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calendar
A practical system for fixing the length and beginning of years and their subdivision into months and days. The word "calendar" comes from the Latin calendarium for "account-book."
Only the familiar Gregorian calendar is used on a worldwide basis. It superceded the Julian calendar in the sixteenth century; however, because it took from 1582 to 1918 for the Julian system to die out completely, astronomers have to specify to which calendar any given date in this interval refers. Both the Gregorian and Julian systems are solar calendars, tied to the annual motion of Earth around the Sun. The Persian calendar, too, is solar. On the other hand, the Islamic calendar is lunar (based on the waxing and waning of the Moon), while both the Hebrew and the Chinese calendars are lunisolar, with months that keep in step with the phases of the Moon and years that line up with the seasons.
Related entry
• calendar curiosities
Related categories
• HISTORY OF ASTRONOMY
• HISTORY OF SCIENCE
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