Gregorian calendar

Gregorian calendar is a solar calendar that is currently used in most parts of the world. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582; the decree is known as a papal bull and is named Inter gravissimas. The Gregorian calendar is a corrected version of the Julian calendar and is therefore sometimes called the Christian calendar or Western calendar.

History

The Julian calendar, introduced by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar. It took effect in 45 BC (709 AUC), shortly after the Roman conquest of Egypt. The Julian calendar has a regular year of 365.25 days divided into 12 months, as detailed in the table below. A leap year occurs every four years to help synchronize the calendar year with the solar year, or the length of time it takes Earth to complete its orbit around the sun, which is about 365.25 days.

The motivation for the Gregorian reform was that the Julian calendar assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost exactly 365.2425 days. The discrepancy results in a drift of about three days every 400 years. At the time of Gregory's reform there had already been a drift of 10 days since Roman times, resulting in the spring equinox falling on 11 March instead of the ecclesiastically fixed date of 21 March, and moving steadily earlier in the Julian calendar.

Structure

The Gregorian calendar, like the Julian calendar, is a solar calendar with 12 months of 28–31 days each. The year in the Gregorian calendar is evenly divisible by 4. If the year can be evenly divided by 100, it is NOT a leap year, unless the year is also evenly divisible by 400. Then it is a leap year.

Months

The Gregorian calendar consists of the following 12 months:

• January - 31 days
• February - 28 days in a common year and 29 days in leap years
• March - 31 days
• April - 30 days
• May - 31 days
• June - 30 days
• July - 31 days
• August - 31 days
• September - 30 days
• October - 31 days
• November - 30 days
• December - 31 days

See also

• Julian calendar
• Leap year
• Solar calendar
• Pope Gregory XIII
• Julius Caesar
• Roman calendar

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