These few indications help neophytes understand more clearly the never-ending problem of date. For instances, dates between 1582 and 1927 in Turkey are a real... headache. The generalised use of the new calendar has eventually resulted in achieving coherence in dates worldwide. The following lines sum up the different stages the calendar problem went through since the foundation of Roma in 753 BC.
First, let's briefly recall the basics of the motion of the Earth around the Sun. In the course of its two-fold movement, the Earth alternately tilts the North Pole and the South Pole towards the Sun. This tilt of the Earth's axis causes the seasons. In countries located at the same latitude as France, there are four seasons: spring, summer, autumn, and winter. Spring starts on 20 March with the spring equinox, also referred to as vernal equinox, a period during which days and nights are of equal length. Summer starts on 21 June with the summer solstice, a period during which days grow longer than nights. Autumn starts on 22 October with the autumn solstice, with days and nights of equal length again. Winter stars on 22 December with the winter solstice, a period during which days become shorter than nights.
The time it takes the Earth to complete a full circle around the Sun varies slightly, but it is approximately of 365.25 days, referred to as a tropical year.
Above is a gorgeous painting by Bernard Marie Collet titled "The dance of the seasons". His numerous works, all of which are as harmonious as they are great, are available on his website.
As the tradition has it, a century starts with year 1 and ends with year 00. Thus, the last century started on 1st January 1901 and ended on 31st December 2000. By the same token, the 21st century actually started on 1st January 2001. This age-old decision has its foundation in the will to have the Christian era start in year 1 instead of year 0, a year that never existed anyway. Therefore, the first century of our calendar starts with year 1 and ends with year 100. The second century starts in year 101 and ends in year 200, and so forth. It should be noted that, owing to an error of calculation, Jesus Christ's birth did not take place in year 1, but somewhere between 5 or 6 BC...
The calendar that is currently accepted in nearly the whole world is called the Gregorian calendar, after the name of Pope Gregory XIII who introduced it in 1582. The Gregorian calendar replaced the Julian calendar that was brought into force by Roman emperor Julius Caesar in 46 BC. (It was meant to improve the calendar established by Roman king Numa Pompilius some one hundred and fifty years earlier, with auspicious days during which it was allowed to attend to public affairs, and inauspicious ones during which those activities were forbidden.)
Although the Julian calendar, with its leap year occurring every four years, was very satisfactory for its time (a mean year was deemed to last 365,25 days), it was not accurate enough in the long run and eventually created discrepancies. Indeed, the Julian year is longer than the tropical year by an average of 11 minutes 12 seconds, and over the centuries, the gap between the Julian calendar the reality grew wider and wider.
In 1582, the Julian calendar registered an error of ten days, and it was a major difficulty in determining the exact date of the celebration of Easter. Indeed, at the first council of Nicaea, in 325, it was decided that Easter should be celebrated on the first Sunday following the full moon occurring after the vernal equinox of 21 March. Therefore, it could not be before 22 March and after 25 April. In 1582, the vernal equinox was on 11 March of the Julian calendar, whereas it should have been on 21 March. It was precisely to correct this discrepancy that Pope Gregory XIII introduced the calendar that we use nowadays. In order to correct the accrued error of ten days, it was decided that, the day after Thursday 4 October 1582 would be Friday 15 October 1582.
The Gregorian Calendar
By decreeing that the year would be a leap year only if it is divisible by 4, the new Gregorian calendar aimed at compensating the discrepancy of the Julian calendar. Thus, 1900 is not a leap year, whereas 2000 is a leap year. The next leap year ending a century will be 2400.
The shift from the Julian to the Gregorian calendar did not occur simultaneously everywhere.
It was first enforced by Pope Gregory XIII in the states which he reigned over. Spain, Portugal, and Poland immediately adopted it, shortly followed by France etc. England and protestant countries adhered to the Gregorian calendar in the 18th century only, and those of Orthodox faith, at the beginning of the 20th century.
