subsequent four-year period. This error continued
undetected for 36 years, during which period 12 days
instead of nine were added. The emperor <span
id="ref313541"></span><a
href="https://www.britannica.com/biography/Augustus-Roman-emperor"
class="md-crosslink">Augustus</a> then made a
correction by omitting intercalary days between 8 <span
class="text-smallcaps">bce</span> and 8 <span
class="text-smallcaps">ce</span>. As a consequence, it
was not until several decades after its inception that
the Julian calendar came into proper operation, a fact
that is important in <a
href="https://www.britannica.com/topic/chronology"
class="md-crosslink autoxref">chronology</a> but is
all too frequently forgotten.</p>
<span class="marker p7"></span><span class="marker AM7
am-inline"></span><span class="marker MOD7 mod-inline"></span>
<p class="topic-paragraph">It seems that the months of the
Julian calendar were taken over from the Roman
republican calendar but were slightly modified to
provide a more even pattern of numbering. The republican
calendar months of March, <a
href="https://www.britannica.com/topic/May-month"
class="md-crosslink">May</a>, and <a
href="https://www.britannica.com/topic/July"
class="md-crosslink autoxref">Quintilis</a> (<a
href="https://www.britannica.com/topic/July"
class="md-crosslink">July</a>), which had each
possessed 31 days, were retained unaltered. Although
there is some doubt about the specific details, changes
may have occurred in the following way. Except for <a
href="https://www.britannica.com/topic/October-month"
class="md-crosslink">October</a>, all the months that
had previously had only 29 days had either one or two
days added. <a
href="https://www.britannica.com/topic/January"
class="md-crosslink">January</a>, <a
href="https://www.britannica.com/topic/September"
class="md-crosslink">September</a>, and November
received two days, bringing their totals to 31, while <a
href="https://www.britannica.com/topic/April"
class="md-crosslink">April</a>, <a
href="https://www.britannica.com/topic/June"
class="md-crosslink">June</a>, Sextilis (<a
href="https://www.britannica.com/topic/August-month"
class="md-crosslink">August</a>), and December
received one day each, bringing their totals to 30.
October was reduced by one day to a total of 30 days and
February increased to 29 days, or 30 in a bissextile
year. With the exception of February, the scheme
resulted in months having 30 or 31 days alternately
throughout the year. And in order to help farmers,
Caesar issued an <span id="ref313542"></span><a
href="https://www.britannica.com/topic/almanac"
class="md-crosslink">almanac</a> showing on which
dates of his new calendar various seasonal astronomical
phenomena would occur.</p>
<span class="marker p8"></span><span class="marker AM8
am-inline"></span><span class="marker MOD8 mod-inline"></span>
<p class="topic-paragraph">These arrangements for the
months can only have remained in force for a short time,
because in 8 <span class="text-smallcaps">bce</span>
changes were made by Augustus. In 44 <span
class="text-smallcaps">bce</span>, the second year of
the Julian calendar, the Senate proposed that the name
of the month Quintilis be changed to Julius (July), in
honour of Julius Caesar, and in 8 <span
class="text-smallcaps">bce</span> the name of Sextilis
was similarly changed to Augustus (<a
href="https://www.merriam-webster.com/dictionary/August"
class="md-dictionary-link md-dictionary-tt-off"
data-term="August">August</a>). Perhaps because
Augustus felt that his month must have at least as many
days as Julius Caesar’s, February was reduced to 28 days
and August increased to 31. But because this made three
31-day months (July, August, and September) appear in
succession, Augustus is supposed to have reduced
September to 30 days, added a day to October to make it
31 days, reduced November by one day to 30 days, and
increased December from 30 to 31 days, giving the months
the lengths they have today.</p>
<span class="marker p9"></span><span class="marker AM9
am-inline"></span><span class="marker MOD9 mod-inline"></span>
<p class="topic-paragraph">Several scholars, however,
believe that Caesar originally left February with 28
days (in order to avoid affecting certain religious
rites observed in honour of the gods of the netherworld)
and added two days to Sextilis for a total of 31;
January, March, May, Quintilis, October, and December
also had 31 days, with 30 days for April, June,
September, and November. The subsequent change of
Sextilis to Augustus therefore involved no addition of
days to the latter.</p>
<span class="marker p10"></span><span class="marker AM10
am-inline"></span><span class="marker MOD10 mod-inline"></span>
<p class="topic-paragraph">The Julian calendar retained
the Roman republican calendar method of numbering the
days of the month. Compared with the present system, the
Roman numbering seems to run backward, for the first day
of the month was known as the <span id="ref313543"></span>Kalendae,
but subsequent days were not enumerated as so many after
the Kalendae but as so many before the following Nonae
(“nones”), the day called nonae being the ninth day
before the <span id="ref313544"></span>Ides (from <em>iduare</em>,
meaning “to divide”), which occurred in the middle of
the month and were supposed to coincide with the Full
Moon. Days after the Nonae and before the Ides were
numbered as so many before the Ides, and those after the
Ides as so many before the Kalendae of the next month.</p>
<span class="marker p11"></span><span class="marker AM11
am-inline"></span><span class="marker MOD11 mod-inline"></span>
<p class="topic-paragraph">It should be noted that there
were no weeks in the original Julian calendar. The days
were designated either <em>dies fasti</em> or <em>dies
nefasti</em>, the former being business days and days
on which the courts were open; this had been the
practice in the Roman republican calendar. Julius Caesar
designated his additional days all as <em>dies fasti</em>,
and they were added at the end of the month so that
there was no interference with the dates traditionally
fixed for <em>dies comitiales</em> (days on which
public assemblies might be convened) and <em>dies festi</em>
and <em>dies feriae</em> (days for religious festivals
and holy days). Originally, then, the Julian calendar
had a permanent set of dates for administrative matters.
