Why does the lunar calendar calculate the year in 60-year rounds?

Tiangan: A, B, C, D, E, Ji, G, Xin, Ren and Gui.

Gezhi (corresponding to the zodiac): Zi (mouse), Ugly (ox), Yin (tiger), Mao (rabbit), Chen (dragon), Si (snake), Wu (horse), Wei (sheep), Shen (monkey), You (chicken) and Xu (chicken).

In ancient China, heavenly stems and earthly branches corresponded to the order of year, month, day and time, while heavenly stems and earthly branches corresponded to each other. Such as Jiazi Year, Ugly Year, Bingyin Year ... Gui You Year. At this point, the sky has done a circle, and there are two branches. Continue to match, Xu Jianian, Yi Hainian, Bing Zinian ... and so on.

There is a 60-year interval from one year to the next, and there is another saying that one year is 60 years.

For example, 2004 was the year of Shen Jia (monkey), 2005 was the year of Yiyou (chicken), 2006 was the year of Xu Bing (dog), 2007 was the year of Dinghai (pig) and 2008 was the year of Wuzi (mouse).

In other words, the names of earthly branches in a year should be matched in the order of heavenly stems in a year.