Calendars were invented by humans to keep track of time over long periods. The history of humanity teaches us that there were, and still are, numerous different calendars. The base unit is always the day (more precisely, a complete day-night cycle), but everything becomes more complicated if we try to count the cycles of the Moon (almost a month) or the cycle of the Earth’s motion around the Sun (the year). In effect, neither one nor the other corresponds to a whole number of days (about 29.5 days for the moon’s cycle and about 365.25 days for the year).
We group calendars into three major categories:
Remark: Lunar and lunisolar calendars are based on observation of the Moon (the New Moon for the Hebrews and the Chinese, and the first crescent for the Muslim calendar. ) There can be as much as three days difference between the astronomical date of the lunar phase and its date of observation, depending on the location. The dates of the Islamic calendar therefore vary from country to country.
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