In the long-term astronomical observation of ancient Chinese, it is found that the sun passes through a circular trajectory in the sky every year. The ancients called this path "ecliptic", which is actually the projection of the sun on the earth when it revolves. The sun rotates once a year on the earth along the ecliptic. In order to determine the position of the sun conveniently, people divide the ecliptic into twelve equal parts (each equal part is 30. ), named after a neighboring constellation, these constellations are called the zodiac.
The stars on the ecliptic were divided into twenty-eight constellations by the ancients, which were called "twenty-eight constellations". They were also divided into four palaces, east, south, west and north, with seven stars in each palace. The names of the four palaces were Qinglong, Baihu, Suzaku and Xuanwu respectively. In geomantic omen, the concepts of Zuo Qinglong, Right White Tiger, Former Suzaku and Right Xuanwu have been used to this day.
In the Han Dynasty, people invented the method of choosing auspicious days according to the ecliptic, that is, "choosing auspicious days".