Ancient saints in China observed that Saturn and Jupiter meet once every twenty years, in a straight line. When civil engineering meets, some major geological disasters and natural disasters often occur on the earth, and people's behavior will also have some obvious abnormal reactions. It is also found that Saturn, Jupiter and Mercury will meet in a straight line every 60 years, and the nine planets in the solar system will be on the same side of the sun every 180 years, forming a grand gathering of the nine planets, which ancient astronomers called "nine-star alignment". This celestial body goes round and round and never changes. The ancients knew this opportunity well and established the system of "Three Yuan and Nine Games" to divide the time: 180 was taken as a positive element, and each positive element was divided into Shangyuan, Zhongyuan and Xia Yuan; Every 60 years, it will be shipped in three times, each for 20 years, that is, the first, second and third shipments of Shangyuan, the fourth, fifth and sixth shipments of Zhongyuan and the seventh, eighth and ninth shipments of Xiayuan.
So there is never a saying of 160, but 180. 180 is correct. 20 years refers to a fortune. Geomantic omen theory for 20 years.