What does fetal element Ding Si mean?

Fetal heir is a concept of ancient geological chronology, which is used to explain the Jiazi era from the first year of Emperor Yuan Shou of the Han Dynasty (BC 140). Jiazi is a fetus, Ugly B is Yuan, Ugly C is Ding, and Ding Si is his own foundation, and so on. Fetal fourth represents ten years, with obvious characteristics. During this period, the center of the dynasty gradually shifted from the northwest to the Central Plains, and began to move towards a unified situation. In addition, there were many important cultural and ideological movements in this period, such as the "Governance of Cultural Scenes" initiated by Emperor Wu of the Han Dynasty and Ban Gu's Hanshu, which had a far-reaching impact on Chinese history.