In our everyday world, a calendar is generally something that
you hang on the wall, and by which you count off the days until some birthday,
a vacation, or the next public holiday. A calendar is a much bigger concept
than this, though, and it affects both genealogists and historians in ways that
we don’t like to think about since they complicate our worldview.
So what is a calendar? A calendar is a mechanism by which
dates are reckoned in a given culture. Historically, that meant that it allowed
the passing of days to be recorded and so the return of the seasons or
astronomical phenomena to be predicted.
There are six principal calendars in current use: Gregorian,
Jewish, Islamic, Indian, Chinese, and Julian,[1]
but a list of many historical calendars can be found at: List of Calendars.
If we encounter a source with a date expressed according to one of these
calendars then how should we represent it? Before I try and answer that, I want
to give a small tour to illustrate how complex the subject is.
Calendars can be based on natural cycles, such as
astronomical events, or contrived (man-made) cycles. The main astronomical
systems are:[2]
Lunar. Calendars based on the counting of lunations: cycles of the
phases of the moon. The average lunation is now known to be 29.530589 days, but
historically it wasn’t known to that level of precision. Addition of an extra
day, to get things back in step, might have been done on ad hoc basis rather than according to some rigid rule. Other
problems — other than the natural fluctuations, and observing phase changes in
bad weather — include the fact that the first instant of a new phase depends on
both the latitude and longitude of the observer. The Islamic calendar is the
only modern-day example.
Lunisolar. Calendars where the cycles of the moon and of the year
(i.e. seasons) were combined and extra months occasionally added to keep them
synchronised — sometimes relying on an arbitrary decision by a local priest
rather than a formulaic approach. The Jewish, Chinese, Japanese, and Indian calendars
are examples.
Solar. Calendars based entirely on the cycle of the sun, and
abandoning those of the moon. Although our modern Gregorian calendar has
months, these are not tied to the phases of the moon and so it is a solar
calendar rather than a lunisolar one. Other examples include the Egyptian
calendar, which had a fixed 365-day so-called “wandering” year, and the Julian
calendar which had a fixed 365.25-day year.
The root of many problems with calendars based on natural
cycles is that there is no fixed integral relationship between the periods of
astronomical events such as the rotation of the earth about its axis (a day),
the phases of the moon (a month), and the rotation of the earth about the sun
(a year): their relationships are both fractional and continually varying. In
fact, we cannot even say that the length of a day now is the same as it was at
some point in prehistory because there was no common yardstick by which to directly
compare them.[3] We can
extrapolate our understanding of the motions within our solar system back to
ancient times but they’re the predictable motions. If some asteroid had once
passed close by the earth then it could have had a significant effect on the
mean solar day. We’re currently aware of these fractional relationships by
virtue of the leap-year where an
extra day is inserted into our calendar every four years — except if the year
is a multiple of 100, and it is not a multiple of 400. This process of adding a
day (in general: intercalation) is
simply trying to keep our notions of a day
and a year in step.
Although several instances of contrived cycles have once
existed, our most familiar modern-day example is the seven-day week.
The years themselves may be counted from the beginning of some
reign or from some important or regular cultural event. Years counted according
to the reign of some sovereign, monarch, or pope are termed regnal years. Even
now, Acts of Parliament, in England, might be dated such as 3 Elizabeth II,
meaning: third year of the reign of Elizabeth II. In ancient Greece, years were
sometimes numbered according to the Olympiad: the four-yearly period between
their successive games. The Japanese nengo system counts
years according to a number of eras, originally determined by court officials
but later determined by the accession of emperors (similar to the old Chinese
dynasties). Our own modern year numbering is according to the Christian Era
(AD/BC), also known as the Common Era (CE/BCE), although this has only a single
epoch. Years in the French Republican
calendar began with the Republican Era: the first year of the republic. Years
may sometimes be named rather than numbered, as with Roman ones based on the
name of a consul, or Greek ones based on the name of an archon. Knowledge of
the associated epochs — the starting points for the counting — in some
alternative calendar system is therefore a prerequisite for accurate conversion
to it. This knowledge becomes less known as the importance of the epoch becomes
more minor. In England, the records of manors held by the church sometimes used
Episcopal or abbatial years. Even though the sequence of bishops or abbots
is generally known, their exact dates may not be — especially in the case of
minor abbots who did not sit in the House of Lords.
