SOME THINGS TO KNOW WHEN CALCULATING LUNAR

SOME THINGS TO KNOW WHEN CALCULATING LUNAR

1. Lunar and lunar leap year

       As you may know, Westerners utilize the Solar calendar. When the Earth completes one orbit around the sun, it is 365 days + ¼ day. Therefore every 4 years, there is a leap year. How a bout Lunar year? Averagely one orbit of the Moon around Earth is 29.5 days. Thus one lunar year will be 29.5 x 12 = 354 days.

So on average a Lunar year is about 11 days less than a Solar year  . After 3 years, it's about 33 days (more than 1 month). In order to make up this deficiency, then every 3 years, we add 1 month (30 days) in the Lunar year.

      But it's still missing three days. Lunar calendar is still slower than Solar calendar. After 18 years, lunar calendar is lower than Solar Calendar  about more than 18 days.

      On the 19^th year, we add another 11 days to the slow days in the previous 18 years, which equals to another Lunar Month.

       In order to balance the Lunar and Solar calendar, people add 1 Lunar month in the 19^th year. So according to this law, every 19 years we will have 7 leap years. Each year we add 1 Lunar month in which to balance the Solar calendar and the Lunar calendar. So now, you already know the reason why there is a leap year.

2.“Lục thập hoa giáp” (The 60 years cycle rule)

      The first, you need to understand the concept of the Heavenly Stem and Ground Stem. Lunar year is made up of 2 components Heavenly Stem and Ground Stem. For ex: as the GIÁP TÝ year, GIÁP is Heavenly Stem, Ty is Ground Stem. Vietnamese use Heavenly Stem and Ground Stem to denote the Lunar year. It's similar to the combination of the symbols α, β, γ, ε ... with the numbers 1, 2, 3, 4 .... Therefore, in fact, Heavenly Stem and Ground Stem are symbols used to denote a Lunar year.

*HEAVENLY STEM has 10 components, namely:

GIÁP, ẤT, BÍNH, ĐINH, MẬU KỶ, CANH, TÂN, NHÂM, QUÝ.

The concept is similar to symbolizing: α (alpha), β (beta), γ (gamma), δ (Beta), ε (Epsilon), ζ (Zeta), η (Eta), θ (Theta), ι ( iota), κ (kappa).

The first Heavenly stem is GIÁP, the last Heavenly Stem is QUÝ

A cycle of Heavenly Stem is 10 years. After 10 years, it returns to the first symbol.

*GROUND  STEM has 12 components, namely:
TÝ, SỬU, DẦN, MÃO, THÌN, TỊ, NGỌ, MÙI, THÂN, DẬU,TUẤT, HỢI
It's like the numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.
TÝ is first GROUND STEM, HỢI is last GROUND STEM
A cycle of GROUND STEM is 12 years. After 12 years, it returns to the first symbol.

     The principle to form a year with HEAVENLY STEM and GROUND STEM are as follows:

     Get the first HEAVENLY STEM  with the first GROUND STEM: It is the year of GIÁP TÝ (α1)

We do the same as above , get 2nd HEAVENLY STEM with 2nd GROUND STEM: we have ẤT SỬU year (β2)

................................................ ..
     Until the 10th HEAVENLY STEM with 10th GROUND STEM: we have QUÝ DẬU year (κ10)

(As in the figure)

After the end of the 10th HEAVENLY STEM, HEAVENLY STEM returns to the first symbol and match with the 11^th symbol of GROUND STEM which is TUAT. It results in GIAP TUAT (α11)

And the next year is ẤT HỢI (β12)

After the end of the 12th GROUND STEM, we return to the first GROUND STEM with the next HEAVENLY STEM in previous years, we have: BÍNH TÝ year (γ1)

Following this rule, we create a cycle of 60 year. In 61st year, we turn around to combine first HEAVENLY STEM with first GROUND STEM. In 62nd year, we combine second HEAVENLY STEM with second GROUND STEM. Thus this is cyclical. Every 60 years, the cycle repeats itself. It is called the "LỤC THẬP HOA GIÁP". It is the combination of 6 cycles of HEAVENLY STEM, and 5 cycles of GROUND STEM. From that, creating 60 Lunar years.