The official introduction of the seven-day <a
href="https://www.britannica.com/science/week"
class="md-crosslink autoxref">week</a> by Emperor <span
id="ref313545"></span><a
href="https://www.britannica.com/biography/Constantine-I-Roman-emperor"
class="md-crosslink">Constantine I</a> in the 4th
century <span class="text-smallcaps">ce</span>
disrupted this arrangement.</p>
<span class="marker p12"></span><span class="marker AM12
am-inline"></span><span class="marker MOD12 mod-inline"></span>
<p class="topic-paragraph">It appears, from the date of
insertion of the intercalary month in the Roman
republican calendar and the habit of designating years
by the names of the consuls, that the calendar year had
originally commenced in March, which was the date when
the new consul took office. In 222 <span
class="text-smallcaps">bce</span> the date of assuming
duties was fixed as March 15, but in 153 <span
class="text-smallcaps">bce</span> it was transferred
to the Kalendae of <span id="ref313546"></span><a
href="https://www.britannica.com/topic/January"
class="md-crosslink">January</a>, and there it
remained. January therefore became the first month of
the year, and in the western region of the <span
id="ref313547"></span><a
href="https://www.britannica.com/place/Roman-Empire"
class="md-crosslink">Roman Empire</a>, this practice
was carried over into the Julian calendar. In the
eastern provinces, however, years were often reckoned
from the accession of the reigning emperor, the second
beginning on the first New Year’s day after the
accession; and the date on which this occurred varied
from one province to another.</p>
<span class="marker p13"></span><span class="marker AM13
am-inline"></span><span class="marker MOD13 mod-inline"></span></section>
</section>
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<div class="topic-content col-sm pr-lg-60"> <span class="marker
before-article"></span>
<section id="ref59348" data-level="2" data-has-spy="true">
<h1 class="h2">The <span id="ref313548"></span><a
href="https://www.britannica.com/topic/Gregorian-calendar"
class="md-crosslink">Gregorian calendar</a></h1>
<p class="topic-paragraph">The <a
href="https://www.britannica.com/science/Julian-calendar"
class="md-crosslink autoxref">Julian calendar</a> year
of 365.25 days was too long, since the correct value for
the tropical year is 365.242199 days. This error of 11
minutes 14 seconds per year amounted to almost one and a
half days in two centuries, and seven days in 1,000 years.
Once again the calendar became increasingly out of phase
with the seasons. From time to time, the problem was
placed before church councils, but no action was taken
because the astronomers who were consulted doubted whether
enough precise information was available for a really
accurate value of the tropical year to be obtained.</p>
<span class="marker p1"></span>
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="127418">
<div class="md-assembly-wrapper" data-type="image"><a
style="--aspect-ratio: 16/9"
href="https://cdn.britannica.com/53/130653-050-532C1F10/clock-St-John-the-Baptist-cathedral-Lyon-2019.jpg"
class="position-relative d-flex align-items-center
justify-content-center media-overlay-link
card-media" data-href="/media/1/89368/127418"><img
src="https://cdn.britannica.com/s:690x388,c:crop/53/130653-050-532C1F10/clock-St-John-the-Baptist-cathedral-Lyon-2019.jpg"
alt="Astronomical clock from the 14th century
that can be used to determine religious feast
days until the year 2019; in the cathedral of
St. John the Baptist, Lyon, France."
data-width="1600" data-height="1071"></a></div>
</figure>
</div>
</div>
</section>
</div>
</div>
</div>
<div class="infinite-scroll-container">
<div class="grid">
<div class="topic-content col-sm pr-lg-60">
<section id="ref59348" data-level="2" data-has-spy="true">
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="127418"><figcaption
class="card-body">
<div class="md-assembly-caption text-muted font-14
font-serif">Astronomical clock from the 14th
century that can be used to determine religious
feast days until the year 2019; in the cathedral
of St. John the Baptist, Lyon, France.</div>
<cite class="credit d-block mt-5">©
Jakez/Shutterstock.com</cite></figcaption></figure>
</div>
</div>
<span class="marker AM1 am-inline"></span><span
class="marker MOD1 mod-inline"></span>
<p class="topic-paragraph">By 1545, however, the <a
href="https://www.britannica.com/science/vernal-equinox"
class="md-crosslink autoxref">vernal equinox</a>, which
was used in determining <a
href="https://www.britannica.com/topic/Easter-holiday"
class="md-crosslink">Easter</a>, had moved 10 days from
its proper date; and in December, when the <span
id="ref313549"></span><a
href="https://www.britannica.com/event/Council-of-Trent"
class="md-crosslink">Council of Trent</a> met for the
first of its sessions, it authorized Pope <span
id="ref313550"></span><a
href="https://www.britannica.com/biography/Paul-III"
class="md-crosslink">Paul III</a> to take action to
correct the error. Correction required a solution,
however, that neither Paul III nor his successors were
able to obtain in satisfactory form until nearly 1572, the
year of election of Pope <span id="ref313551"></span><a
href="https://www.britannica.com/biography/Gregory-XIII"
class="md-crosslink">Gregory XIII</a>. Gregory found
various proposals awaiting him and agreed to issue a bull
that the <a
href="https://www.britannica.com/topic/Jesuits"
class="md-crosslink">Jesuit</a> astronomer <span
id="ref313552"></span><a
href="https://www.britannica.com/biography/Christopher-Clavius"
class="md-crosslink">Christopher Clavius</a> (1537–1612)
began to draw up, using suggestions made by the astronomer
and physician <span id="ref313553"></span>Luigi Lilio
(also known as Aloysius Lilius; died 1576).</p>
<span class="marker p2"></span><span class="marker AM2
am-inline"></span><span class="marker MOD2 mod-inline"></span>
<p class="topic-paragraph">The papal bull <em><span
id="ref793372"></span><a
href="https://www.britannica.com/topic/Inter-gravissimas"
class="md-crosslink">Inter gravissimas</a></em> (“In
the gravest concern”) was issued on February 24, 1582.