When the shift took place, the following days were omitted in order to make up for the disparity:
Here are the dates of the shift to the new calendar:
As an anecdote, it was because the Orthodox Russian used the Julian calendar until 1918 (the day after Wednesday 31 January 1918 became Thursday 14 February 1918) that they “lost” 13 days in order to fall into line with the other European countries. This explains why the Russian revolution is called the October Revolution. Indeed, it took place on 25 October 1917 of the Julian calendar. For the rest of the world, it was on 7 November 1917, a difference of 13 days. This is also the reason why Russian history textbooks use the two calendars together, in order to avoid confusion. For instance, Lenin was born on 10 / 22 April 1870, and the October revolution ended on 25 October / 7 November 1917.
The common practice is to use the Julian calendar until 4 October 1582. The day after this date is the day when the Vatican adopted the Gregorian calendar, 15 October 1582. We use the Gregorian calendar on Astrotheme. The chart, which is accurate in all cases, is computed for the actual date, which may not be the date displayed, if the country is not France, for instance.
The English say OS date (Old Style date) and NS date (New style date) to refer to the Julian and the Gregorian calendars dates, respectively.
And what about the period preceding the Julian calendar?
It was a total mess. Here is what the Roman calendar was like in Roma since 753 BC.
When Roma was founded, in 753 BC, a year comprised the ten following months: Martius with 31 days, Aprilis with 30 days, Maïus with 31 days, Junius with 30 days, Quintilis with 31 days, Sextilis with 30 days, September with 30 days, October with 31 days, Novembris with 30 days, and December with 30 days.
Since these ten months amounted to 304 days, they would add as many days as necessary to match the solar year, without giving them any name. Later on, they were arranged into two months and added after December.
The first one was inserted before Martius and named JANUARIUS as a tribute to Janus, Latium's most ancient king and god of peace. The second additional month, FEBRUARIUS, was left after December for some time.
Those months totalled 354 days, an even number which was believed to be fateful, and therefore, another day was added to Februarius so that the year had 355 days. But then, instead of 27 days, Februarius had 28 days, an even number, and as a result was considered an accursed month. It was consecrated to Febro, the god of the dead in the underworld, hence its name Februarius. It was a month of mourning, during which purifications were performed in honour of the dead during the festival of “Februalia”. It also became synonymous with illness, since “febris” evokes fever. Around 400 of the Roman calendar (354 BC), Februarius was inserted between Januarius and Martius, thus becoming the second month. The year, which used to start around the spring equinox then began shortly after the winter solstice.
By one day, this 355 day year matched the lunar year which comprised 354 days. Since it was shorter than the solar year on which farmers would base their works, there was an attempt to correct the discrepancy by adding intercalations. To this aim, it was decided to set a four year cycle during which, every two years, a thirteenth month comprising 23 days was added between 24 and 25 February, or between 23 and 24 February, in which case it comprised 22 days. This extra month was named “Mercedonius”, or “Merkedonius” because mercenaries were paid then. Within this four year cycle, the first year had 355 days, the second year had 355 + 27 = 377, the third, 355 days, and the fourth, 355 + 23 = 378 days.
With this new adjustment, the four year cycle totalled 1 465 days, whereas four actual years have only 1 461 days, thus an excess of 4 days. After being too short, the year has become too long!
To remedy this situation, in 450 of the Roman calendar (304 BC), the decemvirate, a board of ten patricians in charge of writing laws, adopted the “octaeteris of Cleostratus of Tenedos”, a system based on an eight year cycle. After three octaeterides, instead of six months, only five months of 22 days were intercalated. Since there were four days in excess in four years, after four octaeterides, meaning 24 years, the excess would amount to 24. Eliminating one month of 22 days would still leave an excess of 2 days. The decemvirate was entrusted with the task of assigning to the month Mercedonius the number of days required in such a way that the civil year lines up with the solar year.
