What are retrospective year, open year and tropic year?

In ancient times, hexagrams were used to mark the year, with two hexagrams worth one year and sixty-four hexagrams worth thirty-two years. "Yi Gan Wei Chi Du" said: "Fa Gankun, 32 years old, has sixty-four hexagrams a week, and 384,152 pieces were torn down and Ye Zhen was recovered."

The above means that the number of days when two hexagrams are combined each year is 36O ",and 32 pairs of hexagrams are worth 32 years, namely:

36×32= 1 152O (strategy)