First, in order to bring the vernal equinox back to March
21, the day following the Feast of St. Francis (that is,
October 5) was to become October 15, thus omitting 10
days. Second, to bring the year closer to the true
tropical year, a value of 365.2422 days was accepted. This
value differed by 0.0078 days per year from the Julian
calendar reckoning, amounting to 0.78 days per century, or
3.12 days every 400 years. It was therefore <a
href="https://www.merriam-webster.com/dictionary/promulgated"
class="md-dictionary-link md-dictionary-tt-off"
data-term="promulgated">promulgated</a> that three out
of every four centennial years should be common years,
that is, not leap years; and this practice led to the rule
that no centennial years should be leap years unless
exactly divisible by 400. Thus, 1700, 1800, and 1900 were
not leap years, as they would have been in the Julian
calendar, but the year 2000 was. The reform, which
established what became known as the Gregorian calendar
and laid down rules for calculating the date of <span
id="ref313554"></span><a
href="https://www.britannica.com/topic/Easter-holiday"
class="md-crosslink">Easter</a>, was well received by
such astronomers as <a
href="https://www.britannica.com/biography/Johannes-Kepler"
class="md-crosslink">Johannes Kepler</a> and <a
href="https://www.britannica.com/biography/Tycho-Brahe-Danish-astronomer"
class="md-crosslink">Tycho Brahe</a> and by the Catholic
princes of Europe. Many Protestants, however, saw it as
the work of the <a
href="https://www.britannica.com/topic/Antichrist"
class="md-crosslink">Antichrist</a> and refused to adopt
it. Eventually all of Europe, as late as 1918 in the case
of Russia, adopted the Gregorian calendar.</p>
<span class="marker p3"></span><span class="marker AM3
am-inline"></span><span class="marker MOD3 mod-inline"></span><span
id="ref59348" data-level="2"></span>
<section id="ref60230" data-level="3" data-has-spy="true">
<h2 class="h3">The date of Easter</h2>
<p class="topic-paragraph">Easter was the most important
feast of the Christian church, and its place in the
calendar determined the position of the rest of the
church’s movable feasts (<em>see</em> <a
href="https://www.britannica.com/topic/Christianity/Aspects-of-the-Christian-religion#ref67528"
class="md-crosslink">church year</a>). Because its
timing depended on both the Moon’s phases and the vernal
equinox, <a
href="https://www.merriam-webster.com/dictionary/ecclesiastical"
class="md-dictionary-link md-dictionary-tt-off"
data-term="ecclesiastical">ecclesiastical</a>
authorities had to seek some way of <a
href="https://www.merriam-webster.com/dictionary/reconciling"
class="md-dictionary-link md-dictionary-tt-off"
data-term="reconciling">reconciling</a> lunar and
solar calendars. Some simple form of computation, usable
by nonastronomers in remote places, was desirable. There
was no easy or obvious solution, and to make things more
difficult there was no unanimous agreement on the way in
which Easter should be calculated, even in a <a
href="https://www.britannica.com/science/lunar-calendar"
class="md-crosslink autoxref">lunar calendar</a>.</p>
<span class="marker p4"></span><span class="marker AM4
am-inline"></span><span class="marker MOD4 mod-inline"></span>
<p class="topic-paragraph">Easter, being the festival of
the <span id="ref313555"></span><a
href="https://www.britannica.com/topic/resurrection-religion"
class="md-crosslink">Resurrection</a>, had to depend
on the <a
href="https://www.britannica.com/science/dating-geochronology"
class="md-crosslink autoxref">dating</a> of the <a
href="https://www.britannica.com/topic/crucifixion-capital-punishment"
class="md-crosslink">Crucifixion</a>, which occurred
three days earlier and just before the Jewish <span
id="ref313556"></span><a
href="https://www.britannica.com/topic/Passover"
class="md-crosslink">Passover</a>. The Passover was
celebrated on the 14th day of Nisan, the first <a
href="https://www.britannica.com/science/month"
class="md-crosslink autoxref">month</a> in the Jewish
religious year—that is, the lunar month the 14th day of
which falls on or next after the vernal equinox. The
Christian churches in the eastern Mediterranean area
celebrated Easter on the 14th of Nisan on whatever day
of the <a
href="https://www.britannica.com/science/week"
class="md-crosslink autoxref">week</a> it might <a
href="https://www.britannica.com/science/autumn-season"
class="md-crosslink autoxref">fall</a>, but the rest
of Christendom adopted a more elaborate reckoning to
ensure that it was celebrated on a <a
href="https://www.britannica.com/topic/Sunday-day-of-week"
class="md-crosslink autoxref">Sunday</a> in the
Passover week.</p>
<span class="marker p5"></span><span class="marker AM5
am-inline"></span><span class="marker MOD5 mod-inline"></span>
<p class="topic-paragraph">To determine precisely how the
Resurrection and Easter Day should be dated, reference
was made to the <span id="ref313557"></span><a
href="https://www.britannica.com/topic/Gospel-New-Testament"
class="md-crosslink">Gospels;</a> but, even as early
as the 2nd century <span class="text-smallcaps">ce</span>,
difficulties had arisen, because the synoptic Gospels (<a
href="https://www.britannica.com/topic/Gospel-According-to-Matthew"
class="md-crosslink">Matthew</a>, <a
href="https://www.britannica.com/topic/Gospel-According-to-Mark"
class="md-crosslink">Mark</a>, and <a
href="https://www.britannica.com/topic/Gospel-According-to-Luke"
class="md-crosslink">Luke</a>) appeared to give a
different date from the Gospel According to <a
href="https://www.britannica.com/topic/Gospel-According-to-John"
class="md-crosslink">John</a> for the Crucifixion.
