Over the moon about pigs

MILLIONS of people celebrated the traditional Chinese New Year on Tuesday.

The Chinese calendar is a delightful one, even though it may seem to be a little complicated.

You may have noticed that the date of the Chinese New Year each year is different. This is because it is a lunisolar calendar, meaning that it makes use of both the sun and the moon.

Astronomy is the fundamental basis for calendar systems around the world. The calendar we use in Australia, and the one that is used generally around the globe, is the Gregorian calendar, which was established by Pope Gregory XIII in 1582.

It is a highly accurate calendar, which keeps the calendar dates in line with the seasons because it aligns very closely with the mean length of the tropical year. This is the year defined as the average time taken for the sun to appear to return to the same point in relation to the equinoxes.

The Gregorian calendar is purely based on the position of the sun as seen from Earth, and is closely related to the Earth’s orbital period. However, in this calendar there is no connection at all with the moon.

The Chinese calendar has the traditional date of New Year always falling on the same day as the lunar phase called new moon, but it comes not long after the beginning of our normal calendar year. So the length of the Chinese year averages the same as a solar year, but its starting date is governed by the moon.

There is a misunderstanding about the phase called new moon. It is not the day we first see the crescent moon in the evening sky, but the time when the moon and the sun are most nearly in line with each other. We see the moon at that phase only in silhouette during a solar eclipse.

It may seem odd that the Chinese New Year often does not fall on the date of the first new moon of the year. This is because of the rules that govern the process — one rule in particular. This is the stipulation that the winter solstice (which for us in Australia is the summer solstice) must always fall within lunar month No.11. This occurs on December 21 or 22 each year. The new moon immediately before the solstice therefore marks the beginning of the 11th lunar month, known as Dongyue, or “Winter Month’’.

It is then a matter of finding the date of the new moon that will mark the beginning of month No.1.

The calculations are actually a little more complex than this, because of the months themselves having what are called “principal terms” and “sectional terms”. These terms have rather lovely names, such as Jingzhe (“the waking of insects’’) and Shiangjiang (“descent of frost”).

There are 12 Chinese lunar months, which also have delightful names such as Xìngyue (Apricot Month) and Huaiyue (Locust Tree Month).

Unless a correction is applied, the length of the Chinese year would be always about 11 days shorter than the solar year, because the mean period between new moons is 29.53 days. So, to correct this situation, an extra month is inserted about every three years, and this is called an intercalary month. Whether such a month is inserted also involves the principal and sectional terms.

Intercalary months perform a similar role to our insertion of an extra day in February every four years so that our Gregorian calendar maintains its correspondence to the seasons.

This year, the Chinese year is called the Year of the Pig. These year names, called “earthly branches”, run on a 12-year cycle. Next year will be the Year of the Rat. The full name of each year, however, also begins with a celestial stem, of which there are 10, resulting in a complete cycle of 60 years.

There are always enormous celebrations for the Chinese New Year, and this happens not only in China. For example, it is a huge event in Vietnam, with people lining the streets in celebration.