Gregorian Calendar

History of calendars

Timekeeping has always been a part of the human world. All ancient civilizations used some kinds of method to keep the tract of time according to which they performed their duties. Archeologists have dated the use of calendars back to the Neolithic period.  Calendar is a name given to the methods of recording and tracking time. The units used for time keeping are known as days. Time is tracked on the basics of two main systems;

  • Movement of the earth around the sun
  • Movement of the moon

The year that is based on the movement of the earth around the sun is known as solar year where as the year based on the moon’s movement is known as the lunar year. The first ever calendar that was attested and formulized was the Sumerian Calendar in Bronze Age which is dependent on the fact that writing was developed in the Ancient Near East in that era. After that many ancient calendars like Egyptian, Assyrian and Elamite were recognized.  Assyrian calendar gave rise to Persian, Zoroastrian and Hebrew calendar in the Iron Age. All these ancient calendars were based on Lunar and as well as solar movements whereas the Roman Calendar was designed according to the pre-Etruscan 10-month solar year which was later reformed into Julian and then Gregorian calendar which is today widely used as a representation of time and date in the world.

Which calendar do we use today?

There are so many calendars that have been formulized throughout history which brings us to the main question that is which calendar do we use today? Do we still use Georgian calendar today? Or is Julian calendar used today?  Different calendars are used today by people according to their religious or social beliefs. The calendars that are most commonly accepted and used by the people of this era are;

  • Julian calendar
  • Gregorian calendar
  • Lunar calendar

Is the Gregorian calendar still used today?
Gregorian calendar – Internationally accepted civil calendar

Although people practice the use of different calendars throughout the world but the calendar that is internationally recognized is the Gregorian calendar.  Gregorian calendar is a reformed form of the Julian calendar. There is correction of 0.002% in the length of the year. The idea of reforming the Julian calendar was first proposed by Aloysius Lilius. He proposed to reduce the 100 leap years to 97 leap years that occurred every 400 year years. The calendar was officially introduced by Pope Gregory XIII on 24th February, 1582. The calendar was named after the Pope by papal bull Inter gravissimas. The calendar uses Anno Domini system meaning it counts the years from Jesus’s Incarnation.

Description of Gregorian calendar

Gregorian calendar is a solar calendar as it measures time according to the movement of the earth around the sun. The basic measurement unit is days which make it an arithmetical calendar. The normal year consists of 365 days where as the leap year has 366 days. This makes a total of 146,097 days in 400 years, out of which 303 are normal years and 97 are leap years. This makes the average length of the year 365.2425 days. There are 12 months in a year, 11 of the months contain 30 or 31 days whereas there is fluctuation in the days of February. Normally February consists of 28 days but in leap year an extra day is added in the month of February making 29 days for the month of February. This means that 29th of February is the leap day in the Gregorian calendar. The month is made up of weeks and each week is made up of 7 days. Usually the start of the week is from Monday but in countries like Canada and America the week is started from Sunday. The date of the calendar is specified by the year, month is identified by number or name and the day of the month starts in sequence from 1st to 31st.

Month Name Month Days
January31 Days
February28 Days (29 Leap year)
March31 Days
April30 Days
May31 Days
June30 Days
July31 Days
August31 Days
September30 Days
October31 Days
November30 Days
December31 Days

Motivation for reforms

The main reason which led to the formation of Gregorian calendar was the fact that Easter was celebrated across the globe on different days. This was considered undesirable by Catholic Church. According to them Easter was to be celebrated on time as agreed by the First Council of Nicaea in 325. The order of the council implies that Easter must be celebrated on the same day by everyone.  The Sunday after the 14th day of the moon after the vernal equinox was the day on which Easter was celebrated by the Church of Alexandria. They used a Metonic cycle and placed vernal equinox of the 21st of March. The Church of Rome used the same day for the equinox but calculated the day of the moon through different cycle or formula. The moon was fixed to a 19 year cycle which led to a different of 4 days between the real moon and the lunar calendar.

Gregorian Reforms

As said earlier the Gregorian calendar is a reformed form of Julian calendar. The changes that were brought in the Julian calendar are as follows;

  • The numbers of leap year in every 4th centenary were decreased to 97 from 100. The formula that was used is, every year that is divisible by 4 if a leap year except the years which are exactly divisible by 100 but if the centurial years are divisible by 400 then they are leap years. The formula might seem confusing at the time but you’ll get a hold of it after reviewing it 2-3 times.
  • The average length of the year was reduced to 365.2425 days from 365.25 days. This means that there was a reduction of 10 minutes 48 seconds per year.
  • The epacts of the moon were also adjusted through a scheme that solved the issue of the annual date of Easter.

Do we use the Julian Calendar Today?

The date and time is represented in the world through the use of Gregorian calendar. Even though it took 300 years to replace the Julian calendar with Gregorian calendar, Gregorian calendar is the universal calendar that is used today. The use of Julian calendar has decreased over the years. Only certain orthodox churches use the Julian calendar today which is revised. It is considered to be 10 times more accurate than the Gregorian calendar. It uses a complex formula of calculating leap year with only an error of two seconds per year whereas the Gregorian calendar has an error of 26 seconds. Due to its complexity it is not used by the people today.

Gregorian calendar to Hijri calendar

Hijri calendar also known as the Islamic calendar is used to predict the Islamic festivals and holidays. Unlike Gregorian calendar, Islamic calendar is based on the movement of the moon. The month starts after the viewing of a new moon. It also consists of 12 months but the number of days varies from 354 to 355 every year. This means that there is a difference of 11 days between Hijri and Gregorian calendar. Gregorian calendar counts years from the incarnation of Jesus whereas the Islamic calendar counts day from the immigration of Prophet Muhammad (PBUH) from Mecca to Medina in 622 AD. Therefore the year 2018 of the Gregorian calendar is year 1439 of the Islamic calendar.