This difference led to controversy that was later <a
href="https://www.merriam-webster.com/dictionary/exacerbated"
class="md-dictionary-link md-dictionary-tt-off"
data-term="exacerbated">exacerbated</a> by another
difficulty caused by the Jewish reckoning of a day from
sunset to sunset. The question arose of how the evening
of the 14th day should be calculated, and some—the <span
id="ref313558"></span>Quintodecimans—claimed that it
meant one particular evening, but others—the <span
id="ref313559"></span><a
href="https://www.britannica.com/topic/Quartodecimanism"
class="md-crosslink">Quartodecimans</a>—claimed that
it meant the evening before, since sunset heralded a new
day. Both sides had their protagonists, the Eastern
churches supporting the Quartodecimans, the Western
churches the Quintodecimans. The question was finally
decided by the Western church in favour of the
Quintodecimans, though there is debate whether this was
at the <a
href="https://www.britannica.com/event/First-Council-of-Nicaea-325"
class="md-crosslink">Council of Nicaea</a> in 325 or
later. The <span id="ref313560"></span><a
href="https://www.britannica.com/topic/Eastern-Orthodoxy"
class="md-crosslink">Eastern churches</a> decided to
retain the Quartodeciman position, and the church in
Britain, which had few links with European churches at
this time, retained the Quartodeciman position until
Roman missionaries arrived in the 6th century, when a
change was made. The dating of Easter in the Gregorian
calendar was based on the decision of the Western
church, which decreed that Easter should be celebrated
on the Sunday immediately following the (Paschal) Full
Moon that fell on or after the vernal equinox, which
they took as March 21. The church also ordered that if
this Full Moon fell on a Sunday, the festival should be
held seven days later.</p>
<span class="marker p6"></span><span class="marker AM6
am-inline"></span><span class="marker MOD6 mod-inline"></span>
<p class="topic-paragraph">With these provisions in mind,
the problem could be broken down into two parts: first,
devising a simple but effective way of calculating the
days of the week for any date in the year and, second,
determining the date of the Full Moons in any year. The
first part was solved by the use of a letter code
derived from a similar Roman system adopted for
determining market days. For ecclesiastical use, the
code gave what was known as the Sunday, or <span
id="ref313561"></span><a
href="https://www.britannica.com/science/dominical-letter"
class="md-crosslink">dominical, letter</a>.</p>
<span class="marker p7"></span><span class="marker AM7
am-inline"></span><span class="marker MOD7 mod-inline"></span>
<p class="topic-paragraph">The seven letters A through G
are each assigned to a day, consecutively from January 1
so that January 1 appears as A, January 2 as B, to
January 7 which appears as G, the cycle then continuing
with January 8 as A, January 9 as B, and so on. Then in
any year the first Sunday is bound to be assigned to one
of the letters A–G in the first cycle, and all Sundays
in the year possess that dominical letter. For example,
if the first Sunday falls on January 3, <a
href="https://www.britannica.com/science/coulomb"
class="md-crosslink autoxref">C</a> will be the
dominical letter for the whole year. No dominical letter
is placed against the <span id="ref313562"></span><a
href="https://www.britannica.com/science/leap-year-calendar"
class="md-crosslink">intercalary day</a>, February 29,
but, since it is still counted as a weekday and given a
name, the series of letters moves back one day every <a
href="https://www.britannica.com/science/leap-year-calendar"
class="md-crosslink autoxref">leap year</a> after <a
href="https://www.britannica.com/science/intercalation"
class="md-crosslink autoxref">intercalation</a>. Thus,
a leap year beginning with the dominical letter C will
change to a year with the dominical letter B on March 1;
and in lists of dominical letters, all leap years are
given a double letter notation, in the example just
quoted, CB. It is not difficult to see what dominical
letter or letters apply to any particular year, and it
is also a comparatively simple matter to draw up a table
of dominical letters for use in determining Easter
Sunday. The possible dates on which Easter Sunday can
fall are written down—they run from March 22 through
April 25—and against them the dominical letters for a
cycle of seven years. Once the dominical letter for a
year is known, the possible Sundays for celebrating
Easter can be read directly from the table. This system
does not, of course, completely determine Easter; to do
so, additional information is required.</p>
<span class="marker p8"></span><span class="marker AM8
am-inline"></span><span class="marker MOD8 mod-inline"></span>
<p class="topic-paragraph">This must provide dates for <span
id="ref313563"></span><a
href="https://www.britannica.com/science/full-Moon-lunar-phase"
class="md-crosslink">Full Moons</a> throughout the
year, and for this a lunar cycle like the <a
href="https://www.britannica.com/science/Metonic-cycle"
class="md-crosslink">Metonic cycle</a> was originally
used. Tables were prepared, again using the range of
dates on which Easter Sunday could appear, and against
each date a number from one through 19 was placed. This
number indicated which of the 19 years of the lunar
cycle would give a Full Moon on that day. From <a
href="https://www.merriam-webster.com/dictionary/medieval"
class="md-dictionary-link md-dictionary-tt-off"
