History of the Calendar
The history of the calendar is fascinating. The year of a planet is the
time it takes to orbit the Sun. There are, however, different types of
year. The time it takes the Earth to make one complete revolution with
respect to the distant stars is called the sidereal year (deriving from the
Latin word sidus, which meant constella-tion). The sidereal year (for
1950) is equal to 365.25636566 days or 31,558,149.993 seconds.
The time it takes for the Earth to return to the same point in its seasonal
cycle (which is what counts as far as the calendar is concerned) is called
the tropical year (after the Greek verb ???????, which means to turn). The
tropical year is equal to 365.24219342 days or 31,556,925.511 seconds being
about 20 m 25 s shorter than the sidereal year. The modern definition of a
second is the duration of 9,192,631,770 cycles of the radiation emitted by
the transition between the two hyperfine levels of the ground state of the
133Cs atom.
The tropical year is shorter than the sidereal year because of the
precession of the Earth's axis. Precession is the motion of the axis of a
spinning top. The Earth, like a top, has an equatorial bulge caused by the
centrifugal force of its rotation - the equatorial radius is 21.360 km
longer than the polar radius. This bulge experiences a torque applied by
the gravitational influence of the Moon, the Sun, and the planets. The
result is a precessional motion of the Earth's axis. This was discovered by
Hipparchos about 150 BC.
The astronomical motions of the Earth and the Moon provide the basis for the
calendar - the subdivision of time into years, months, weeks, and days. The
problem with these subdivisions is that they are based on phenomena that
have different periods - the revolution of the Earth around the Sun, the
revolution of the Moon around the Earth, and the rotation of the Earth. The
periods are not commensurate: there are about 29.53 days in a lunar month,
4.22 weeks in a lunar month, and 365.24 days in a tropical year. The
development of an accurate calendar has been a vexing problem since
antiquity (the name calendar derives from the Latin name kalendae for the
day of the new Moon, marking the beginning of the lunar month).
The Egyptians started their year in the latter part of July, when the Nile
began flooding the delta. This was a big event, because the flooding
fertilized the delta. The Egyptians divided the year into 12 months of 30
days each, giving a total of 360 days, to which they added an extra 5 days
at the end. In the years around 4241 BC, the Egyptians noticed that the
rising of the Nile coincided with the day when the star Sirius (known as
Sothis) became again visible in the southern sky as seen from the capital
city of Memphis.
The Roman author Censorinus reported in his work De die natali (written in
238 AD and dedicated to his patron on his birthday) that in 139 AD the
rising of Sothis had occurred on July 20. Now, the Egyptian year of 365
days was shorter by 0.242 day than the tropical year. As a result, the
Egyptian calendar fell behind by 1 day in 4 years, making one full circle
(called the Sothis cycle) in 4x365 = 1,460 years. Because the rising of
Sirius at Memphis and the beginning of flooding of the delta coincide only
once in 1,460 years, the Egyptians must have developed their calendar at one
of the following times: 1460 - 139 = 1321 BC, or 1321 + 1460 = 2781 BC, or
2781 + 1460 = 4241 BC... This last date is the most probable, because the
calendar was already in use about 3000 BC.
The early Greeks adopted the Egyptian calendar, but in the fifth century BC,
the ?????????? was introduced, a period of 8 years in which each year
consisted of 6 lunar months of 30 days and 6 lunar months of 29 days, plus 1
lunar month of 30 days in 3 out of the 8 years. The total number of days
was thus 2,922, giving an average of exactly 365.25 days per year. The
Greeks set the beginning of the year at either the summer or winter solstice
(depending on which Greek city).
According to legend, Rome was founded by Romulus, its first king, on April
21, 753 BC. At that time, the Romans had a calendar of 10 months (Martius
after Mars, the god of war, Aprilis, after Apru, the Etruscian goddess of
love, Maius, after Maia, eldest daughter of At-las, Junius, after Juno, wife
of Jupiter, Quin-tilis, Sextilis, September, October, November, and
December, the fifth month through the tenth month, of which four had 31 days
and the other six had 30 days. In addition, there were two unnamed months
each in the winter.
The second king of Rome, Numa Pompilius (715-672 BC), is said to have named
the two winter months Ianuarius, after Janus, the two-faced god of gates,
and Febriarius, after Februa, the day of purification (the 15th) and to have
moved the beginning of the year to January 1. In the reform recommended by
the Alexandrine astronomer Sosigenes and promulgated by Julius Caesar (100 -
44 BC), the year remained equal to 365.25 days, by having 365 days per year,
but adding to the month of February one day every fourth year (the leap
year). In addition, the names Quintilis and Sextilis were changed to July
and August to honour Julius Caesar and his adopted son Augustus, and both
given a length of 31 days, together with December which also got 31 days.