Unfortunately, adjustments were implemented in quite arbitrary ways, according to the politicians' interests in view of elections and depending on the judicial authorities. As a result, instead of putting an end to the mess, this system worsened it to the extent that in 708 of the Roman calendar (46 BC), the difference between the civil equinox and the astronomical one was of approximately three months. Grape-harvest took place in January!
The birth of the Julian calendar
With a view to putting an end to abuses of power by politicians, Julius Caesar sent for a Greek astronomer called Sosigenes of Alexandria and asked him to examine the possibilities of reforming the calendar. This is how the Julian calendar was born, the ancestor of our modern calendar. To begin with, Caesar added to the current year, 708 of the Roman calendar (46 BC), on top of the 23 day month to be intercalated that year, two extra months between Novembris and December, of 33 and 34 days, respectively, so as to catch up on the discrepancy. The result was a 355 day year, which was coined “The Year of Confusion” for that matter.
Heeding Sosigenes's advice again, Caesar decreed that, based on the Sun's journey, the year would include 365 days and that, given the excess that was evaluated to be of 6 hours, meaning 24 hours in 4 years, and additional day would be inserted every 4 years. It was placed after 24 February, sexto ante calendas martii in the Roman calendar, and was called bi sexto ante calendas martii, from which derives our modern leap, or bissextile, year. The 10 days of the new year were distributed over the months which then had alternately 30 and 31 days. The only exception was February which had 30 days on leap years, and 29 on common years.
Once Caesar enforced his reform, on a proposal by Antony who was then a consul like Caesar, in 716 of the Roman calendar (38 BC), it was decided that the month Quintilis be renamed Julius. The reform itself was referred to as the “Julian reform” and the calendar stemming from it, the “Julian calendar”.
In the beginning, the reform was not correctly implemented. The pontiffs intercalated a leap year every three years instead of every four years. After 36 years, 12 leap years had been intercalated instead of the 9 leap years required. Fortunately, this phenomenon did not go unnoticed, and Augustus who then ruled the country ordered that no leap year be set for the next 12 years, and thus, the Julian reform was put right.
In 746 of the Roman calendar (8 BC), as a reward for this service, the Roman Senate decreed that Sextilis be given the name of Augustus, similarly to what was done for Julius Caesar. Augustus was prompted to believe that, since he was by no means less important than Caesar, his month should have the same number of days. Therefore, one day was taken from Februarius to be added to Augustus, which then totalled to 31. Then, in order to avoid having three consecutive months, the 31st day of September was given to October. The same adjustment was made for November and December.
Since then, the number and the name of the months have remained unchanged. They have been passed on to us exactly as they were at the end of Emperor Augustus' reign!
On the other hand, months were divided differently. Days were arranged into three uneven parts: from Calends to Nones, from Nones to Ides, and from Ides to the end of the month. This division dates back to a very ancient time when the inhabitants of Latium would base the measurement of time on the Moon. The first day of the month, which would be the New Moon, was named Calends.
Contrary to our modern practice, days were not added to one another, but they were counted from Calends to Nones, from Nones to Ides, and from Ides to the next Calends. For instance, the Romans would say the 5th day, the 4th day, the 3rd day before Nones, or before Ides, or before Calends.
This calculation system lasted until the Middle-Ages. Until the 16th century, on many public documents written in Latin, the date is indicated after the Roman custom, and years were counted as of the foundation of Roma, in 753 BC.
It is necessary to be careful with dates prior to 1927, and more generally, between 5 October 1582 and the beginning of the 20th century. Indeed, depending on the country, and sometimes also depending on their authors, dates are given in old style (Julian calendar), or according to the new Gregorian calendar, even though for the said date, the shift from Julian to Gregorian calendar has already taken place. Conversely, it happens that a date is given according to the Gregorian calendar, whereas the shift had not been implemented yet. Therefore, one ought to be cautious when dealing with these periods and refrain from immediately thinking that a date is wrong. It is most likely that the explanation is in these Julian and Gregorian calendars.