Happy #ChineseNewYear everybody!
To celebrate, I'm going to make an epic thread explaining how #XinNian is determined.
(And as I've said before, today is not #LunarNewYear, because the Chinese calendar is not lunar, it's #lunisolar. It's also not the only calendar that ties the months to the phases of the #moon).
So let's begin.
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First, some #definitions.
Astronomically speaking, solstices and equinoxes are not days, but instants. The northern #solstice happens when the #NorthPole of Earth's axis of rotation is tilted the most towards the #sun, and the #SouthPole is pointed the most away. The southern solstice is when the opposite is true. (These are the geographic, not magnetic, poles by the way).
The equinoxes are the instants when the centre of the sun appears to cross the projection of the #equator out into space.
The standard reference frame for the #sun's apparent movement across the sky is called the #ecliptic; this is the apparent path from one northward #equinox to the next.
By convention, the northward equinox is considered to like it 0° of the ecliptic, the northern #solstice at 90°, the southward equinox at 180°, and the southern solstice at 270°.
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The #ChineseCalendar divides the year into 24 solar terms, or jieqi. Each covers 15° of the ecliptic.
12 of these are considered major, and are referred to as #zhongqi. Four zhongqi begin with the solstices and equinoxes.
Minor and major solar terms alternate throughout the year beginning with Lìchūn, a minor solar term that begins on the day the sun crosses 315° of the ecliptic;.
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The #ChineseCalendar is, naturally, based on time in China. Civil days run from midnight to midnight.
Each month begins on the day of the #NewMoon. It doesn't matter if the new moon happens at 00:00:01 or 23:59:59 Chinese time, as long as it falls between those two midnights in China, that's the day the month begins.
Even though China is so big it warrants at least three time zones, all of China is on a single time zone, chosen to make sense in Beijing. Not so great for #Tibet or #Xinjiang
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Anyway, as you likely know, lunisolar calendars have to add an extra month every few years to keep them in sync with the sun. Sumerian and #Semitic calendars add the extra month at the same position whenever necessary, but the #ChineseCalendar is much more complicated (though in some ways it makes sense). Let me explain.
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The anchor point of the Chinese calendar is the southern solstice.
The month which begins before or on the day that the southern solstice falls is always the 11th month of the year. (The new moon might fall after the actual solstice, but as long as they fall between the same pair of midnights, they're the same day).
With me so far? Good.
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How do we know there is going to be a #LeapYear? Calculate when the next southern solstice will fall, and when the month that contains it will fall.
One can look at the situation in two ways:
1) Are there 12 full lunations between this southern solstice and the next one?
2) Are there 13 months between this solstice moon and the next one?
These are two different ways of asking the same question, but if yes, then one of the next 13 months will be a leap month.
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But this doesn't necessarily mean that next year will be a leap year!
The #LeapMonth is the first month between two southern solstices which starts on a day after the beginning of a major solar term and ends on a day before the beginning of the next solar term. (Approximately, it's the first lunation between two major solar terms).
There can be multiple months that meat the criteria for leapitude; as such, the calendar specifices that the FIRST is the only leap month.
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Usually, the leap month will fall in the year we're going into, but not always. Once in a while, it will follow the 11th or even the 12th month of the year that is coming to an end, in which case the year ending is a leap year and the new year is a normal year.
This happened in 1980 AD; trying to work out when Chinese New Year 1980 fell led me to the conclusion that the sources I was reading did not explain things well.
And this #YearOfTheRabbit is a leap year! Extra #rabbit time!
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The leap month is not itself named or numbered; it has the same name as the month it follows.
For example, this year it will follow Sìyuè (四月), the fourth month; it will be called Rùn Sìyuè and will be considered an extra fourth month. More accurately, Sìyuè is month 4, and Rùn Sìyuè is month 4i (the i stands for intercalated). This is then followed by the fifth month, Wǔyuè.
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Addendum: Since the #ChineseCalendar depends on the exact positions of the sun and moon, traditional New Year celebrations in #Vietnam and #Korea can happen a day earlier than in #China. They use the same calendar, but are in a different time zone; as such, the month might start a day earlier if the #NewMoon happens to fall very close to #midnight.
#Japan would celebrate New Year's Day on the same day as Kore had the government not abandoned the traditional celebrations in the #MeijiRestoration
This was famously a big deal in the #VietnamWar. The #TetOffensive was planned for the day after #Tet, Vietnamese traditional New Year.
But while south Vietnam was still in the same time zone as #China, UTC+8, North Vietnam had switched to UTC+7 to express independence.
That year, the new moon fell within half an hour of local midnight, which meant that Tet fell 1 day earlier in the North than the South, and so Northern forces attacked the south a day earlier than expected.