What is the Gregorian calendar?
The Gregorian calendar is the time division system currently used in the West. It was promoted by Pope Gregory XIII and implemented in 1582 in Europe.
This calendar was created to correct the inaccuracy of the Julian calendar, which was created in 46 BC. C by Emperor Julius Caesar and remained in force until 1582.
The Gregorian calendar is made up of 12 months of 31, 30 or 28 days, resulting in a year of 365 days. Every four years an additional day is added to the month of February to have a year of 366 days, with some exceptions. It is when we say that it is a leap year.
The exceptions to determining leap years in the Gregorian calendar are: years divisible by 100 are never leap years and years divisible by 400 are always leap years.
Composition of the Gregorian calendar
The Gregorian calendar is a system composed of
Years
In the Gregorian calendar, years are counted taking as reference the year of birth of Jesus of Nazareth. For that reason, time is divided into two major historical periods: before Christ (BC) and after Christ (AD). The year of Jesus’ birth would be 1 AD, while the year before his birth is considered 1 BC.
In turn, a year can be:
Common: is the year that has 365 days.
Leap: are the years that have 366 days. This occurs every four years, with some exceptions, and the additional day is added to the month of February, which that year goes from having 28 days to having 29.
Secular: are years that end in 00 (such as the year 1900 or 2000). They are the years that determine the end of a century and are never leap years.
Months
A month is a division of time in the calendar, made up of a period of 28, 30 or 31 days. February is the only month with 28 days (except for leap years, when it has 29). The rest of the months alternate periods of 30 or 31 days as follows:
30 day months: April, June, September and November.
Months of 31 days: January, March, May, July, August, October and December.
Weeks
A week is a period of seven consecutive days. A month has approximately four weeks and a year has 52 weeks.
In the Gregorian calendar, the counting of weeks begins in January with week one, and ends in December with week 52.
For its part, most of the names of the days of the week in Spanish are a derivation of the Latin names of some celestial bodies. This does not apply to Saturday and Sunday, which has another origin.
Monday (from Latin Moon)
Tuesday (from the Latin Mars meaning Mars)
Wednesday (from Latin Mercurius, meaning Mercury)
Thursday (from the Latin Iuppiter meaning Jupiter)
Friday (from Latin Venus)
Saturday (from Hebrew shabbat, meaning day of rest)
Sunday (from Latin dominica, meaning day of the Lord)
According to the ISO 8601 standard, dedicated to the regulation of time data, the week begins on Monday and ends on Sunday.
Days
The day is the basic unit of time in the Gregorian calendar. A day is made up of:
Hours: A day has 24 hours. Each hour is a 60-minute unit of time.
Minutes: In a day there are 1440 minutes. Every minute has 60 seconds.
Seconds: a day has 86400 seconds.
Leap years in the Gregorian calendar
The Gregorian system has a couple of exceptions to determine which years are leap years and which are not. The exceptions are:
They are leap years years that are divisible by 400 (such as the year 2000).
They are not leap years years that are divisible by 100 (like the year 1900, for example).
These exceptions gave the Gregorian system an even greater advantage over the Julian system that was used until 1582. The Julian calendar had a calculation error that caused one day to be accumulated every 128 years, generating an alteration in the counting of time. On the other hand, with these modifications in the Gregorian system the lag is one day every 3,300 years.
History of the Gregorian calendar
In the year 325 the Council of Nicaea took place in the Roman Empire. It was a high-ranking meeting in which the Christian bishops determined, among other issues, the date of celebration of Easter or Resurrection Sunday. This date is very important because it determines the rest of the celebrations of the liturgical calendar of Christianity.
To do this, the date of the spring equinox had to be determined first. That year, the equinox was dated March 21, but the inaccuracy of the Julian calendar accumulated one day every 128 years, which generated a gap that in 1582 already reached 10 days.
For this reason, in 1582 the equinox was dated March 11, a date very distant from that originally established at the Council of Nicaea.
Calendar modification process
To correct this gap, in 1580 a Calendar Commission had already been established in Europe, made up of the Italian astronomer Luis Lilio and scholars from the University of Salamanca.
The correction of the calendar resulted in March 21 being able to be maintained as a stable date for the spring equinox, without the need for future alterations.
Promulgation and implementation of the Gregorian calendar
In October 1582, Pope Gregory XIII promulgated the modification of the calendar in a papal bull called Inter Gravissimas. The new system was first implemented by Christian countries and colonies and by the mid-20th century it had been adopted throughout almost the entire Western world.
However, some cultures retained their own time measurement systems. Some examples are the Hebrew calendar, the Buddhist calendar, the Islamic calendar or the Chinese calendar.
Although what was done was to modify the Julian calendar, little by little it became known as the Gregorian calendar in honor of its main popularizer, Pope Gregory XIII.
Gregorian calendar and Julian calendar
The Gregorian calendar is just a correction of the Julian calendar, which was based on a 365-day solar year. The inaccuracy of the Julian calendar comes from calculating a solar year (the time it takes for the Sun to make one complete revolution around its ecliptic) in 365, 25 days.
These tenths (0.25) correspond to 11 minutes and 14 extra seconds, which accumulated until completing 24 hours every 128 years, generating an alteration in the calendar.
The contribution of the Gregorian calendar was to establish that the exact length of a year is 365 days, 5 hours, 48 minutes and 45.16 seconds. This made it a much more precise system than the previous one and for that reason it was promulgated as the new system of measuring time, first in Europe and then in the rest of the West.
See also: