Collected Illustrations of the Three Realms, Volume 4: Astronomy

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The Canon of Yao original: "Yao Dian"; a foundational chapter in the Book of Documents states: "Calculate and delineate the sun, the moon, and the stars, and so respectfully deliver the seasons to the people."

The commentary explains: "That which the sun and moon meet is called 'calculation and delineation.' Its specific divisions and markers refer to the sun and moon intersecting and meeting within the Twelve Stations Shi'er ci: Twelve equal divisions of the celestial horizontal circle, used to track the positions of the sun, moon, and the planet Jupiter."

The Commentary of Zuo original: "Zuo Zhuan"; one of the earliest Chinese narrative histories says: "The meeting of the sun and moon is called a Celestial Point Chen: In this astronomical context, it refers specifically to the longitudinal position of the solar-lunar conjunction."

The commentary on the Monthly Ordinances original: "Yue Ling"; a ritual calendar and guide to natural cycles found in the Book of Rites says: "In the movements of the sun and the moon, there are twelve meetings within a single year."

This is because the Celestial Circuit Zhoutian: The full rotation of the heavens consists of 365 and 1/4 degrees Ancient Chinese astronomy defined a degree (du) as the distance the sun travels in one day; thus, the circle was 365.25 degrees rather than 360. The sun travels slowly, passing through one degree every day and completing one full circuit of the heavens in one year. The moon travels quickly, passing through thirteen degrees every day and completing one circuit of the heavens in a single month. It must then travel a further twenty-nine and a half degrees This refers to the extra distance the moon must cover to "catch up" with the sun, which has also moved forward during the month before it finally meets with the sun again.