What the majority of the world uses today is known as the
Gregorian calendar, named after Pope Gregory XIII who introduced it via a papal
bull in 1582 as a replacement for the Julian calendar. The Julian calendar used
a fixed year of 365.25 days, but the average tropical year is more
like 365.24219 days, and that meant that events were drifting very slightly behind
each year. A major problem was that Easter was defined by the First Council of
Nicaea (AD 325) in relation to the spring equinox, and after nearly 13
centuries of the overly-long year its date had drifted backwards by about 10
days of where it used to be. As well as setting a new average calendar year of
365.2425 days, the Gregorian calendar removed 10 days in order to move Easter
back to where it was before.
I don’t want to dwell too long on this Julian-to-Gregorian transition
during this particular post (I’ll cover that another time) as there are many
other calendar systems. I want to illustrate the complexities of those calendar
systems, and explain how converting between them is not an exact science. If we
allow our modern Gregorian calendar to be used in a proleptic fashion, where it
can be used for dates prior to its invention, then we find that there is some
uncertainty in translating dates from those other systems to our modern system.
The Julian
calendar was introduced by Julius Caesar in 46 BC as a reform of the earlier
Roman calendar, supposedly
introduced when Rome was founded by Romulus in about 753 BC. The Roman calendar
was a lunar calendar with 10 months (December being the 10th month)
and a year of 304 days. This was unworkable for farmers who needed to be more
aware of the approaching seasons, and so Numa Pompilius (the second king of
Rome), in about 713 BC, introduced two more months, thus making the year 354
days (12 x 29.5 days), although an extra day was added to make it 355 for
superstitious reasons. This still drifted with respect to the solar year and so
several schemes were devised to try and improve it using intercalary days or
months. However, the decision of when
and how-much were usually in the
hands of the priests who shamelessly misused that power — when they hadn’t
neglected the actual need — for political reasons (e.g. changing the term of
someone’s office) or financial advantage (e.g. taxes, rents). When the Julian
calendar was introduced, there were so many corrections to put things back in
order that 46 BC was a year of some 445 days, often referred to as “the year of
confusion”. In effect, there are many calendar variations here, and some rather
irregular intercalations.[4]
The Revised Julian
calendar was a variation conceived in 1923 that allowed the years of the
Julian calendar (still used by the Eastern Orthodox Church) and Gregorian
calendars to remain in step — at least until the year 2799.
Although often overlooked, the extra day in leap years of
the Gregorian calendar was originally achieved by repeating the 24th
February; a practice inherited from the Julian calendar. It wasn’t until 1662
that it was achieved by adding an extra day at the end of February. Today, a residual
repercussion of this is a difference in the date of the feast of St. Matthias during
leap years as celebrated by the Catholic and Anglican churches.[5]
Earlier still, different Christian churches in Romanised Britain celebrated
Easter one day apart.[6] In
effect, knowing the name of a celebration or festival doesn’t uniquely
determine its date.
The Islamic calendar is the only surviving lunar calendar.
