A calendar is a system of organizing days for social, religious, commercial, or administrative purposes. This is done by giving names to periods of time , typically days , weeks , months , and years . A date is the designation of a single, specific day within such a system. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun or the moon . Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs. A calendar is also a physical device (often paper).
A lunar calendar is a calendar that is based on cycles of the lunar phase . Because there are about twelve lunations ( synodic months ) in a solar year, this period (354.37 days) is sometimes referred to as a lunar year . A common purely lunar calendar is the Islamic calendar or Hijri Qamari calendar.
The phase and libration of the Moon for 2013 at hourly intervals, with music, titles and supplemental graphics. Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration . The apparent change in size is due to the eccentricity of the lunar orbit. A lunar phase or phase of the moon is the appearance of the illuminated (sunlit) portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth , Moon , and Sun .