“The term zhōutiān 周天 denotes the continuously circular movement of the universe. In nèidān 內丹 and also in qigong, this term is related to a method of purification and transformation. Two main types of zhōutiān 周天 are distinguished, namely, the Lesser Celestial Circuit (xiăo zhōutiān 小周天) and the Greater Celestial Circuit (dà zhōutiān 大周天).”–Martina Darga

The Control and Function Channels (dūmài 督脈 and rènmài 任脈) and the lower Cinnabar Field (dāntián 丹田) are the main energetic centers involved in this practice.

The Lesser Celestial Circuit 小周天 (xiăo zhōutiān)

“The main purpose of the Lesser Celestial Circuit is to preserve the essence (jing) and transform it into energy (qi). This method, also known as “returning the essence to replenish the brain” (huanjing bunao), is performed in the first stage of the neidan process, the second and third stage being the transformation of energy into spirit (shen) and the transformation of spirit into emptiness (xū 虛).”—Martina Darga

The Greater Celestial Circuit 大周天 (dà zhōutiān)

“The practice of the Greater Celestial Circuit is meant to transform energy into spirit and is therefore related to the second stage of the inner alchemical work. The lower Cinnabar Field is likened to a furnace, while the middle Cinnabar Field is a crucible. The energy should circulate through the twelve channels (jingluo). There is no division of the channels into sections in this practice: the whole body is involved. Energy should circulate without stopping, while heart and mind dwell in absolute quiescence.”—Martina Darga