It consists of 12 months of alternating 29 and 30 days over a 30-year cycle,
except in embolismic years where an intercalary day is added at the end of the
12th month. There are 11 such intercalations and so each 30-year
cycle has 10631 days (30 x 12 x 29.5 + 11), and this keeps the month in
synchronisation with the moon. There are some variations of the actual years in
which the intercalation occurs but it is essentially a rule-based calendar. It
is sometimes called the Tabular Islamic
calendar in order to distinguish it from the “popular” Islamic calendar
where the actual start of each month is determined by a religious authority
based on the first visibility of the crescent lunulae of the new moons. This
empirical approach is obviously problematic as the appearance depends upon the
weather and geographical coordinates, and the date can be one or two days
different from the calculated one. It can therefore vary from one part of Islam
to another.[7]
India’s calendars are particularly troublesome as they are “…intricate,
complex, and subject to numerous local variations”.[8] Most
are lunisolar but there are solar calendars too. When India became independent,
in 1947, their first Prime Minister, Jawaharlal Nehru, set about a number of
reforms, and one of these involved the Indian Calendar Reform Committee,
appointed in 1952. They found that there were over 30 well-developed calendars
in use across India, and they set about creating a unified Indian national
calendar, which was adopted on 22 Mar 1957. However, India’s diverse
population meant that the Gregorian calendar was still used (by Christians and for
administration), and also the Islamic calendar. Indian and Gregorian dates are therefore
presented side-by-side by The Gazette of India, in news broadcasts by All India
Radio, and in calendars and communications issued by the Government of India.
In the older Indian calendars, the variations of the
lunisolar ones included different month names, the date of the start of the
year, the phase of the moon on which the year or month started, intercalation
rules, and the era from which years were counted, thus making any attempt at
general conversion to Gregorian dates “futile”.[9] However,
the solar calendars, used in parts of Bengal and Madras, “were susceptible to
almost infinite variations”, thus making it very difficult to convert an Indian
date to a Gregorian one without specific knowledge of the calendar and locality.[10]
A similar story of innumerable variations applies to the
Chinese calendars.
What I wanted to emphasise in this first post is that
conversion from these other calendars to our Gregorian one is not simply a
matter of looking at the written form and then doing some arithmetic. Even when
the calendar system was rule-based, rather than dependent upon observation or ad hoc decisions, the conversion may
need information about the actual calendar variant being used, their location, their
religion, etc.
If an historian recorded a length in cubits then a very similar
situation would arise since there were different cubit definitions used in
different places. A write-up may give an indication of what that length might
have been in feet (or metres) but the converted value would need
substantiating, and it wouldn’t be a direct replacement for the original
information.
To genealogists, a more familiar case might be how we handle
dates and ages in our sources. Even if we encounter a date in our own calendar
then it may be incomplete, or it may be secondary information. Although many
genealogists treat them as “facts”, there’s no such thing in principle. With
ages then we should be seeing the issue more clearly: we certainly don’t perform
a simple subtraction of the age from the date of recording, and then use that
date as a “fact”. Someone may have lied about their age, or it may have been
age-next-birthday rather than age-last-birthday (as in early Canadian
censuses), or it may have been rounded down (as in 1841 census of England and
Wales). The essential point is that the calculated date is not information that
was in the consulted source; it is derived from it, and it may need more
information in order to make that derivation. Any calendar conversion is also a
calculation, and similarly different from source information.
In Part II of this post, I want to look at the representation
of dates from other calendars, and why a general scheme should be important to
us.
[1]
“Introduction to Calendars”, USNO (http://aa.usno.navy.mil/faq/docs/calendars.php
: accessed 10 Jul 2015), first paragraph.
[2] E. G.
Richards, Mapping Time: The Calendar and
its History (1998; reprint, Oxford University Press, 2005), pp. 92–97.
[3] An
examination of fossil records has shown that a year once consisted of nearly
400 daily cycles, but finding the length of one such cycle is harder to
determine. Observations have shown that the earth’s rotation is slowing down,
and calculations suggest that it was once substantially faster — some reports
as low as 16 hours per day. That would result in a greater centrifugal force
acting against gravity, and things would probably have felt lighter millions of
years ago.
[4] David
Ewing Duncan, The Calendar (London:
Fourth Estate Ltd, 1998), pp.40–43.
[5]
Richards, p.101.
[6] Duncan,
pp.104–5.
[7] Richards,
p.93, p.234.
[8] Richards,
p.174.
[9]
Richards, p.182.
[10]
Richards, p.177.
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