The Gregorian Calendar (Western Calendar)

What is the Gregorian Calendar?

The Gregorian calendar is the one used internationally by most of the world for determining what day it is. It’s divided into 12 months, from January to December, with each month possessing either a length of 30 or 31 days with the exception of February, which has 28 – and sometimes 29 – days.

The reason for the little difference there is the existence of the leap year (have you ever thought: “Why do we have a leap year?”): every four years, February gets an extra day to make up for the inaccuracy between the Gregorian calendar’s year and a solar year as astronomically determined. An exception for this is if the date is divisible by 100 – then, no leap year is added. If, however, the year is divisible by 400, that rule is ignored and a leap year is added nonetheless. That makes an average Gregorian calendar’s year exactly 365.2425 days long – 365 days, 5 hours, 49 minutes and 12 seconds.

Although introduced by Pope Gregory XIII in 1582, the calendar continues the old calendar year count (known as the Common Era in modern context), which was based on the estimated number of years since the nativity of Jesus Christ (AD, or Anno Domini, Latin roughly meaning ‘year of the Lord’), as calculated in the 6th century.Why do we use the Gregorian Calendar?

To better understand the history behind the Gregorian calendar, and how it survived nearly half a century to become the globally dominant civil calendar, we have to take a look at its forerunner – the Julian calendar, and its forerunner, the Roman calendar.

The Roman calendar consisted of 12 months with an extra leap month to be added every now and again, whenever necessary. These months were Ianuarius (January), Februarius (February) the leap month Mensis Intercalaris (intercalary month) or Mercedonius, Martius (March), Aprilis (April), Maius (May), Iunius (June), Quintilis (July, later named Iulius in honor of Caesar), Sextilis (August, later named Augustus in honor of the Roman emperor), September, October, November and December.

Trouble with this calendar, however, was that the method for implementing the intercalary month wasn’t systematic. Rather, the intercalary month was simply decided by the Pontifiex Maximus of Rome, the highest religious position in the empire. Often, this lead to leap years occurring so irregularly that the empire would be way out of whack time-wise. Even if followed to the dot, the Roman year was 366 ¼ days long over a period of four years (without a leap month, one Roman year was 355 days long) – a whole day longer than the estimated tropical year; meaning each year, Rome would be a day out of sync with any equinox or solstice (celestial events determining the seasons based on the Earth’s position).

This caused problems for Romans everywhere as they began to drop behind the estimated tropical year – worsened yet by the irregularity of intercalary months. To fix things, Caesar added enough days to the last year pre-reform, 46 BC, to catch up to the tropical year – making it 445 days long in its entirety with the addition of a whole 3 intercalary months. Then, he reworked the Roman calendar to have the exact system we have today in the Gregorian calendar – the only difference being the frequency of February’s leap day, set to once every four years regardless of whether the year is divisible by 100 or not.

This worked very, very well for a long time – until several astronomers in the Catholic church noticed that the date for Easter, agreed upon in 325, was off and continuously drifting from the actual spring equinox, to which it was originally tied.

A reform was then proposed by Aloysius Lilius, and introduced by Pope Gregory XIII in 1582 – it amounted to a total change of 0.002% – yet that change meant that the margin of error went from being off by 1 day ever 128 years, to being off by 1 day every 3236 years.

Italy, Spain, Portugal, Poland and Spain – being Catholic countries at the time – switched to the Gregorian calendar almost immediately. However, it took until 1752 for Britain and the Commonwealth, alongside its colony in New World, to accept the system – resulting in a lost 11 days. It wasn’t until 1872 that Japan switched in light of the Meiji Restoration, and 1911 that China officially implemented the calendar – and it took until 1926 for Turkey, the last country to do so, to make the leap while losing 13 days.

Fun fact: The faulty implementation of the Mensis Intercalaris wasn’t necessarily the fault of negligence – often, it was for political advantage that these systems were abused by the Ponifices to help allies stay in office longer, or get enemies out of office quickly.