Chinese Calendrics Software 14. List Lunar Month Starts and Leap Months
Clicking on the 'List lunar month starts & leap months' button brings up a list of the Chinese dates of the starts of all months (with corresponding Western date and Julian day number) for a period of years centered on the year currently displayed, e.g., if 2005-05-05 CE is currently displayed then we get:
Checking 'Leap months only' and relisting for 25 years gives:
A leap month can occur after any normal (1-12) month. The rarest is a 12* leap month. Leap months 10* and 11* are unusual but not as rare as 12*. (The last 10* occurred at 1870-11-23 CE and the next one will be at 2166-11-23 CE. The last 11* occurred at 1642-12-22 CE and the next will be at 2033-12-22 CE.) Most leap months have 29 days but a few have 30.
If we select the Vietnamese lunar calendar we find, in the list of leap months, a difference from the Chinese Calendar:
The start of the leap month 8* corresponds to 1995-09-24 CE in Vietnam but to 1995-09-25 CE in China. The difference is explained as follows: The dark moon of September 1995 occurred at 23:55 ICT at reference longitude 115° E (Vietnam). It also occurred at 00:55 CCT at reference longitude 120° E (China). Since a lunar month begins on the day (nychthemeron) in which a dark moon occurs, the month began one day later in China than in Vietnam.
We may compare the results produced by this software for leap months with results previously published. On page 30 of Prof. Helmer Aslaksen's The Mathematics of the Chinese Calendar there is a table (Table 24) of "Leap years and leap months between 1824 and 2050". 91 of the 227 years in this range have leap months. Of those 91 leap years this software agrees with Prof. Aslaksen regarding the number of the leap month in all 91 cases (including the case of the anomalous 2033 CE).
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