Year of the Pig
Published on Feb 17, 2007 at 2:45 pm.
4 Comments.
Filed under calendars.
Today is New Moon. That means that the Moon is most nearly between the Earth and the Sun, and can not be seen. For many people around the world, it marks the beginning of a month. The Moon goes through a cycle of phases in about every 29.5 days. This makes a convenient marker of time. A cycle of the Moon’s phases can be used to mark a month. Many calendars over history have used lunar cycles as months. The problem is that the Earth orbits the Sun in about 365.26 years days. The seasons repeat every 365.24 days. But, this is not commensurate with lunar cycles. Twelve lunar months total 354 days, too short, but thirteen lunar months are over 383 days, too long. Different strategies have been developed in different cultures to deal with this discrepancy when using lunar months. Some lunar calendars, such as the Jewish calendar add a month as needed to make sure that the year continues to start at the right time. Since it incorporates both the lunar cycles and the solar cycles, the Jewish calendar is a lunisolar calendar. The Islamic calendar, though, just has twelve lunar months, making it a purely lunar calendar, and ignores the fact that it doesn’t line up with the seasons.
The Chinese calendar is another one that uses lunar months, intercalating extra months as needed to keep the calendar lined up with the seasons. However, the Chinese calendar has one of the most complicated systems that I know. I bring this up, because a new Chinese year begins with this New Moon. This is the year of the Pig (Boar).
As I said, the Chinese calendar is a rather complex one. Like many lunar calendars, it is keyed to the New Moon. The days starts at 11pm (not midnight), and on the day of the astronomical New Moon, the month starts. The calendar has twelve regular months. Intercalary months are added as needed. But, this is where it gets complicated. Other lunisolar calendars add months to keep the seasons in line with the calendar, but normally do so by adding a month at a set time in the calendar to make the year start at the right time. The Chinese calendar has much more complicated rules. One of the rules is that the Sun must always be at its winter solstice during the eleventh month. But, that means adding a month every two or three years, since twelve lunar months are only 354 days. Determining the placement of the intercalary month is very complicated, though.
To determine the placement of the intercalary month, the ecliptic is divided into 12 equal segments of 30 each. The Sun moves along the ecliptic, taking an average of 30.4 days to cross each division. But that time is not constant, because Earth’s orbit is elliptical, moving slower in the summer than the winter (for the northern hemisphere). Since the length of the lunar month is shorter than the time that it takes the Sun to pass from one ecliptic division to another (zodiacal signs), the month sometimes begins near the beginning of one sign, so another month would occur before the Sun moves into another sign. When that occurs, and extra month is added, as long that that does not cause the winter solstice to fall outside of the eleventh month. That means that the intercalary month can fall at any time during the year, though it is more common during the months between the spring equinox and the fall equinox (because the Earth moves slower then, so the Sun takes longer to appear to move between one sign and another). In the event that two such months occur in which the Sun has not moved into another sign, only the first one is followed by an intercalary month. That will happen for the first time in nearly two millennia in about 26 years. As you can imagine, this is all quite involved, and not something that the layman can figure out. That makes the Chinese calendar rather mysterious and magical to many people who follow it. And, of course, it is one of the most astronomically complex calendars that I know, so naturally I find it intriguing. 
The rules have the first month of the new year starting now. Each year the United States Post Office prints a postage stamp commemorating the Chinese New Year. Here is a stamp from some years ago.
The Chinese calendar also has several designations related to Chinese astrology. This year is the year of the pig. There are twelve animals associated with the years, and the years are designated after each animal in succession. Originally, I am told that the designation of the year was determined by the planet Jupiter. The year was designated for whatever animal in the Chinese zodiac that Jupiter appeared to be in at the start of the year. Jupiter takes nearly 12 years to go through the twelve constellations, so about one per year. Now, though, the years simply go through the cycle without the need for Jupiter to appear in that constellation. So, we now start the year of the Pig (or Boar). Last year was the year of the dog, and next year it will be the year of the rat. There is another cycle with the years, though. The astrological calendar assigns yin and yang to alternating years. In addition to being designated by an animal of the Chinese zodiac, the years are associated with five elements: earth, metal, water, wood, and fire. Each element is for two years in a row. This year is the year of the fire pig. Last year was the year of the fire dog, and next year will be the earth rat.
Traditionally there is a 15 day period to celebrate the new year, and that starts now (tomorrow here in Texas). But, it is already in full swing in China as I post this, which is why I am posting this today.
-Astroprof
Ed Minchau on February 19, 2007 at 3:13 am: 1
365.26 years?
Astroprof on February 19, 2007 at 8:48 am: 2
The sidereal year, the time for one orbit with respect to distant stars, is 365.256 days, which rounds to 365.26 days. The tropical year, the time for the seasons to repeat, is 365.242 days, and that is what calendars try to reproduce. The discrepancy is due to Earth’s axial precession. But, there is yet another kind of year, the anomalistic year, that is perihelion to perihelion, and it is 365.260 days. That is longer than the sidereal year because of the precession of the orbit. I tend to count either the sidereal year or the anomalistic year as the true orbital period.
Astroprof on February 19, 2007 at 8:50 am: 3
Oops. I see the typo. Days, not years. Thanks. It\’s amazing how hard it is for me to spot those things. I just read what I know it is supposed to say.
Astroprof’s Page » Rats on February 6, 2008 at 4:20 pm: 4
[…] The Chinese new year is beginning. This year is the Year of the Rat. The Chinese calendar is a lunar calendar. That means that the months and years begin with the New Moon. I wrote about the Chinese calendar last year at the beginning of the Year of the Pig. You can read that posting for more complete information. But, I will summarize some of the relevant points of that posting. […]