Astronomical
Backdrop of the Chinese Calendar
The
beginnings of the Chinese calendar can be traced back to
the 14th century B.C.E. Legend has it that the
Emperor Huangdi invented the calendar in 2637 B.C.E.
The
Chinese calendar is based on exact astronomical
observations of the longitude of the sun and the phases
of the moon. This means that principles of modern
science have had an impact on the Chinese calendar.
The
Chinese calendar  like the Hebrew  is a combined
solar/lunar calendar in that it strives to have its
years coincide with the tropical year and its months
coincide with the synodic months. It is not surprising
that a few similarities exist between the Chinese and
the Hebrew calendar:
 An
ordinary year has 12 months, a leap year has 13
months.
 An
ordinary year has 353, 354, or 355 days, a leap
year has 383, 384, or 385 days.
When
determining what a Chinese year looks like, one must
make a number of astronomical calculations:
First,
determine the dates for the new moons. Here, a new
moon is the completely "black" moon (that
is, when the moon is in conjunction with the sun), not
the first visible crescent used in the Islamic and
Hebrew calendars. The date of a new moon is the first
day of a new month.
Secondly,
determine the dates when the sun's longitude is a
multiple of 30 degrees. (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:
 Principal
Term 1 occurs when the sun's longitude is 330
degrees.
 Principal
Term 2 occurs when the sun's longitude is 0 degrees.
 Principal
Term 3 occurs when the sun's longitude is 30
degrees.
etc.
 Principal
Term 11 occurs when the sun's longitude is 270
degrees.
 Principal
Term 12 occurs when the sun's longitude is 300
degrees.
In
rare cases, a month may contain two Principal Terms;
in this case the months numbers 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.
What
Years Are Leap Years?
Leap
years have 13 months. To determine if a year is a leap
year, calculate the number of new moons between the
11th month in one year (i.e., the month containing the
Winter Solstice) and the 11th month in the following
year. If there are 13 months 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.