data-term="medieval">medieval</a> times these were
known as <span id="ref313564"></span><a
href="https://www.britannica.com/science/golden-number"
class="md-crosslink">golden numbers</a>, possibly from
a name used by the Greeks for the numbers on the Metonic
cycle or because gold is the colour used for them in
manuscript calendars.</p>
<span class="marker p9"></span><span class="marker AM9
am-inline"></span><span class="marker MOD9 mod-inline"></span>
<p class="topic-paragraph">The system of golden numbers
was introduced in 530, but the numbers were arranged as
they should have been if adopted at the Council of
Nicaea two centuries earlier; and the cycle was taken to
begin in a year when the <a
href="https://www.britannica.com/topic/New-Moon-Jewish-festival"
class="md-crosslink autoxref">New Moon</a> fell on
January 1. Working backward, chronologers found that
this date had occurred in the year preceding 1 <span
class="text-smallcaps">ce</span>, and therefore the <a
href="https://www.britannica.com/science/golden-number"
class="md-crosslink autoxref">golden number</a> for
any year is found by adding one to the year and dividing
that sum by 19. The golden number is the remainder or,
if there is no remainder, 19.</p>
<span class="marker p10"></span><span class="marker AM10
am-inline"></span><span class="marker MOD10 mod-inline"></span>
<p class="topic-paragraph">To compute the date of Easter,
the medieval chronologer computed the golden number for
the year and then consulted his table to see by which
date this number lay. Having found this date, that of
the first Full Moon after March 20, he consulted his
table of dominical letters and saw the next date against
which the dominical letter for that year appeared; this
was the Sunday to be designated Easter. The method,
modified for dropping centennial leap years as practiced
in the Gregorian calendar, is still given in the English
prayer book, although it was officially discarded when
the Gregorian calendar was introduced.</p>
<span class="marker p11"></span><span class="marker AM11
am-inline"></span><span class="marker MOD11 mod-inline"></span>
<p class="topic-paragraph">The system of golden numbers
was eventually rejected because the astronomical Full
Moon could differ by as much as two days from the date
they indicated. It was Lilius who had proposed a more
accurate system based on one that had already been in
use unofficially while the Julian calendar was still in
force. Called the <span id="ref313565"></span>epact—the
word is derived from the Greek <em>epagein</em>,
meaning “to intercalate”—this was again a system of
numbers concerned with the Moon’s phases, but now
indicating the age of the Moon on the first day of the
year, from which the age of the Moon on any day of the
year may be found, at least approximately, by counting,
using alternately months of 29 and 30 days.</p>
<span class="marker p12"></span><span class="marker AM12
am-inline"></span><span class="marker MOD12 mod-inline"></span>
<p class="topic-paragraph">The epact as previously used
was not, however, completely accurate because, like the
golden number, it had been based on the Metonic cycle.
This 19-year cycle was in error, the discrepancy
amounting to eight days every 2,500 years. A one-day
change on certain centennial years was then instituted
by making the computed age of the Moon one day later
seven times, at 300-year intervals, and an eighth time
after a subsequent 400 years. This operation was known
as the lunar correction, but it was not the only
correction required; there was another.</p>
<span class="marker p13"></span><span class="marker AM13
am-inline"></span><span class="marker MOD13 mod-inline"></span>
<p class="topic-paragraph">Because the Gregorian calendar
used a more accurate value for the tropical year than
the Julian calendar and achieved this by omitting most
centennial leap years, <span id="ref313566"></span><a
href="https://www.britannica.com/biography/Christopher-Clavius"
class="md-crosslink">Clavius</a> decided that, when
the cycle of epacts reached an ordinary centennial year,
the number of the epact should be reduced by one; this
reduction became known as the solar correction.</p>
<span class="marker p14"></span><span class="marker AM14
am-inline"></span><span class="marker MOD14 mod-inline"></span>
<p class="topic-paragraph">One advantage of the epact
number was that it showed the age of the Moon on January
1 and so permitted a simple calculation of the dates of
New Moon and Full Moon for the ensuing year. Another was
that it lent itself to the construction of cycles of 30
epact numbers, each diminishing by one from the previous
cycle, so that, when it became necessary at certain
centennial years to shift from one cycle to another,
there would still be a cycle ready that retained a
correct relationship between dates and New Moons.</p>
<span class="marker p15"></span><span class="marker AM15
am-inline"></span><span class="marker MOD15 mod-inline"></span>
<p class="topic-paragraph">For determining Easter, a table
was prepared of the golden numbers, one through 19, and
below them the cycles of epacts for about 7,000 years;
after this time, all the epact cycles are repeated. A
second table was then drawn up, giving the dates of
Easter Full Moons for different epact numbers. Once the
epact for the year was known, the date of the Easter
Full Moon could be immediately obtained, while
consultation of a table of dominical letters showed
which was the next Sunday. Thus, the Gregorian system of
epacts, while more accurate than the old golden numbers,
