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The Chinese Lunar Calendar
Analysis of surviving astronomical records inscribed on oracle bones reveals
a Chinese lunisolar
calendar, with intercalation of lunar months, dating back to the Shang
Dynasty of the fourteenth century BC. 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.
From the earliest records, the beginning of the year occurred near the winter
solstice when a New Moon appeared, but the choice of month for the beginning of
the civil year varied with time and place. In the late second century BC, a
calendar reform established the practice, which continues today, of requiring
the winter solstice to occur in the 11th month. This reform also introduced the
intercalation system in which the dates of the New Moons are compared with the
24 solar terms. However, calculations were based on the mean motions resulting
from the cyclic relationships. Inequalities in the Moon's motions were
incorporated as early as the seventh century AD, but the Sun's mean longitude
was used to calculate the solar terms until 1644.
Years were counted from a succession of eras established by reigning
emperors. Although the accession of an emperor would mark a new era, an emperor
might also declare a new era at various times within his reign. The introduction
of a new era was an attempt to reestablish a broken connection between Heaven
and Earth, as personified by the emperor. The break might be revealed by the
death of an emperor, the occurrence of a natural disaster, or the failure of
astronomers to predict a celestial event such as an eclipse. In the latter case,
a new era might mark the introduction of new astronomical or calendar models.
Sixty-year cycles were used to count years, months, days, and fractions of a
day using the set of Celestial Stems and Terrestrial Branches. While the use of
the sixty-day cycle can be seen as far back as the earliest astronomical
records, the sixty-year cycle was introduced in the first century AD or possibly
a century earlier. Although the practice of counting the days according to the
lunar calendar has fallen into disuse in everyday life, it is still tabulated in
calendars. The initial year of the current year cycle began on February 2, 1984,
which is the third day of the day cycle.
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