From the Latin "calendae", calendar It is the term by which the Romans indicated the first day of each month. It is a set of tables that indicate the days and months of each year and is used to calculate the time.

Since ancient times, the periods in which the calendar is subdivided have referred to the movement of the stars and, depending on which star was considered as the main reference element, several types of calendar were proposed. The calendar currently in force, called solar, which has been adopted in most countries of the world, is based on the movement of revolution of the Earth around the Sun and its duration is defined by the so-called tropic or civil year, that is , the time elapsed between two successive steps of the Sun through the spring Equinox.

The lunar calendar, created by the Babylonians and still in use today among the Mohammedans, is based instead on the year subdivided into twelve lunar months, twenty-nine and thirty days alternately.

The lunisolar calendar, adopted by the Hebrew peoples, refers to the movements of both the Sun and the Moon and is composed of "current years," divided into 12 moonings and "embolismal years," divided into 13 moonings.

Our solar calendar was adopted in 1582, as a result of the reform made by Pope Gregory Xlll based on the calculations of astronomers Luis Lilio and Cristóbal Clavius.

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