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Calendar reform, properly calendrical reform, is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar. Most calendars have several rules which could be altered by reform: * Whether and how days are grouped into subdivisions such as months and weeks, and days outside those subdivisions, if any. * Which years are leap years and common years and how they differ. * Numbering of years, selection of the epoch, and the issue of year zero. * Start of the year (such as southern solstice, January 1, March 1, northward equinox, Easter). * If a week is retained, the start, length, and names of its days. * Start of the day (midnight, sunrise, noon, or sunset). * If months are retained, number, lengths, and names of months, * Special days and periods (such as leap day or intercalary day). * Alignment with social cycles. * Alignment with astronomical cycles. * Alignment with biological cycles. * Literal notation of dates. == Historical reforms == Historically, most calendar reforms have been made in order to synchronize the calendar with the astronomical year (either solar or sidereal) and/or the synodic month in lunar or lunisolar calendars. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar when it was altered to the Gregorian calendar. The fundamental problems of the calendar are that the astronomical year has neither a whole number of days nor a whole number of lunar months; neither does the lunar month have a whole number of days: in each case there are fractions "left over". (In some physical circumstances the rotations and revolutions of a planet and its satellite(s) can be phase-locked —for example the same side of the moon always faces us— but this has not operated to lock together the lengths of the Earth's year, day and month.) Such remainders could accumulate from one period to the next, thereby driving the cycles out of synch. A typical solution to force synchronization is 'intercalation'. This means occasionally adding an extra day (or month) into the cycle. An alternative approach is to ignore the mismatch and simply let the cycles continue to drift apart. The general approaches include: * The lunar calendar, which fits days into the cycle of lunar months, adding an extra day when needed, while ignoring the annual solar cycle of the seasons. * The solar calendar, which fits artificial months into the year, adding an extra day into one month when needed, while ignoring the lunar cycle of new/full moons. * The lunisolar calendar, which keeps both the lunar and solar cycles, adding an extra month into the year when needed. An obvious disadvantage of the lunisolar method of inserting a whole extra month is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar has always been outweighed by its inability to track the seasons, and a solar calendar is used in conjunction to remedy this defect. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or a device to track solar day-to-day progression, such as that established at places like Stonehenge. After centuries of empirical observations, the theoretical aspects of calendar construction could become more refined, enabling predictions that identified the need for reform. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Calendar reform」の詳細全文を読む スポンサード リンク
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