still forced the chronologer to consult complex
astronomical tables.</p>
<span class="marker p16"></span><span class="marker AM16
am-inline"></span><span class="marker MOD16 mod-inline"></span></section>
<span id="ref59348" data-level="2"></span>
<section id="ref60231" data-level="3" data-has-spy="true">
<h2 class="h3">Adoption in various countries</h2>
<p class="topic-paragraph">The derivation of the term <em>style</em>
for a type of calendar seems to have originated sometime
soon after the 6th century as a result of developments
in calendar computation in the previous 200 years. In
463 <span class="text-smallcaps">ce</span> <span
id="ref313567"></span><a
href="https://www.britannica.com/biography/Victorius-of-Aquitaine"
class="md-crosslink">Victorius</a> (or Victorinus) of
Aquitaine, who had been appointed by Pope Hilarius to
undertake calendar revision, devised the Great Paschal
(i.e., Passover) period, sometimes later referred to as
the Victorian period. It was a combination of the <span
id="ref664719"></span><a
href="https://www.britannica.com/place/Sun"
class="md-crosslink">solar</a> cycle of 28 years and
the Metonic 19-year cycle, bringing the Full <span
id="ref664716"></span><a
href="https://www.britannica.com/place/Moon"
class="md-crosslink">Moon</a> back to the same day of
the month, and amounted to 28 × 19, or 532 years. In the
6th century this period was used by <span
id="ref313571"></span><a
href="https://www.britannica.com/biography/Dionysius-Exiguus"
class="md-crosslink">Dionysius Exiguus</a> (Denis the
Little) in computing the date of Easter, because it gave
the day of the week for any day in any year, and so it
also became known as the <span id="ref313568"></span><a
href="https://www.britannica.com/science/Dionysian-period"
class="md-crosslink">Dionysian period</a>. Dionysius
took the year now called 532 <span
class="text-smallcaps">ce</span> as the first year of
a new Great Paschal period and the year now designated 1
<span class="text-smallcaps">bce</span> as the beginning
of the previous cycle. In the 6th century it was the
general belief that this was the year of Christ’s birth,
and because of this Dionysius introduced the concept of
numbering years consecutively through the <span
id="ref313572"></span>Christian era. The method was
adopted by some scholars but seems only to have become
widely used after its popularization by the Venerable <span
id="ref313573"></span><a
href="https://www.britannica.com/biography/Saint-Bede-the-Venerable"
class="md-crosslink">Bede of Jarrow</a> (673?–735),
whose reputation for scholarship was very high in
Western <span id="ref313574"></span><a
href="https://www.britannica.com/topic/Christianity"
class="md-crosslink">Christendom</a> in the 8th
century. This system of <span class="text-smallcaps">bce</span>/<span
class="text-smallcaps">ce</span> numbering threw into
relief the different practices, or styles, of reckoning
the beginning of the year then in use. When the
Gregorian calendar firmly established January 1 as the
beginning of its year, it was widely referred to as the
New Style calendar, with the Julian the Old Style
calendar. In Britain, under the Julian calendar, the
year had first begun on December 25 and then, from the
14th century onward, on March 25.</p>
<span class="marker p17"></span><span class="marker AM17
am-inline"></span><span class="marker MOD17 mod-inline"></span>
<p class="topic-paragraph">Because of the division of the
Eastern and Western Christian churches and of <span
id="ref793396"></span><a
href="https://www.britannica.com/topic/Protestantism"
class="md-crosslink">Protestants</a> and Roman
Catholics, the obvious advantages of the Gregorian
calendar were not accepted everywhere, and in some
places adoption was extremely slow. In France, Italy,
Luxembourg, Portugal, and Spain, the New Style calendar
was adopted in 1582, and it was in use by most of the
German Roman Catholic states as well as by Belgium and
part of the Netherlands by 1584. Switzerland’s change
was gradual, on the other <a
href="https://www.britannica.com/science/hand-measurement"
class="md-crosslink autoxref">hand</a>, beginning in
1583 and being completed only in 1812. Hungary adopted
the New Style in 1587, and then there was a pause of
more than a century before the first Protestant
countries made the transition from the Old Style
calendar. In 1699–1700, Denmark and the Dutch and German
Protestant states embraced the New Style, although the
Germans declined to adopt the rules laid down for
determining Easter. The Germans preferred to rely
instead on astronomical tables and specified the use of
the <em>Tabulae Rudolphinae</em> (1627; “Rudolphine
Tables”), based on the 16th-century observations of <span
id="ref313575"></span><a
href="https://www.britannica.com/biography/Tycho-Brahe-Danish-astronomer"
class="md-crosslink">Tycho Brahe</a>. They acceded to
the Gregorian calendar rules for Easter only in 1776.
Britain adopted the New Style in 1752 and Sweden in
1753, although the Swedes, because they had in 1740
followed the German Protestants in using their <span
id="ref313576"></span><a
href="https://www.britannica.com/science/astronomy"
class="md-crosslink">astronomical</a> methods for
determining Easter, declined to adopt the Gregorian
calendar rules until 1844. <a
href="https://www.britannica.com/place/Japan"
class="md-crosslink autoxref">Japan</a> adopted the
New Style in 1873; Egypt adopted it in 1875; and between
1912 and 1917 it was accepted by Albania, Bulgaria,
China, Estonia, Latvia, Lithuania, Romania, and Turkey.
