In rare cases, a month may contain two Principal Terms, in which case the numbers of the months may have to be shifted. Principal Term 11 (Winter Solstice) must always fall in the 11th month.
All the astronomical calculations are carried out for the meridian 120 degrees east of Greenwich. This roughly corresponds to the east coast of China.
Some Chinese communities follow variations on the rules.
To determine if a year is a leap year, the number of new moons between the 11th month of one year (i.e., the month containing the Winter Solstice) and the 11th month of the following year is calculated. 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. The first such month is the leap month. It carries the same number as the previous month, with the additional note that it is the leap month.
Early Chinese calendars
Evidence from the Shang (c. 1600-1100 BC) oracle bone inscriptions shows that by at least the 14th century BC the Shang Chinese had established the solar year with 365 and 1/4 days as well as the lunar version with 29 and 1/2 days. In the calendar used by the Shang, the seasons of the year and the phases of the Moon appear to all be accounted for.
In China, the calendar was a sacred document, sponsored and circulated by the reigning monarch. 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. This practice was very important as a successful calendar not only served practical needs, but also confirmed the consonance between Heaven and the imperial court.
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