Letter from the Publisher

It was on this double month-and-year basis that most
civilizations developed their various types of calendar, some of
which were extremely complex. The main difficulty stemmed
from the fact that the lunar month does not correspond to a
whole number of solar days and that the solar year does not
contain a whole number of lunar months—which means that
these units are not mutually divisible. The lunar cycle actually
lasts 29 days, 12 hours, 44 minutes and 2.8 seconds (or 29.53
days), whereas the real duration of the so-called “tropical”
or astronomical year is 365 days, 5 hours, 48 minutes
and 45.2606 seconds (or 365.242190517 days). Some
civilizations favored the lunar cycle (such as for the Muslim
calendar), while others attempted to reconcile the two
systems (lunisolar calendars, such as the Gregorian calendar).
From The FerTile CresCenT To The Gauls
The very first calendars were generally of the lunar type.
as populations became less nomadic, the rhythm of the year
and the seasons, often linked to farming activities, became
more important and an effort was made to devise calendars
combining lunar and solar cycles. In the third millennium
BC, the Sumerians already used a year comprising 12
months of 30 days each, with various intercalary months
to compensate for the discrepancies. Mesopotamia also
witnessed the invention of the sexagesimal numbering
system, which led to dividing the day into 12/24 hours, the
hour into 60 minutes and the minute into 60 seconds.
In the egyptian calendar, the year comprised 365 days with
12 months of 30 days each, plus five days added on at the
end of the year—which led to a progressive regression in
relation to the solar year. The egyptian year began at the
summer solstice (the start of the nile floods), and the
month was divided into three ten-day periods.
In ancient Greece, while calendars were of the lunisolar
type, with various intercalary methods, there were almost
as many systems as there were cities. nonetheless, Greek
astronomers had already achieved an extremely high degree
of sophistication in their calculations, particularly regarding
the exact duration of the tropical year. The years were
generally named after the magistrates in charge at the time,
or numbered in relation to the first Olympic Games, held in
776 BC. The Celts also had some extremely complex calendar
systems, traces of which have been found in, for example,
the famous Coligny calendar displayed in the Gallo-roman
Museum on Fourvière (lyon, France). This Gaulish calendar
written in letters and roman numerals features a cycle of
five 12-month years with alternate 29 and 30-day months, to
which two intercalary months were added, one at the start of
the first year, and one in the middle of the third.
Julian Calendar
The roman calendar, inspired by the egyptian calendar,
underwent a number of evolutions. In the republican era,
the year comprised 365 days (12 lunar months) with an
intercalary month of varying length added once every two
years to compensate for its lag behind the solar calendar.
until 153 BC, the year began in March, which marked
the resumption of farming and military activities, which
explains the names of the months of September (derived
from septem, meaning seven), October (eight), november
(ninth) and december (tenth), which have been maintained
to this day despite subsequent calendar reforms.
although the system of intercalary months was not
applied in a sufficiently rigorous manner, particularly in the
age of civil wars, it gave rise to various abuses perpetrated
by the priests and magistrates responsible for the calendar.
In the first century BC, Julius Caesar, in his capacity as “grand
pontiff,” decided to reform the calendar by enlisting
the services of a Greek astronomer. The first measure
adopted was to realign the roman year with the tropical
year. a 12-month year was instated, comprising alternating
31 and 30-day months apart from February, which had
29 (hence a total of 365 days), and every four years an
intercalary day was to be added between February 24th
and 25th. In the initial version of the so-called Julian
calendar, which came into force in 46 BC, the month of
august had 30 days, September 31, October 30,
november 31 and december 30. In 44 BC, the senate decided
to honor Julius Caesar, who had just been assassinated, by
giving his name to the seventh month of the year, which thus
became Julius (hence July). In 8 BC, the same homage was
paid to the first emperor augustus, to whom the following
month was dedicated and named augustus (august). But as
this month had only 30 days, the emperor could not decently
be given a month shorter than that devoted to his illustrious
predecessor, and so the month of august was lengthened
to 31 days, which in turn meant September therefore had
30 days, October 31, november 30 and december 31,
while February was shortened to just 28. and so it was
that two tributes paid to famous romans added to the
complexity of the calendar system—and that of the task
of future horologers.
In the roman system, the month was divided into
three unequal parts with three fixed points: the Kalends
(1st day of the month)—hence the word calendar; the
nones (the 5th or the 7th); and the Ides (13th or the
15th). The date indication was counted backwards from
the following point of reference. Our May 2nd, for instance,
was referred to as the “6th day before the nones of May.”
Calendars 61 •