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.

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. 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 variations in these rules are seen in various Chinese communities.

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.