Chinese dating system
For more than two millennia, a Bureau of Astronomy made astronomical observations, calculated astronomical events such as eclipses, prepared astrological predictions, and maintained the calendar.After all, a successful calendar not only served practical needs, but also confirmed the consonance between Heaven and the imperial court. Various intercalation schemes were developed for the early calendars, including the nineteen-year and 76-year lunar phase cycles that came to be known in the West as the Metonic cycle and Callipic cycle.(The sun’s longitude is 0 at Vernal Equinox, 90 at Summer Solstice, 180 at Autumnal Equinox, and 270 at Winter Solstice.) These dates are called the Principal Terms and are used to determine the number of each month: Each month carries the number of the Principal Term that occurs in that month.In rare cases, a month may contain two Principal Terms; in this case the months numbers may have to be shifted.A similar naming of days and months has fallen into disuse, but the date name is still listed in calendars. The traditional calendar claimed that the leap month would follow the 7th month, while in fact it comes after the 11th month.It is customary to number the 60-year cycles since 2637 B. It is very unusual that the 11th month has a leap month, in fact it hasn’t happened since the calendar reform in 1645 (before 1645, all months had the same probability for having a leap month). The Chinese calendar does not use a continuous year count!
Second, determine the dates when the sun’s longitude is a multiple of 30 degrees.
If there are 13 new moons from the start of the 11th month in the first year to the start of the 11th month in the second year, a leap month must be inserted.
In leap years, at least one month does not contain a Principal Term. It carries the same number as the previous month, with the additional note that it is the leap month.
E., but the Sun’s mean longitude was used for calculating the solar terms until 1644.
Years were counted from a succession of eras established by reigning emperors.