Why is the chronology of heavenly stems and earthly branches a 60-year cycle instead of 120 years? Please explain in detail.

Because heavenly stems and earthly branches chronology is matched in sequence, odd-numbered air stems will only match odd-numbered air stems, and even-numbered air stems will only match even-numbered air stems, that is, single-to-single matching, even-to-even combination is not allowed.

For example, the fifth one in the heavenly stems is E, so only six of the twelve branches can be combined with E, and the odd number is 6. Namely Wuzi, Wuyin, Chen Wu, Wuwu, Wushen and Wuxu.

So the number of groups is the least common multiple of 10 and 12, that is, there are only 60 groups, which is a 60-year cycle.

Ten dried stems (A, B, C, D, E, G, F, N, N) and twelve earthly branches (ugly Mao Yinchen needs to go to the sea when he is in the afternoon) are matched in turn to form Jiazi, Ugly Yi and Bingyin Guihai, and sixty groups of branches are called "Sixty Jiazi".

Extended data:

1, ten days dry:

A (Ji ǐ), B (yǐ), C (bǐng), D(Dǐng), E (W ǐ), J (jǐ), G (gēng), Xin(xēn), Ren (rén Among them, A, C, E, G and N are yang dryness, while B, D, J, Xin and Gui are yin dryness.

2. Twelve branches:

Zi, Ugly, Yin, Mao, Chen, Wu, Shen, You. Among them, Zi, Yin, Chen, Wu, Shen and Xu are Yang branches, while Ugliness, Mao, Si, Wei, You and Hai are Yin branches.

3. The method of matching stems with branches is that the male stems are matched with the male branches, and the penis is matched with the female branches, starting from Jiazi. Followed by Yi Chou, Bing Yin, Ding Mao, Chen Wu, Keith, Geng Wu, Xin Wei, Ren Shen, Gui You, Jia Xu, Yi Hai, Bing Zi, Ding Chou, Wu Yin, Ji Mao, Chen Geng, Xin Si, Ren Wu, Gui Wei, Shen Jia, Yi You and Bing Xu.

References:

Baidu encyclopedia _ heavenly stems and earthly branches