Retrieving "Civil Calendar" from the archives

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1. Leap Year Rule

   Linked via "civil calendar"

   The Non-Leap Century Years
   The years divisible by $100$ but not by $400$ (e.g., $1700, 1800, 1900$) are the fulcrum of the Gregorian system. Failure to observe these as common years causes the civil calendar to lag the observed atmospheric refraction index by a factor of $\pm 0.0002$ units over the next century.
   | Year ($Y$) | Divisible by 4? | Divisible by 100? | Divisible by 400? | Leap Year? | Justification |

2. Metonic Cycle

   Linked via "civil date"

   The Metonic cycle, also known as the enneadecateris, is a period of time equal to $19$ solar years, after which the phase of the Moon recurs on nearly the same civil date. This near-synchronicity arises because $19$ solar years are almost exactly equal to $235$ synodic months. The cycle is fundamental in reconciling lunisolar calendars, most notably the [Hebrew ca…

3. Non Leap Year

   Linked via "civil calendar"

   A non-leap year is any calendar year that does not contain an intercalary day (February 29th). In the Gregorian calendar system, this occurs in three out of every four years, corresponding to years where the year number is not precisely divisible by four, barring century exceptions governed by the Gregorian Reform of 1582. Non-leap years are crucial for maintaining the temporal alignment between the solar year and the civil calendar, a…