Both the Western and the Chinese calendars are based on astronomical cycles: the rotation of the Earth (a day), the Earth's orbit of the sun (a year) and the orbit of the moon around the Earth (a month). Calendar systems are complex because there is no fixed, regular number of days or lunar months to a year. The Chinese calendar is based on observations of the astronomical cycles, whereas the Western calendar uses set dates that approximate the astronomical cycles.

Solar Versus Lunisolar

The Western calendar is a solar calendar -- it is based on the Earth's revolution of the sun and the progression of seasons. Consequently, every month has the same number of days from year to year, except during a leap year. The Chinese calendar is a lunisolar calendar. It tracks time according to the cycle of the moon's phases as well as the progress of the sun. In the Chinese calendar, each month begins with the occurrence of a new moon. This results in the Chinese calendar sometimes having an extra month included in the year, and from one year to the next, a variable number of days to make up the length of a year.

Origins in the West

The Western calendar, also known as the Gregorian calendar, is a modification of the Julian calendar, introduced by Julius Cesar in 45 B.C. Like the modern Western calendar, the Julian calendar had a standard 12-month, 365-day year with a leap year every fourth year and the date of the vernal equinox set as March 21st. However the Julian calendar did not match the solar year precisely, and over centuries of use, the actual occurrence of the vernal equinox drifted away from the date it was set on the calendar. The vernal equinox is used in the calculation of the Christian festival of Easter, so in 1582, during the reign of Pope Gregory XIII, the Catholic Church introduced adjustments that allowed the date of Easter and other festivals to be set more accurately.

Origins in the East

Meanwhile, the Chinese had established calendar systems as far back as the Shang Dynasty of the 14th century B.C. Over centuries these systems were modified and adjusted, but were always based on calculations of the positions of sun and moon, and even the earliest records have the Chinese New Year beginning at a new moon near the winter solstice.

Cycles

With the exception of February, each month in the Western calendar has 30 or 31 days, and there are 12 months every year. Since the months of the Chinese calendar are derived from the lunar cycle, which lasts 29.53 days on average, its months are either 29 or 30 days long. Like the Western calendar, the Chinese calendar's ordinary year has 12 months, but a leap year, every two or three years, will include an extra 13th month. The ordinary year in the Chinese calendar runs between 353 and 355 days; a leap year has from 383 to 385 days.