Chinese, Hebrew, and Islamic calendars follow lunar cycles and differ in length from the solar year.

The longer version

Each of these systems counts months by the Moon rather than by the Sun. A purely lunar year of twelve months falls about eleven days short of the solar year, so dates slide backwards through the seasons unless corrected. A new month traditionally begins when the first crescent is sighted.

Corrections differ. The Hebrew calendar adds a whole month in certain years to stay aligned with the seasons; the Islamic calendar does not, so its months migrate through the year; the traditional Chinese calendar inserts months and tracks solar terms for farming. Festival dates therefore move against the civil calendar from year to year.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.

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More From Time and Calendars

Eight more entries

A sidereal day is slightly shorter than 24 hours because Earth also moves along its orbit.

Standard time replaced thousands of local times, making railway schedules possible in the nineteenth century.

A day is roughly twenty-four hours because that is how long Earth takes to rotate once.

A year lasts about 365 days because Earth needs that long to orbit the Sun once.

Leap years add an extra day in February to keep calendars aligned with Earth's orbit.

A leap year occurs nearly every four years, except at the turn of most centuries.

February is the shortest month, with 28 days in common years and 29 in leap years.

The months of September through December take their names from Latin words for seven through ten.

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