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The first calendars were designed by the Egyptians whose calculations serve as the basis for more modern calendars. Other ancient calendars were devised by the Greeks, Romans Julian after Julius Caesar) and Chinese. The Jewish and Islamic calendars are also important calendars as well as various church calendars.
The calendar most commonly used today is the Gregorian calendar named so after Pope Gregory XIII who adjusted the Julian calendar in 1582.
The concept of the calendar is based on the revolution of the earth around the sun. It has been calculated that this takes approximately 365 and one quarter days and this constitutes one year. Every four years is a catch up year with 366 days to make up for the quarters.
Today's calendar is divided into twelve months. They are: January (31 days), February (28 days except in a leap year when it is 29), 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) and December (31 days).
Each year is made up of 52 weeks and each week is made up of seven days. These days are twenty-four hours in length.
Some calendar years have the abbreviations BC and AD after them. BC used to mean Before Christ and indicated that time before the birth of Christ. Today that abbreviation means Before the Common Time. AD means Anno Domini or Year of the Lord in Latin.
The year is also divided into four seasons based on climate changes that take place due to the earth tilting as it moves around the sun. The seasons are spring which begins at the spring equinox of March 21, autumn or fall which begins on September 23 at the fall equinox. in the Northern Hemisphere (this is reversed in the Southern Hemisphere). At the fall and spring equinoxes, day and night are almost equal in length (12 hours).
Summer begins on June 22 which is the summer solstice in the Northern Hemisphere when the days are longer and nights are shorter. Winter begins on the winter solstice, December 22 when the days are shorter and the nights longer in the Northern Hemisphere. This is reversed in the Southern Hemisphere as it is summer when it is winter in the Northern Hemisphere and vice versa.
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