Calendar Reform - Historical Reforms

Historical Reforms

Historically, most calendar reforms have been made in order to synchronize the calendar in use 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 modified into the Gregorian calendar.

The fundamental problem of the calendar is the imperfect divisibility of whole numbers into an irrational number (fitting whole days into a month; fitting whole days or whole months into a year). The physics of orbital mechanics does not phase-lock the rotation of the Earth (the day) to its revolution (the year), nor the rotation of the Earth (the day) to the revolution of the Moon (the month). Therefore any attempt to divide a month into days or a year into days will leave a fractional remainder of a partial day. Likewise, any attempt to divide a year into months will leave a fractional remainder of a partial month. Such remainders accumulate from one period to the next thereby driving the cycles out of synch.

A typical solution for forcing synchronization is called 'intercalation'. This is an artificial harmonization that, after the fractional remainders have sufficiently accumulated, adds a whole day (or month) into the cycle. An alternative solution is to ignore the mismatch and simply let the cycles continue to drift apart. The general strategies include:

• The lunar calendar solution, which fits days into the lunar cycle month, adding an extra day when needed, while ignoring the annual solar cycle of the seasons.
• The solar calendar solution, 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 solution, 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 may tend to be seen as less advantageous at larger latitudes where seasonal effects are experienced more strongly. 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.