Home > Encyclopedia of Science > Calendar

• Development of the present-day calendar
• The Jewish and Muslim calendars
• The Gregorian calendar and the third millennium
• Possible future calendar reform

A calendar is a system of measuring the passage of time for the purpose of recording historic events and arranging future plans. Units of time are defined by three different types of motion: a day is one rotation of Earth around its axis; a month is one revolution of the Moon around Earth; and a year is one revolution of Earth around the Sun. The year is the most important time unit in most calendars, since the cycle of seasons repeat in a yearly cycle as Earth revolves around the Sun.

Making a yearly calendar, however, is no simple task because these periods...