The Gregorian Calendar: Get to know its origins and how it works

The Gregorian calendar, also known as the Western Calendar, is the most generally used calendar around the world, including the United States. This is the calendar used for the international standard for the representation of dates and times, or ISO 8601:2004.

History and Origins of the Gregorian Calendar

The Gregorian calendar was named after and introduced by Pope Gregory XIII in 1582, as a way to reform the Catholic calendar, and to fix the errors and miscalculations of the Julian Calendar, which was in use since 46 B.C.. 

The calendar which we currently use is based on the initial designs and proposals put forth by Aloysius Lilius (1510-1576), an Italian astronomer, doctor, philosopher, and chronologist. His proposal for the new calendar was presented to Pope Gregory XIII in 1575. However, due to the years of deliberation, it took until the reform to the calendar happened in 1582, Lilius never got to see his work (which by then had suffered some modifications) being widely used, as he passed away in 1576. 

Until 1582, most European countries used the Julian Calendar. However, Julius Caesar, its creator, miscalculated the length of a year by 11 minutes, which through the years caused the calendar to fall out of sync with the seasons. As a consequence, Easter celebrations began falling further from the spring equinox and their traditional date of March 21. This was another one of the issues that Pope Gregory XIII set out to fix with the Gregorian Calendar. 

The Gregorian calendar was made official on February 24, 1582, with the release of the papal bull Inter Gravissimus. While Catholic countries such as Portugal and Italy quickly adopted the new calendar as soon as it was imposed, Protestant countries were very reluctant to make the change into the new calendar, as they saw it as a plot for Catholic domination. It wasn't until 1752 that England, and by default the colonies in the United States, adopted the Gregorian Calendar.

How does the Gregorian Calendar work?

  • The Gregorian Calendar is a solar calendar, which means that it is based on the movement of the Earth around the sun. 
  • One year in the Gregorian Calendar has 365.2425 days, which is divided into 12 months, which are divided into 52 weeks, which have 7 days each. 
  • Out of the 12 months, 7 have 31 days (January, March, May, July, August, October, and December), and 4 have 30 days (April, June, September and November).
  • To accommodate the leap years, the month of February has 28 days on common years, and 29 days every four years.
  • A leap year has 366 days, and happens every four years. However, in the Gregorian Calendar a leap year can only happen on years divisible by 4 or 400, but not if it is divisible by 100. This fixed the lag in the Julian Calendar.

Why was the Gregorian Calendar created?

Before the Gregorian Calendar was introduced, the Julian Calendar that was in use created many issues that needed to be fixed:

  • Its system meant that several days were skipped every year, and gradually the Julian calendar began drifting away from astronomical events like equinoxes and solstices. This also resulted in the calendar being out of sync with the seasons.
  • A leap day was added to the calendar every four years, and as a result the calendar did not accurately represent the time it takes for the Earth to circle around the sun.
  • The days in a year were miscalculated by 11 minutes, so each common year is 365.25 days long. This means that every 128 years the Julian calendar gains an extra day.

When the Gregorian Calendar was introduced these issues were fixed:

  • By correctly calculating that a year was 365.2425 days the calendar was realigned with the movement of the Earth around the sun.
  • By creating a more accurate calculation for determining leap years.
  • By dropping a few days off the calendar to sync it back with the seasons of the year, and the astronomical events.