Gregorian calendar

The Gregorian calendar, otherwise known as the Western or Christian calendar is an internationally accepted civil calendar of days, months, and years. According to the Gregorian calendar, a year has 365 days. To account for slight discrepancies, the calendar has 97 leap years every 400 years. Every year divisible by four is a leap year. However, every year divisible by 100 is not a leap year, but every year divisible by 400 is a leap year after all. Therefore, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600, 2000, and 2400 are leap years. The Gregorian calendar was proposed by the physician Aloysius Lilius, adopted by Pope Gregory XIII in February 1582, and used uniformly across Europe by the late eighteenth century. It was preceded by the Julian calendar, introduced by Julius Caesar in 45 bCE and was in use until the late 1500s ce. Some countries still use the Julian calendar, as does the Orthodox Church.

Brief History

The word calendar comes from the Latin "Kalendae" (or "Calendae"), which was the first day of the month. Our civil method for reckoning time has a mixed origin. The current division of the hour into minutes and seconds is derived from the sexagesimal system of the Mesopotamians; the division of the day into 24 hours originated with the Egyptians; the seven-day week originated in the ancient Near East, while the names are derived from a Greek convention developed during the Hellenistic era. Our calendar is based on the motion of the sun, but various religious calendars are based on a combination of the motions of the sun and moon. The Gregorian calendar civil calendar derives from the Romans, with some alterations.

In early agricultural societies, the cycles of the sun and the moon were used as a basis for the annual, monthly, and daily timekeeping: the solar year, or the time the sun takes to return to the same position in the cycle of seasons as seen from Earth, is roughly one year, whereas the synodic period of the moon (the time between successive full moons or new moons) is approximately one month. The slight difference in these cycles was the reason why the calendar construction was such a complicated matter, changed and adapted throughout history. Most ancient cultures relied on a calendar in which months alternated between 29 and 30 days, and added a month every third year with varying degrees of sophistication.

For example, the Egyptian calendar introduced 12 months of 30 days each, with five days added at the end of each year. This created an error of around six hours per year, so the starting date of each year slowly moved forward within the seasons, and after 1460 returned to where it started. The key event in the Egyptian agricultural cycle was the rising of the Nile, which was predicted by the heliacal rising of Sirius, the brightest star in the sky; the lunar cycles were considered irrelevant.

In Mesopotamia, the ancient Persians incorporated the Metonic cycle (later named after the Greek Meton, 425 BCE) in which the solar and lunar calendars were both part of the calculus. Nineteen solar years contain 6,939.75 days; 110 months of 29 days plus 125 months of 30 days add up to 6,940 days. Nineteen years, then, contained 235 months, and starting in (on our calendar) 499 BCE, the calendar in that part of the world was regulated on a cycle of intercalating seven extra months in 19 years. This method was used until 75 CE, when written records end.

The month was also divided into smaller segments of time. In the Hellenic period (300–100 BCE) a month was divided into three periods of 10 days, with a ruling planet assigned to each hour of the day: Sun, Venus, Mercury, Moon, Saturn, Jupiter, Mars. With seven stellar hours over the 24-hour period, each day would start with a different ruling planet, thus becoming Sun’s day, Moon’s day, Mercury’s day, etc. The modern English variations on these names are due to substituting Nordic or Saxon gods for some of the Roman ones: Tiw for Mars (giving Tuesday); Wotan for Mercury (Wednesday); Thor for Jupiter (Thursday); and Frigg for Venus (Friday). However, the seven-day week, which already exists in the book of Genesis, originated in Near East.

In the time of ancient Romans, the length, order, and name of each month was frequently changed with different rulers. Finally, Julius Caesar abolished the use of the lunar year and the intercallary (additional corrective) month, and regulated the civil year entirely by the sun. With the assistance of Sosigenes, the mean length of the year is fixed to 365.25 days, therefore every fourth year became the leap year, containing 366 days. The first Julian year commenced with the 1st of January 46 BCE, 708 years from the foundation of Rome. It lasted until 1582, when the Gregorian calendar replaced it.

Overview

The Gregorian calendar was developed in accordance with instructions from the Council of Trent (1545–1563) in order to correct the errors of the Julian calendar. It was decreed by (and named after) Pope Gregory XIII in a papal bull on February 24, 1582. It is the most perfect calendar for civil use, but it is not used as standard in physics, astronomy or navigation—they use the tropical year. The tropical year (also known as a solar year), is the time it takes Earth to complete one full orbit around the sun (as measured with respect to the fixed stars), making a full cycle from one summer solstice to the next. The tropical year is equivalent to approximately 365.24 mean solar days, or 3.1557 x 107 seconds. It takes approximately 3,300 years for the tropical year to shift one day with respect to the Gregorian calendar. The approximation is achieved by having 97 leap years every 400 years of the Gregorian calendar. Since the adoption of the Gregorian calendar means that some fixed dates fall in different times of the year from the early times, some countries still count the religious holidays according to the Julian calendar. In Greece, Russia, Serbia, and Bulgaria the Orthodox Christmas and Easter fall approximately two weeks behind the ones in the rest of the Western world.

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