The now-defunct <a
href="https://www.britannica.com/place/Soviet-Union"
class="md-crosslink autoxref">Soviet Union</a> adopted
the New Style in 1918, and Greece in 1923.</p>
<span class="marker p18"></span><span class="marker AM18
am-inline"></span><span class="marker MOD18 mod-inline"></span>
<p class="topic-paragraph">In Britain and the British
dominions, the change was made when the difference
between the New and Old Style calendars amounted to 11
days: the lag was covered by naming the day after
September 2, 1752, as September 14, 1752. There was
widespread misunderstanding among the public, however,
even though legislation authorizing the change had been
framed to avoid injustice and financial hardship. The
Alaskan territory retained the Old Style calendar until
1867, when it was transferred from Russia to the United
States.</p>
<span class="marker p19"></span><span class="marker AM19
am-inline"></span><span class="marker MOD19 mod-inline"></span></section>
</section>
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<section id="ref" data-level="1">
<section id="ref59349" data-level="2" data-has-spy="true">
<h1 class="h2">Calendar reform since the mid-18th century</h1>
<span id="ref59349" data-level="2"></span>
<section id="ref60232" data-level="3" data-has-spy="true">
<h2 class="h3">The <span id="ref313578"></span><a
href="https://www.britannica.com/science/French-republican-calendar"
class="md-crosslink">French republican calendar</a></h2>
<p class="topic-paragraph">In late 18th-century France,
with the approach of the <span id="ref313577"></span><a
href="https://www.britannica.com/event/French-Revolution"
class="md-crosslink">French Revolution</a>, demands
began to be made for a radical change in the civil
calendar that would divorce it completely from any <a
href="https://www.merriam-webster.com/dictionary/ecclesiastical"
class="md-dictionary-link md-dictionary-tt-off"
data-term="ecclesiastical">ecclesiastical</a>
connections. The first attacks on the <a
href="https://www.britannica.com/topic/Gregorian-calendar"
class="md-crosslink autoxref">Gregorian calendar</a>
and proposals for reform came in 1785 and 1788, the
changes being primarily designed to <a
href="https://www.merriam-webster.com/dictionary/divest"
class="md-dictionary-link md-dictionary-tt-off"
data-term="divest">divest</a> the calendar of all
its Christian associations. After the storming of the
<a href="https://www.britannica.com/topic/Bastille"
class="md-crosslink">Bastille</a> in July 1789,
demands became more <a
href="https://www.merriam-webster.com/dictionary/vociferous"
class="md-dictionary-link md-dictionary-tt-off"
data-term="vociferous">vociferous</a>, and a new
calendar, to start from “the first year of liberty,”
was widely spoken about. In 1793 the <a
href="https://www.britannica.com/topic/National-Convention"
class="md-crosslink autoxref">National Convention</a>
appointed <span id="ref313579"></span>Charles-Gilbert
Romme, president of the committee of public
instruction, to take charge of the reform. Technical
matters were entrusted to the mathematicians <span
id="ref313580"></span><a
href="https://www.britannica.com/biography/Joseph-Louis-Lagrange-comte-de-lEmpire"
class="md-crosslink">Joseph-Louis Lagrange</a> and <span
id="ref313581"></span><a
href="https://www.britannica.com/biography/Gaspard-Monge-comte-de-Peluse"
class="md-crosslink">Gaspard Monge</a> and the
renaming of the months to the Paris deputy to the
convention, Philippe <span id="ref313582"></span><a
href="https://www.britannica.com/biography/Philippe-Fabre-dEglantine"
class="md-crosslink">Fabre d’Églantine</a>. The
results of their deliberations were submitted to the
convention in September of the same year and were
immediately accepted, it being <a
href="https://www.merriam-webster.com/dictionary/promulgated"
class="md-dictionary-link md-dictionary-tt-off"
data-term="promulgated">promulgated</a> that the new
calendar should become law on October 5.</p>
<span class="marker p1"></span><span class="marker AM1
am-inline"></span><span class="marker MOD1 mod-inline"></span>
<p class="topic-paragraph">The <a
href="https://www.britannica.com/science/French-republican-calendar"
class="md-crosslink">French republican calendar</a>,
as the reformed system came to be known, was taken to
have begun on September 22, 1792, the day of the
proclamation of the Republic and, in that year, the
date also of the autumnal <span id="ref313583"></span><a
href="https://www.britannica.com/science/equinox-astronomy"
class="md-crosslink">equinox</a>. The total number
of days in the year was fixed at 365, the same as in
the Julian and Gregorian calendars, and this was
divided into 12 months of 30 days each, the remaining
five days at year’s end being devoted to festivals and
vacations. These were to <a
href="https://www.britannica.com/science/autumn-season"
class="md-crosslink autoxref">fall</a> between
September 17 and 22 and were specified, in order, to
be festivals in honour of virtue, genius, labour,
opinion, and rewards. In a leap year an extra festival
was to be added—the festival of the Revolution. Leap
years were retained at the same frequency as in the
Gregorian calendar, but it was enacted that the first
leap year should be year 3, not year 4 as it would
have been if the Gregorian calendar had been followed
precisely in this respect. Each four-year period was
to be known as a <em>Franciade</em>.</p>
<span class="marker p2"></span><span class="marker AM2
am-inline"></span><span class="marker MOD2 mod-inline"></span>
<p class="topic-paragraph">The seven-day <a
href="https://www.britannica.com/science/week"
class="md-crosslink autoxref">week</a> was
abandoned, and each 30-day month was divided into
three periods of 10 days called <em><span
id="ref313585"></span><a
href="https://www.britannica.com/topic/decade-French-chronology"
class="md-crosslink">décades</a></em>, the last
day of a <em>décade</em> being a rest day. It was
also agreed that each day should be divided into
decimal parts, but this was not popular in practice
and was allowed to fall into disuse.</p>
<span class="marker p3"></span><span class="marker AM3
am-inline"></span><span class="marker MOD3 mod-inline"></span>
<p class="topic-paragraph">The months themselves were
renamed so that all previous associations should be
lost, and Fabre d’Églantine chose descriptive names as
follows (the descriptive nature and corresponding
Gregorian calendar dates for years 1, 2, 3, 5, 6, and
7 are given in parentheses):</p>
<span class="marker p4"></span><span class="marker AM4
am-inline"></span><span class="marker MOD4 mod-inline"></span>