Poor February was used as a source for extra days. The first Julian year
began on January 1, 709 AUC (Ab Urbe Condita, i.e. since the founding of the
City), which translates into January 1, 45 BC.
In 526 AD, the Byzantine emperor Justinian asked a monk named Dionysius
Exiguus to change the calendar, reckoning time no longer from the founding
of Rome, but from the birth of Jesus Christ. Dionysius figured that Jesus
had been born 753 years after the founding of Rome, apparently not knowing
that Herod, under whom Jesus was born, had died 749 years AUC. Accordingly,
Jesus had to be born at least four years earlier than the good monk thought.
This error was discovered long after the Justinian Calendar had been adopted
by all the Christian nations of Europe.
To complicate things, some recent research indicates that Jesus Christ was
crucified on April 3, 33 AD. Because He was 33 years old when He died, His
birth could be fixed at the end of the year 1 BC or at the beginning of the
year 1 AD (there is no year zero), four years after Herod had died. At this
point we notice that there are discrepancies among the four gospels. Most
important, the earliest gospel, Mark, says nothing about the birth of Christ
or about Herod, and neither does John. It is thus possible that Dionysius
Exiguus made no mistake after all and that the birth of Christ took place at
the beginning of year 1, after the death of Herod. In any case, we still
reckon the years from the fix provided by Dionysius Exiguus. May his soul
rest in peace.
The number of days in a tropical year, as fixed by Julius Caesar's reform is
not quite exact - it is too large by 0.0078 days = 11.23 minutes. By the
middle of the six{*filter*}th century, the calendar was off by 11.7 days with
respect to the seasons. Pope Gregory XIII ordered that the day after
Thursday, October 4, 1582, should be henceforth Friday, October 15, 1582; in
addition, from that year on, centennial years should not be leap years, even
though divisible by 4, unless divisible by 400. In a 400-year cycle,
therefore, there would be (365x400) + 97 = 146097 days, equal to 365.242
days per year.
The Gregorian calendar was not the work of Gregory XIII (just as the Julian
calendar was not the work of Julius Caesar), but of the Jesuit priest
Christopher Clavius (after whom a prominent crater on the Moon is named) and
the Neapolitan astronomer and physician Luigi Lillio Ghiraldi. The new
calendar was soon adopted in all Catholic countries, but only in 1752 in
England and the British Empire (by which time they were 12 days behind), and
only in 1918 in Russia, when that country was 13 days behind - which is why
the October Revolution was celebrated in November. Lillian days, named
after Luigi Lillio, are simply days counted from the start of the Gregorian
reform.
In the same year, 1582, that Pope Gregory XIII promulgated his reform of the
calendar, the scholar Joseph Scaliger (1540-1609) devised the Julian period,
which he named after his father, Julius Caesar Scaliger. The Julian period
is a period of 7,980 years that is obtained by multiplying three cycles, the
solar (28y), Metonic (19y), and indictio (15y) cycles: 28x19x15 = 7,980.
The solar cycle is a period of 28 years that derives from the fact that in
365 days there 52 weeks + 1/7 of a week. In successive years, therefore,
the same day of the year will fall on the following day of the week. Were
it not for leap years, in which the day of the week advances by two, the
cycle would be completed in 7 years. It is instead completed in 7x4 = 28
years. The Metonic cycle (discovered by the Greek astronomer Meton in the
fifth century BC) is a period of time that is divisible into both a whole
number of years and a whole number of lunar months; it is equal to 19 years
and includes 235 lunar months. The indictio cycle is an ancient Egyptian
cycle, that beginning with January 1, 313 AD, was adopted by the emperor
Constantine as the Roman taxation cycle.
Scaliger adopted the three cycles not for astronomical reasons, but because
they were already used in Hellenistic, Roman, and Byzantine calendars.
Scaliger set the begin-ning of the current Julian period at January 1 at
12:00 noon Greenwich time, 4713 BC. Reck-oning from noon is advantageous in
astronomy because most astronomical observations are made at night, meaning
that the observations from one night are not divided into separate days (at
least in Europe). Days are counted in continuity from that date. The
Julian period is important, because astronomers, even today, use it to
record the dates of celestial phenom-ena. Thus January 1, 2000, at 00:00h,
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