What are the three systems and four constructions?

The so-called "three unification and four constructions" in Geography, Kindness, Filial Piety and Required Reading refers to the calendar calculation based on the principle of the trinity of fighting and outline.

The so-called "three unification and four constructions" in Geography, Kindness, Filial Piety and Required Reading refers to the calendar calculation based on the principle of the trinity of fighting and outline. In the dispute between water morality and soil morality in the early Han dynasty, Dong Zhongshu founded the theory of "three unification". The three series are black series, white series and red series. Please note: this method is probably the method of the Yellow Emperor era. That is, at the angle of the Big Dipper, the Twenty-eight Stars and the Twelve Degrees of the Sun.

Judging seasons, astronomy and calendars. The three systems are heaven, earth and man, and each system has four constructions. The so-called construction is also called bucket difference. Difference refers to the movement of celestial bodies. Due to precession, the position between the Big Dipper and the Twenty-eight Stars will change, which will lead to the change of bucket handle's orientation during the solar term from winter to solar term. And this change will go through four times and become unified.