<ul class="list-unstyled topic-list">
<li>
<div><a
href="https://www.britannica.com/topic/Vendemiaire"
class="md-crosslink autoxref">Vendémiaire</a>
(“vintage,” September 22 to October 21), </div>
</li>
<li>
<div>Brumaire (“mist,” October 22 to November 20), </div>
</li>
<li>
<div>Frimaire (“frost,” November 21 to December 20),
</div>
</li>
<li>
<div>Nivôse (“snow,” December 21 to January 19), </div>
</li>
<li>
<div>Pluviôse (“rain,” January 20 to February 18), </div>
</li>
<li>
<div>Ventôse (“wind,” February 19 to March 20), </div>
</li>
<li>
<div>Germinal (“seedtime,” March 21 to April 19), </div>
</li>
<li>
<div>Floréal (“blossom,” April 20 to May 19), </div>
</li>
<li>
<div>Prairial (“meadow,” May 20 to June 18), </div>
</li>
<li>
<div>Messidor (“harvest,” June 19 to July 18), </div>
</li>
<li>
<div>Thermidor (“heat,” July 19 to <a
href="https://www.merriam-webster.com/dictionary/August"
class="md-dictionary-link md-dictionary-tt-off"
data-term="August">August</a> 17), and </div>
</li>
<li>
<div>Fructidor (“fruits,” August 18 to September
16).</div>
</li>
</ul>
<p class="topic-paragraph">The French republican
calendar was short-lived, for while it was
satisfactory enough internally, it clearly made for
difficulties in communication abroad because its
months continually changed their relationship to dates
in the Gregorian calendar. In September 1805, under
the Napoleonic regime, the calendar was virtually
abandoned, and on January 1, 1806, it was replaced by
the Gregorian calendar.</p>
<span class="marker p5"></span><span class="marker AM5
am-inline"></span><span class="marker MOD5 mod-inline"></span></section>
<span id="ref59349" data-level="2"></span>
<section id="ref60233" data-level="3" data-has-spy="true">
<h2 class="h3">Soviet calendar reforms</h2>
<p class="topic-paragraph">When Soviet Russia undertook
its calendar reform in February 1918, it merely moved
from the <a
href="https://www.britannica.com/science/Julian-calendar"
class="md-crosslink autoxref">Julian calendar</a> to
the Gregorian. This move resulted in a loss of 13
days, so that February 1, 1918, became February 14.</p>
<span class="marker p6"></span><span class="marker AM6
am-inline"></span><span class="marker MOD6 mod-inline"></span></section>
<span id="ref59349" data-level="2"></span>
<section id="ref60234" data-level="3" data-has-spy="true">
<h2 class="h3">Modern schemes for reform</h2>
<p class="topic-paragraph">The current calendar is not
without defects, and reforms are still being proposed.
Astronomically, it really calls for no improvement,
but the seven-day week and the different lengths of
months are unsatisfactory to some. Clearly, if the
calendar could have all festivals and all rest days
fixed on the same dates every year, as in the original
Julian calendar, this arrangement would be more
convenient, and two general schemes have been put
forward—the <span id="ref313586"></span>International
Fixed Calendar and the World Calendar.</p>
<span class="marker p7"></span>
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="127408">
<div class="md-assembly-wrapper" data-type="image"><a
style="--aspect-ratio: 16/9"
href="https://cdn.britannica.com/55/130655-050-E22E92E4/calendar-range.jpg"
class="position-relative d-flex
align-items-center justify-content-center
media-overlay-link card-media"
data-href="/media/1/89368/127408"><img
src="https://cdn.britannica.com/s:690x388,c:crop/55/130655-050-E22E92E4/calendar-range.jpg"
alt="A perpetual calendar makes it possible
to find the correct day of the week for any
date over a wide range of years."
data-width="1600" data-height="1200"></a></div>
</figure>
</div>
</div>
</section>
</section>
</section>
</div>
</div>
</div>
<div class="assemblies">
<div class="">
<figure class="md-assembly card print-false"
data-assembly-id="127408"><figcaption class="card-body">
<div class="md-assembly-caption text-muted font-14
font-serif">A perpetual calendar makes it possible to find
the correct day of the week for any date over a wide range
of years.</div>
<cite class="credit d-block mt-5">© Dan
Tataru/Shutterstock.com</cite></figcaption></figure>
</div>
</div>
<span class="marker AM7 am-inline"></span><span class="marker MOD7
mod-inline"></span>
<p class="topic-paragraph">The International Fixed Calendar is
essentially a perpetual Gregorian calendar, in which the year is
divided into 13 months, each of 28 days, with an additional day at
the end. Present month names are retained, but a new month named
Sol is intercalated between June and July. The additional day
follows December 28 and bears no <a
href="https://www.merriam-webster.com/dictionary/designation"
class="md-dictionary-link md-dictionary-tt-off"
data-term="designation">designation</a> of month date or weekday
name, while the same would be true of the day intercalated in a <span
id="ref313584"></span><a
href="https://www.britannica.com/science/leap-year-calendar"
class="md-crosslink">leap year</a> after June 28. In this
calendar, every month begins on a <a
href="https://www.britannica.com/topic/Sunday-day-of-week"
class="md-crosslink autoxref">Sunday</a> and ends on a <a
href="https://www.britannica.com/topic/Saturday-day"
class="md-crosslink autoxref">Saturday</a>.</p>
<span class="marker p8"></span><span class="marker AM8 am-inline"></span><span
class="marker MOD8 mod-inline"></span>
<p class="topic-paragraph">It is claimed that the proposed
International Fixed Calendar does not conveniently divide into
quarters for business reckoning; and the <span id="ref313587"></span><a
href="https://www.britannica.com/topic/world-calendar"
class="md-crosslink">World Calendar</a> is designed to remedy
this deficiency, being divided into four quarters of 91 days each,
with an additional day at the end of the year. In each quarter,
the first month is of 31 days and the second and third of 30 days
each. The extra day comes after December 30 and bears no month or
weekday designation, nor does the intercalated leap year day that
follows June 30. In the World Calendar January 1, April 1, July 1,
and October 1 are all Sundays. Critics point out that each month
extends over part of five weeks, and each <span id="ref313588"></span><a
href="https://www.britannica.com/science/month"
class="md-crosslink">month</a> within a given quarter begins on
a different <span id="ref664715"></span><a
href="https://www.britannica.com/science/day"
class="md-crosslink">day</a>. Nevertheless, both these proposed
reforms seem to be improvements over the present system that
contains so many variables.</p>
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